JUSTUS-LIEBIG-UNIVERSITÄT GIESSEN

Submitted manuscripts

: Franz, V. H. (submitted; Preprint at arXiv:0710.2024). Ratios: A short guide to confidence limits and proper use.
[ web-site ]
Researchers often calculate ratios of measured quantities. Specifying confidence limits for ratios is difficult and the appropriate methods are often unknown. Appropriate methods are described (Fieller, Taylor, special bootstrap methods). For the Fieller method a simple geometrical interpretation is given. Monte Carlo simulations show when these methods are appropriate and that the most frequently used methods (index method and zero-variance method) can lead to large liberal deviations from the desired confidence level. It is discussed when we can use standard regression or measurement error models and when we have to resort to specific models for heteroscedastic data. Finally, an old warning is repeated that we should be aware of the problems of spurious correlations if we use ratios.
: Kirchner, H., Franz, V. H., Thorpe, S. J., & Brenner, E. (submitted). Animals make you move: Fast online control in natural scenes.
: Schenk, T., Franz, V. H., & Bruno, N. (submitted). Converging evidence for a theoretical alternative: an integrative view of the visual cortex accounts better for the neuropsychological and psychophysical data.
: Eloka, O., & Franz, V. H. (submitted). Effects of object shape on the visual guidance of action.

Journal papers

: Hesse, C., & Franz, V. H. (in press). Grasping remembered objects: Exponential decay of the visual memory. Vision Research.
: Fiehler, K., Bannert, M. M., Bischoff, M., Blecker, C., Stark, R., Vaitl, D., Franz, V. H., & Rösler, F. (in press). Working memory maintenance of grasp-target information in the human posterior parietal cortex. NeuroImage.
: Janczyk, M., Franz, V. H., & Kunde, W. (2010). Grasping for parsimony: Do some motor actions escape dorsal processing? Neuropsychologia, 48, 3405-3415.

It is an open question whether the visual transformations guiding human actions are similar to those generating visual perception. The Action-Perception model assumes a strict division of labor: the ventral cortical stream generates perception while the dorsal stream guides actions. However, only skilled and natural actions are assumed to be under dorsal control, while awkward and left-handed actions should be under ventral control in the same way as perception. Here, we used a combination of Garner-Interference and the psychological refractory period (PRP) paradigm to test this notion. We found that all types of grasping (left-handed, awkward, using a tool) behave in a way similar to skilled right-handed grasping: other than perception they show no Garner-Interference, but similar to perception they show a limitation of processing capacities as indicated by the PRP paradigm. This behavior suggests that similar processes guide all these actions.
: Brouwer, A. M., Franz, V. H., & Gegenfurtner, K. R. (2009). Differences in fixations between grasping and viewing objects. Journal of Vision, 9, 1-24.
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Where exactly do people look when they grasp an object? An object is usually contacted at two locations, whereas the gaze can only be at one location at the time. We investigated participants´ fixation locations when they grasp objects with the contact positions of both index finger and thumb being visible and compared these to fixation locations when they only viewed the objects. Participants grasped with the index finger at the top and the thumb at the bottom of a flat shape. The main difference between grasping and viewing was that after a saccade roughly directed to the object´s center of gravity, participants saccaded more upward and more into the direction of a region that was difficult to contact during grasping. A control experiment indicated that it was not the upper part of the shape that attracted fixation, while the results were consistent with an attraction by the index finger. Participants did not try to fixate both contact locations. Fixations were closer to the object´s center of gravity in the viewing than in the grasping task. In conclusion, participants adapt their eye movements to the need of the task, such as acquiring information about regions with high required contact precision in grasping, even with small (graspable) objects. We suggest that in grasping, the main function of fixations is to acquire visual feedback of the approaching digits.
: Bruno, N., & Franz, V. H. (2009). When is grasping affected by the Müller-Lyer illusion? A quantitative review. Neuropsychologia, 47, 1421-1433.

Milner and Goodale (1995) [Milner, A. D., & Goodale, M. A. (1995). The visual brain in action. Oxford, UK: Oxford University Press] proposed a functional division of labor between vision-for-perception and vision-for-action. Their proposal is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, it has remained controversial in the prediction that actions are not affected by visual illusions. Following up on a related review on pointing (see Bruno et al., 2008 [Bruno, N., Bernardis, P., & Gentilucci, M. (2008). Visually guided pointing, the Müller-Lyer illusion, and the functional interpretation of the dorsal-ventral split: Conclusions from 33 independent studies. Neuroscience and Biobehavioral Reviews, 32(3), 423-437]), herewe re-analyze 18 studies on grasping objects embedded in the Müller-Lyer (ML) illusion.We find that median percent effects across studies are indeed larger for perceptual than for grasping measures. However, almost all grasping effects are larger than zero and the two distributions show substantial overlap and variability. A fine-grained analysis reveals that critical roles in accounting for this variability are played by the informational basis for guiding the action, by the number of trials per condition of the experiment, and by the angle of the illusion fins. When all these factors are considered together, the data support a difference between grasping and perception only when online visual feedback is available duringmovement. Thus, unlike pointing, grasping studies of the Müller-Lyer (ML) illusion suggest that the perceptual and motor effects of the illusion differ only because of online, feedback-driven corrections, and do not appear to support independent spatial representations for vision-for-action and vision-for-perception.
: Franz, V. H., Hesse, C., & Kollath, S. (2009). Visual illusions, delayed grasping, and memory: No shift from dorsal to ventral control. Neuropsychologia, 47, 1518-1531.

We tested whether a delay between stimulus presentation and grasping leads to a shift from dorsal to ventral control of the movement, as suggested by the perception-action theory of Milner and Goodale (Milner, A.D., & Goodale, M.A. (1995). The visual brain in action. Oxford: Oxford University Press.). In this theory the dorsal cortical stream has a short memory, such that after a fewseconds the dorsal information is decayed and the action is guided by the ventral stream. Accordingly, grasping should become responsive to certain visual illusions after a delay (because only the ventral stream is assumed to be deceived by these illusions).We used the Müller-Lyer illusion, the typical illusion in this area of research, and replicated the increase of the motor illusion after a delay. However, we found that this increase is not due to memory demands but to the availability of visual feedback during movement execution which leads to online corrections of themovement. Because suchonline corrections are to be expected if themovement is guided by one single representation of object size,we conclude that there is no evidence for a shift fromdorsal to ventral control in delayed grasping of the Müller-Lyer illusion.We also performed the first empirical test of a critique Goodale (Goodale, M.A. (2006, October 27). Visual duplicity: Action without perception in the human visual system. The XIV. Kanizsa lecture, Triest, Italy.) raised against studies finding illusion effects in grasping: Goodale argued that these studies used methods that lead to unnatural grasping which is guided by the ventral stream. Therefore, these studies might never have measured the dorsal stream, but always the ventral stream.We found clear evidence against this conjecture.
: Hesse, C., & Franz, V. H. (2009). Corrective processes in grasping after perturbations of object size. Journal of Motor Behavior, 41, 253-273.

Researchers proposed that humans may achieve grip adaptation to a new object size by reprogramming and substituting the initially planned motor program. The authors investigated corrective processes in grasping by using a size perturbation paradigm. In 3 experiments, they investigated how grip adjustments are influenced by different perturbation times (early or late), the visibility of the moving hand, and different perturbation sizes (small or large). Results indicated that individuals execute corrections faster after late perturbations. The availability of visual information about the hand had minimal effect on the corrections, suggesting that feedforward mechanisms are involved. Moreover, participants achieved adjustments mainly by smooth changes of the aperture over time, contradicting the researchers´ assumption that a new movement is programmed and superimposed.
: Hesse, C., & Franz, V. H. (2009). Memory mechanisms in grasping. Neuropsychologia, 47, 1532-1545.

The availability of visual information influences the execution of goal-directed movements. This is very prominent in memory conditions, where a delay is introduced between stimulus presentation and execution of the movement. The corresponding effects could be due to a decay of the visual information or to different processing mechanisms used for movements directed at visible (dorsal stream) and remembered (ventral stream) objects as proposed by the two visual systems hypothesis. In three experiments, the authors investigated grasping under full vision and three different delay conditions with increasing memory demands. Results indicate that the visuomotor information used for grasping decays rapidly. No evidence was found for qualitative changes in movement kinematics and the use of different representations for visually guided and memory guided movements. Findings rather suggest that delayed grasping is similar to grasping directed to larger objects under full vision. Therefore, the authors propose that grasping after a delay is guided by classic memory mechanisms and that this is reflected in an increasing maximum grip aperture in grasping.
: Kleinholdermann, U., Stockmeyer, K., Gegenfurtner, K. R., & Franz, V. H. (2009). Grasping isoluminant stimuli. Experimental Brain Research, 197(1), 15-22.

We used a virtual reality setup to let participants grasp discs, which differed in luminance, chromaticity and size. Current theories on perception and action propose a division of labor in the brain into a color proficient perception pathway and a less color-capable action pathway. In this study, we addressed the question whether isoluminant stimuli, which provide only a chromatic but no luminance contrast for action planning, are harder to grasp than stimuli providing luminance contrast or both kinds of contrast. Although we found that grasps of isoluminant stimuli had a slightly steeper slope relating the maximum grip aperture to disc size, all other measures of grip quality were unaffected. Overall, our results do not support the view that isoluminance of stimulus and background impedes the planning of a grasping movement.
: von Luxburg, U., & Franz, V. H. (2009). A geometric approach to confidence sets for ratios: Fieller's theorem, generalizations, and bootstrap. Statistica Sinica, 19(3), 1095-1117.
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We present a geometric method to determine confidence sets for the ratio E(Y)/E(X) of the means of random variables X and Y. This method reduces the problem of constructing confidence sets for the ratio of two random variables to the problem of constructing confidence sets for the means of one-dimensional random variables. It is valid in a large variety of circumstances. In the case of normally distributed random variables, the so constructed confidence sets coincide with the standard Fieller confidence sets. Generalizations of our construction lead to definitions of exact and conservative confidence sets for very general classes of distributions, provided the joint expectation of (X,Y) exists and the linear combinations of the form aX + bY are well-behaved. Finally, our geometric method allows to derive a very simple bootstrap approach for constructing conservative confidence sets for ratios which perform favorably in certain situations, in particular in the asymmetric heavy-tailed regime.
: de Grave, D. D. J., Hesse, C., Brouwer, A. M., & Franz, V. H. (2008). Fixation locations when grasping partly occluded objects. Journal of Vision, 8(7), 1-11.
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When grasping an object, subjects tend to look at the contact positions of the digits (A. M. Brouwer, V. H. Franz, D. Kerzel, & K. R. Gegenfurtner, 2005; R. S. Johansson, G. Westling, A. Bäckström, & J. R. Flanagan, 2001). However, these contact positions are not always visible due to occlusion. Subjects might look at occluded parts to determine the location of the contact positions based on extrapolated information. On the other hand, subjects might avoid looking at occluded parts since no object information can be gathered there. To find out where subjects fixate when grasping occluded objects, we let them grasp flat shapes with the index finger and thumb at predefined contact positions. Either the contact position of the thumb or the finger or both was occluded. In a control condition, a part of the object that does not involve the contact positions was occluded. The results showed that subjects did look at occluded object parts, suggesting that they used extrapolated object information for grasping. Additionally, they preferred to look in the direction of the index finger. When the contact position of the index finger was occluded, this tendency was inhibited. Thus, an occluder does not prevent fixations on occluded object parts, but it does affect fixation locations especially in conditions where the preferred fixation location is occluded.
: Duemmler, T., Franz, V. H., Jovanovic, B., & Schwarzer, G. (2008). Effects of the Ebbinghaus illusion on children's perception and grasping. Experimental Brain Research, 186, 249-260.
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We investigated the development of the Ebbinghaus illusion in children's perception and grasping. A previous study (Hanisch et al. 2001) had reported negative illusion effects on 5- to 12-year-olds' grasping as compared to their perception. We attempted to replicate this finding and to test different hypotheses based on a direct influence of the context elements on the trajectories of the fingers which could explain this reversal of the illusion effects. For 5- to 7- and 9- to 11-year-olds we observed the classical illusion effects in perception. Illusion effects were perfectly similar for perception and grasping in 9- to 11-year-olds, while there was a non-significant trend toward smaller illusion effects in grasping for the 5- to 7-year-olds. This could be due to a slightly different effect of the illusion on younger children's grasping. However, it seems clear that there are no qualitative changes, as a reversal of the illusion effects in grasping of younger children. Finally, we show that our grasping data conform well to the motor literature for children's grasping, thereby strengthening our conclusions.
: Franz, V. H., & Gegenfurtner, K. R. (2008). Grasping visual illusions: Consistent data and no dissociation. Cognitive Neuropsychology, 25(7), 920-950. (Available online first; DOI:10.1080/02643290701862449)

The finding that the Ebbinghaus/Titchener illusion deceives perception but not grasping is usually seen as strong evidence for Goodale and Milner's notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. We argue that these results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.
: Hesse, C., de Grave, D. D. J., Franz, V. H., Brenner, E., & Smeets, J. B. J. (2008). Planning movements well in advance. Cognitive Neuropsychology, 25, 985-995.

It has been suggested that the metrics of grasping movements directed to visible objects are controlled in real time and are therefore unaffected by previous experience. We tested whether the properties of a visually presented distractor object influence the kinematics of a subsequent grasping movement performed under full vision. After viewing an elliptical distractor object in one of two different orientations participants grasped a target object, which was either the same object with the same orientation or a circular object without obvious orientation. When grasping the circular target, grip orientation was influenced by the orientation of the distractor. Moreover, as in classical visuomotor priming, grasping movements were initiated faster when distractor and target were identical. Results provide evidence that planning of visually guided grasping movements is influenced by prior perceptual experience, challenging the notion that metric aspects of grasping are controlled exclusively on the basis of real-time information.
: Gegenfurtner, K. R., & Franz, V. H. (2007). A comparison of localization judgments and pointing precision. Journal of Vision, 7, 1-12.
[ web-site ]
We compared the precision of perceptual localization and manual pointing. A Gaussian blob was presented 6 degree to the right or left of a central fixation spot on a CRT screen. Above and below the blob, vertical lines were displayed. On each trial, the blob was slightly offset to the right or left with respect to the lines. The subjects had to judge whether the blob appeared to the right or to the left of the vertical lines. At the same time, they had to point to the center of the blob with their index finger. Precision for perceived position was significantly better than precision for pointing. Performance in these two tasks correlated highly between the subjects. Overall, subjects pointed more leftward on trials where they judged the blob to be to the left of the lines. There was also a significant correlation for each subject between the pointing error and the perceived location error, calculated by partialling out the effect of the physical offset. The results are in agreement with the idea that the signals determining the perceived location of an object are used to guide the motor system in pointing toward it.
: Kleinholdermann, U., Brenner, E., Franz, V. H., & Smeets, J. B. J. (2007). Grasping trapezoidal objects. Experimental Brain Research, 180, 415-420.
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When grasping rectangular or circular objects with a precision grip the digits close in on the object in opposite directions. In doing so the digits move perpendicular to the local surface orientation as they approach opposite sides of the object. This perpendicular approach is advantageous for accurately placing the digits. Trapezoidal objects have non-parallel surfaces so that moving the digits in opposite directions would make the digits approach the contact surfaces at an angle that is not 90 degrees. In this study we examined whether this happens, or whether subjects tend to approach trapezoidal objects' surfaces perpendicularly. We used objects of different sizes and with different surface slants. Subjects tended to approach the object's surfaces orthogonally, suggesting that they aim for an optimal precision of digit placement rather than simply closing their hand as it reaches the object.
: de Grave, D. D. J., Franz, V. H., & Gegenfurtner, K. R. (2006). The influence of the Brentano illusion on eye and hand movements. Journal of Vision, 6, 727-738.
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When making an eye movement and a hand movement toward a visual target, the movements could be guided by visual judgments of direction and distance (or length) of the required displacement (vector coding), estimates of the final position (position coding), or both. Using the same information for the eyes and the hand is efficient; however, if this information contains an error, this causes both the eye and the hand to be incorrect. In this study, we tried to find out whether saccades and pointing movements use the same source of information when eye and hand movements are performed either concurrently or separately. Four experiments have been performed using the Brentano illusion, which primarily influences judgments of length but not those of position. This illusion only influences movements if the illusory length is relevant for the task, demonstrating that vector coding is involved. Subjects made saccades, pointing movements, or both between vertices of the Brentano illusion. The illusion influenced saccades and pointing movements when these movements were performed concurrently and separately, showing that the eye and the hand use vector coding. However, depending on the task, eye and hand movements were influenced to a different extent. This favors the interpretation that the eyes and the hand use a common motor command but each with a different relative contribution of vector coding.
: Himmelbach, M., Karnath, H.-O., Perenin, M.-T., Franz, V. H., & Stockmeier, K. (2006). A general deficit of the 'automatic pilot' with posterior parietal cortex lesions? Neuropsychologia, 44, 2749-2756.
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Lesions of the parieto-occipital cortex (POJ) in humans cause gross deviations of reaching movements and impaired grip formation if the targets are located in the subjects' peripheral visual field. Movements to central targets are typically less impaired. This disorder has been termed 'optic ataxia'. It has been suggested that a general deficit of online corrections under central as well as peripheral viewing conditions might be sufficient to explain this discrepancy. According to this hypothesis, patients with optic ataxia should demonstrate an impaired online correction of grip aperture under central viewing conditions if the target object changes its size during the grasping movement. We investigated this prediction in a patient with optic ataxia (I.G.) in a virtual visuo-haptic grasping task. We imposed an isolated need for online corrections of the hand aperture independently of positional changes of the target object. While we found some general inaccuracies of her grasping movements, the patient did not show a specific impairment of online adjustment of grip aperture. On the contrary, I.G. smoothly adjusted her grip aperture comparable to healthy subjects. A general deficit of fast movement correction affecting targets in peripheral as well as central visual fields thus does not appear to account for the overt visuomotor deficits in optic ataxia. Rather, it seems more likely that an anatomical dissociation between visuomotor pathways related to actions in the central and in the peripheral visual field underlies the dissociation of visuomotor performance depending on the retinotopic target position in optic ataxia.
: Berndt, I., Franz, V. H., Bülthoff, H. H., Götz, K. G., & Wascher, E. (2005). Effects of rearranged vision on event-related lateralizations of the EEG during pointing. Biological Psychology, 68(1), 15-39.
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We used event-related lateralizations of the EEG (ERLs) and reversed vision to study visuomotor processing with conflicting proprioceptive and visual information during pointing. Reversed vision decreased arm-related lateralization, probably reflecting the simultaneous activity of left and right arm specific neurons: neurons in the hemisphere contralateral to the observed action were probably activated by visual feedback, neurons in the hemisphere contralateral to the response side by the somatomotor feedback. Lateralization related to the target in parietal cortex increased, indicating that visual to motor transformation in parietal cortex required additional time and resources with reversed vision. A short period of adaptation to an additional lateral displacement of the visual field increased arm-contralateral activity in parietal cortex during the movement. This is in agreement with the Clower et al. study (1996), which showed that adaptation to a lateral displacement of the visual field is reflected in increased parietal involvement during pointing.
: Brouwer, A., Thornton, I. M., & Franz, V. H. (2005). Forward displacement in grasping and visually judging pliers. Visual Cognition, 12(5), 801-816.
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Observers often tend to misremember the visual stopping point of a movement in the direction of movement (representational momentum). We investigated whether this forward displacement also occurs in grasping. We asked participants to close virtual pliers after the pliers had been opening or closing. The participants' thumbs and index fingers were attached to robot arms which allowed us to provide haptic feedback about the location of the pliers' handles. In a visual task, participants judged the remembered final opening width of the pliers relative to a comparison stimulus. For grasping, we found forward displacement: participants opened their fingers wider if the pliers had been opening compared to when they had been closing. In contrast, we did not find clear forward displacement in the visual task. The effects in grasping and the visual task were not correlated between participants. These results seem to argue against the existence of one form of anticipation that serves both perception and grasping.
: Franz, V. H., Scharnowski, F., & Gegenfurtner, K. R. (2005). Illusion effects on grasping are temporally constant, not dynamic. Journal of Experimental Psychology: Human Perception and Performance, 31(6), 1359-1378.
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The authors tested whether the effects of the Ebbinghaus illusion on grasping are corrected during late phases of the movement. Surprisingly, the grasp aperture was corrected neither under no-vision (N=52) nor under full-vision (N=48) conditions. The authors show that previous reports of a correction (e.g., S. Glover & P. Dixon, 2002a) are due to 2 artifacts: (a) inclusion of time points at which the target object was already touched and (b) erroneous statistics. This removes the central evidence on which S. Glover and P. Dixon's (2001a) planning-control model of action is based. In addition, the authors' results can help to refine more classic notions of motor control (e.g., R. Woodworth, 1899). In consequence, the authors reject S. Glover and P. Dixon's (2001a) planning-control model but not classic online-control theories.
: Hartung, B., Schrater, P. R., Bülthoff, H. H., Kersten, D., & Franz, V. H. (2005). Is prior knowledge of object geometry used in visually guided reaching? Journal of Vision, 5(6), 504-514.
[ web-site ]
We investigated whether humans use prior knowledge of the geometry of faces in visually guided reaching. When viewing the inside of a mask of a face, the mask is often perceived as being a normal (convex) face, instead of the veridical, hollow (concave) shape. In this hollow-face illusion, prior knowledge of the shape of faces dominates perception, even when in conflict with information from binocular disparity. Computer images of normal and hollow faces were presented, such that depth information from binocular disparity was consistent or in conflict with prior knowledge of the geometry. Participants reached to touch either the nose or cheek of the faces or gave verbal estimates of the corresponding distances. We found that reaching to touch was dominated by prior knowledge of face geometry. However, hollow faces were estimated to be flatter than normal faces. This suggests that the visual system combines binocular disparity and prior assumptions, rather than completely discounting one or the other. When comparing the magnitude of the hollow-face illusion in reaching and verbal tasks, we found that the flattening effect of the illusion was similar for verbal and reaching tasks.
: Brouwer, A., Franz, V. H., & Thornton, I. M. (2004). Representational momentum in perception and grasping: Translating versus transforming objects. Journal of Vision, 4(7), 575-584.
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Representational momentum is the tendency to misremember the stopping point of a moving object as further forward in the direction of movement. Results of several studies suggest that this effect is typical for changes in position (e.g., translation) and not for changes in object shape (transformation). Additionally, the effect seems to be stronger in motor tasks than in perceptual tasks. Here, participants judged the final distance between two spheres after this distance had been increasing or decreasing. The spheres were two separately translating objects or were connected to form a single transforming object (a dumbbell). Participants also performed a motor task in which they grasped virtual versions of the final objects. We found representational momentum for the visual judgment task for both stimulus types. As predicted, it was stronger for the spheres than for the dumbbells. In contrast, for grasping, only the dumbbells produced representational momentum (larger maximum grip aperture when the dumbbells had been growing compared to when they had been shrinking). Because type of stimulus change had these different effects on representational momentum for perception and action, we conclude that different sources of information are used in the two tasks or that they are governed by different mechanisms.
: Franz, V. H. (2004). Is there a dynamic illusion effect in the motor system? Behavioral and Brain Sciences, 27(1), 34-35.
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Glover's planning-control model is based on his finding that visual illusions exert a larger effect in early phases than in late phases of a movement. But evidence for this dynamic illusion effect is weak, because: (a) it appears difficult to replicate; (b) Glover overestimates the accuracy of his results; and (c) he seems to underestimate the illusion effect at late phases.
: Franz, V. H. (2003). Planning versus online control: Dynamic illusion effects in grasping? Spatial Vision, 16(3-4), 211-223.
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The planning/control model of action assumes that grasping is sensitive to the context of an object only in early stages of the movement (planning), but not in later stages (control). In consequence, the effects of context-induced illusions (as the Ebbinghaus/Titchener illusion) should decrement during a grasping movement. Here, we tested this claim by reanalysing a large data set (N=26) on grasping in the Ebbinghaus illusion. Contrary to the predictions of the planning/control model, we found that the effects of the illusion did not decrease over time. Instead, the illusion effects stayed remarkably constant.
: Franz, V. H. (2003). Manual size estimation: A neuropsychological measure of perception? Experimental Brain Research, 151(4), 471-477.
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Manual size estimation (participants indicate the size of an object with index finger and thumb) is often interpreted as a measure of perceptual size information in the visual system, in contrast to size information used by the motor system in visually guided grasping. Because manual estimation is a relatively new measure, I compared it to a more traditional perceptual measure (method of adjustment). Manual estimation showed larger effects of the Ebbinghaus (or Titchener) illusion than the traditional perceptual measure. This inconsistency can be resolved by taking into account that manual estimation is also unusually responsive to a physical variation of size. If we correct for the effect of physical size, manual estimation and the traditional perceptual measure show similar illusion effects. Most interestingly, the corrected illusion effects are also similar to the illusion effects found in grasping. This suggests that the same neuronal signals which generate the illusion in the traditional perceptual measure are also responsible for the effects of the illusion on manual estimation and on grasping.
: Franz, V. H., Bülthoff, H. H., & Fahle, M. (2003). Grasp effects of the Ebbinghaus illusion: Obstacle-avoidance is not the explanation. Experimental Brain Research, 149(4), 470-477.
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The perception-versus-action hypothesis states that visual information is processed in two different streams, one for visual awareness (or perception) and one for motor performance. Previous reports that the Ebbinghaus illusion deceives perception but not grasping seemed to indicate that this dichotomy between perception and action was fundamental enough to be reflected in the overt behavior of non-neurological, healthy humans. Contrary to this view we show that the Ebbinghaus illusion affects grasping to the same extent as perception. We also show that the grasp effects cannot be accounted for by non-perceptual obstacle avoidance mechanisms as has recently been suggested. Instead, even subtle variations of the Ebbinghaus illusion affect grasping in the same way as they affect perception. Our results suggest that the same signals are responsible for the perceptual effects and for the motor effects of the Ebbinghaus illusion. This casts doubt on one line of evidence, which used to strongly favor the perception-versus-action hypothesis.
: Berndt, I., Franz, V. H., Bülthoff, H. H., & Wascher, E. (2002). Effects of pointing direction and direction predictability on event-related lateralizations of the EEG. Human Movement Science, 21(3), 387-410.
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In two experiments, we investigated hemispheric electroencephalography (EEG) differences in 9(12) healthy volunteers during pointing to lateral and central targets. The questions addressed were whether horizontal pointing direction and the predictability of pointing direction modulated hemispheric differences (event-related lateralizations of the EEG = ERLs). To vary pointing direction predictability, targets were displayed either randomly at one of nine different positions on a screen (random) or at the same horizontal position in five subsequent trials (sequenced) while vertical positions varied randomly. Event-related lateralizations (ERLs) varied with pointing direction. This was true across changes in target eccentricity and pointing distance. Foci of the ERLs were in premotor and posterior parietal cortex, which might reflect the critical involvement of these areas in the control of visually guided reaching. Direction predictability reduced the parietal and premotor ERL before pointing onset, probably reflecting a lesser effort in visuomotor transformation. Predictability also added an overlying N2pc component to the early ERL after target onset and increased direction effects during movement.
: Franz, V. H. (2001). Action does not resist visual illusions. Trends in Cognitive Sciences, 5(11), 457-459.
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Recent TICS articles discussed the psychophysical evidence in favor of Goodale and Milner's action vs. perception hypothesis. Carey argued that most of the studies investigating the effects of visual illusions on grasping can be reconciled with the notion that the action system resists visual illusions. Bruno suggested a new interpretation of the action vs. perception hypothesis in order to incorporate most of the empirical findings. Here, I argue that action does not resist visual illusions. Even more, the effects on the motor system seem to be comparable to the effects on the perceptual system. This challenges the action vs. perception hypothesis in its current form.
: Franz, V. H., Fahle, M., Bülthoff, H. H., & Gegenfurtner, K. R. (2001). Effects of visual illusions on grasping. Journal of Experimental Psychology: Human Perception and Performance, 27(5), 1124-1144.
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In 2 experiments, the Müller-Lyer illusion (F. C. Müller-Lyer, 1889; N = 16) and the parallel-lines illusion (W. Wundt, 1898; N = 26) clearly affected maximum preshape aperture in grasping (both ps < .001). The grasping effects were similar but not perfectly equal to the perceptual effects. Control experiments show that these differences can be attributed to problems in matching the perceptual task and the grasping task. A model is described stating the assumptions that are needed to compare the grasping effects and the perceptual effects of visual illusions. Further studies on the relationship between perception and grasping are reviewed. These studies provide no clear evidence for a dissociation between perception and grasping and therefore do not support the action versus perception hypothesis (A. D. Milner & M. A. Goodale, 1995).
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (2000). Grasping visual illusions: No evidence for a dissociation between perception and action. Psychological Science, 11(1), 20-25.
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Neuropsychological studies prompted the theory that the primate visual system might be organized into two parallel pathways, one for conscious perception and one for guiding action. Supporting evidence in healthy subjects seemed to come from a dissociation in visual illusions: In previous studies, the Ebbinghaus (or Titchener) illusion deceived perceptual judgments of size, but only marginally influenced the size estimates used in grasping. Contrary to those results, the findings from the present study show that there is no difference in the sizes of the perceptual and grasp illusions if the perceptual and grasping tasks are appropriately matched. We show that the differences found previously can be accounted for by a hitherto unknown , nonadditive effect in the addition. We conclude that the illusion does not provide evidence for the existence of two distinct pathways for perception and action in the visual system.
: Miller, J., Franz, V., & Ulrich, R. (1999). Effects of auditory stimulus intensity on response force in simple, go/no-go, and choice RT tasks. Perception & Psychophysics, 61(1), 107-119.
[ pdf-file ]
In four experiments, increasing the intensities of both relevant and irrelevant auditory stimuli was found to increase response force (RF) in simple, go/no-go, and choice reaction time (RT) tasks. These results raise problems for models that localize the effects of auditory intensity on purely perceptual processes, indicating instead that intensity also affects motor output processes under many circumstances. In Experiment 1, simple RT, go/no-go, and choice RT tasks were compared, using the same stimuli for all tasks. Auditory stimulus intensity affected both RT and RF, and these effects were not modulated by task. In Experiments 2-4, an irrelevant auditory accessory stimulus accompanied a relevant visual stimulus, and the go/no-go and choice tasks were used. The intensity of the irrelevant auditory accessory stimulus was found to affect RT and RF, although the sizes of these effects depended somewhat on the temporal predictability of the accessory stimulus.

Conference proceedings

: Kerzel, D., Franz, V. H., & Gegenfurtner, K. R. (Eds.). (2004). Experimentelle Psychologie / 46. Tagung experimentell arbeitender Psychologen. Lengerich: Pabst Verlag.
[ web-site ]

Theses

: Franz, V. H. (2004, October). Informationsverarbeitungsprozesse in Wahrnehmung & Handlung. Begleitschrift zur publikationsbasierten Habilitation an der Universität Gießen, Gießen, Germany.
: Franz, V. H. (2000, July). The relationship between visually guided motor behavior and visual perception. MPI Series in Biological Cybernetics, No. 1; Doctoral Dissertation, University of Bochum, Bochum, Germany. Berlin: Logos.
[ pdf-file ]
Neuropsychological studies prompted the hypothesis that visual information is processed in two anatomical and functional distinct streams in the primate brain. The perception versus action hypothesis states that the dorsal stream transforms visual information for the guidance of motor actions while the ventral stream uses visual in- formation for object recognition and conscious perception (Goodale & Milner, 1992; Milner & Goodale, 1995). Critical evidence in healthy observers was reported by Aglioti, DeSouza, and Goodale (1995). They found that grasping is not, or only little affected by visual illusions. In their study, the Ebbinghaus Illusion deceived perceptual judgments of size, but only marginally influenced the size estimates used in grasping the same objects. This dissociation between perceiving the size of an object and grasping it was interpreted as strong evidence for the perception versus action hypothesis because it assumes that mainly the ventral stream is affected by visual illusions, but not the dorsal stream. In the present study this claim is tested extensively. A mathematical model is formulated that explicitly states the assumptions that are needed to compare the effects of visual illusions on perception and on grasping. Experiments show that the Ebbinghaus Illusion, the Müller-Lyer Illusion, and the Parallel-Lines Illusion affect maximum preshape aperture in grasping. In the Ebbinghaus Illusion there is a very good match between the perceptual effect and the grasping effect. In the Müller-Lyer Illusion the grasping effect is larger than the perceptual effect and in the Parallel-Lines Illusion it is smaller. Further experiments show that these differences can be attributed to problems in matching the perceptual task and the grasping task. Furthermore, it is shown that the larger perceptual effect that was found in previous studies for the Ebbinghaus Illusion (e.g., Aglioti et al., 1995) was most likely caused by a non-additive effect that selectively increased the effect of the illusion on perception. The literature on the effects of visual illusions on grasping is reviewed and it is concluded that grasping reliably is affected by visual illusions. In particular, there is no evidence that grasping is less deceived by visual illusion than perception. This contradicts the predictions of the perception versus action hypothesis and is compatible with the more parsimonious account that the same visual signals that are used for object recognition and conscious perception are also used to guide motor actions.
: Franz, V. H. (1995). Human information processing: Discrete or continuous? Diploma thesis, University of Konstanz, Konstanz, Germany.

Increasing the intensity of visual and auditory targets increased response force in a simple reaction time (RT) task. For both, task-relevant and irrelevant auditory stimuli this effect was also obtained in go-no-go and choice RT tasks. The effect was not modulated by the task. These results bear on the discrete stage model (Sternberg, 1969) as long as it is assumed that intensity solely influences early stages while response force is determined in late stages. Results are consistent with a dual route model in which the effect of intensity is mediated via an arousal route bypassing normal information processing channels and connecting early stages with late stages (e.g. Sanders, 1983). However, results are also consistent with continuous models (e.g. McClelland, 1979). Further tests of the dual route model using response force are proposed.

Invited talks

: Franz, V. H. (2010, May 17). Dual pathway hypotheses for perception and action: A critical view. University Medical Center Hamburg-Eppendorf, Hamburg, Germany.

The finding that certain visual illusions deceive perception but not grasping is usually seen as strong evidence for Goodale and Milner's notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. The most prominent illusion in this context is the Ebbinghaus/Titchener illusion. I will use this illusion as an example and will argue that the results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.
: Franz, V. H. (2009, November 3). Dual pathway hypotheses for perception and action: A critical view. University of Washington, Seattle, WA, USA.

The finding that certain visual illusions deceive perception but not grasping is usually seen as strong evidence for Goodale and Milner's notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. The most prominent illusion in this context is the Ebbinghaus/Titchener illusion. I will use this illusion as an example and will argue that the results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.
: Franz, V. H. (2009, October 27). Confidence intervals in within-subject designs. University of Washington, Seattle, WA, USA.

Repeated measures designs are common in experimental psychology. In recent years psychologist increasingly have used confidence intervals and error bars to picture the variability of data in these designs. Due to the special correlational structure in repeated measures designs, the calculation of confidence intervals is not trivial. The classic solution was provided by Loftus & Masson (1994). This solution has certain limitation (e.g., it uses a pooled error term which might obscure differences in variability between factor levels and is, strictly speaking, limited to one within-subjects factor). Also, the method's perceived complexity has sometimes led practitioners to use a simplified variant, based on a per-subject normalization of the data. I will argue that this normalization method can lead to biased results and will suggest a generalization to the classic method that is very simple, intuitive and might even be used in more general cases of multi-factor designs.
: Franz, V. H. (2009, July 21). Kognition in Wahrnehmung und Handlung. Universität Potsdam, Potsdam, Germany.

Ich gebe einen Überblick über meine Arbeiten zur visuellen Informationsverarbeitung beim Menschen. Beispielhaft werden drei Themen vertiefend behandelt: Der Einfluss optischer Täuschungen in Wahrnehmung und Handlung, die Verbindung einzelner Hinweisreize (Cues) im Rahmen bayesianischer Modelle, und der Einfluss von Gedächtnisprozessen auf Wahrnehmung und Handlung. Am Ende spreche ich aktuelle, laufende Forschungsprojekte stichpunktartig an, um einen Überblick über meine derzeitige Forschungsrichtung zu geben.
: Franz, V. H. (2009, July 8). Das dichotome Gehirn: Gibt es eine Dissoziation zwischen Wahrnehmung und Handlung bei optischen Täuschungen? Technische Universität Berlin, Berlin, Germany.
: Franz, V. H. (2009, July 7). Das dichotome Gehirn: Gibt es eine Dissoziation zwischen Wahrnehmung und Handlung bei optischen Täuschungen? Technische Universität Dortmund, Dortmund, Germany.
: Franz, V. H. (2009, April 8). Wahrnehmung und Handlung bei der Interaktion mit virtuellen und realen Umgebungen. Universität Hamburg, Hamburg, Germany.
: Franz, V. H. (2009, January 27). Wahrnehmung und Handlung bei der Interaktion mit virtuellen und realen Umgebungen. Bergische Universität Wuppertal, Wuppertal, Germany.
: Franz, V. H. (2009, January 20). Das dichotome Gehirn: Gibt es eine Dissoziation zwischen Wahrnehmung und Handlung bei optischen Täuschungen? Universität Kiel, Kiel, Germany.
: Franz, V. H. (2008, December 16). Wahrnehmung und Handlung bei der Interaktion mit virtuellen und realen Umgebungen. Otto-Friedrich Universität Bamberg, Bamberg, Germany.
: Franz, V. H. (2008, October 22). Konfidenzintervalle für Messwiederholungsdesigns. Universität zu Köln, Köln, Germany.
: Franz, V. H. (2008, October 20). Wahrnehmung und Handlung bei der Interaktion mit virtuellen und realen Umgebungen. Universität Regensburg, Regensburg, Germany.
: Franz, V. H. (2008, June 6). Kognition in Wahrnehmung und Handlung. Ludwig-Maximilians Universität München, München, Germany.
: Franz, V. H. (2008, April 26). Konfidenzintervalle für Messwiederholungsdesigns. Technische Universität Dresden, Dresden, Germany.
: Franz, V. H. (2008, February 11). Kognition und Handlung: Die kognitive Bedingtheit motorischer Interaktionen mit virtuellen und realen Umgebungen. Universität Duisburg-Essen, Duisburg, Germany.
: Franz, V. H. (2007, June 19). Das dichotome Gehirn: Zwei-Pfad Theorien in den Neurowissenschaften. Universität Bielefeld, Bielefeld, Germany.
: Franz, V. H. (2007, June 5). Dual pathway hypotheses for perception and action: A critical view. Friedrich-Schiller Universität Jena, Jena, Germany.

A number of prominent theories assume that the visual system is divided into two parallel subsystems. For example, Milner and Goodale proposed two separate systems for action and perception: The vision-for-action system is assumed to be located in the dorsal cortical stream and to use visual information to guide motor actions. The vision-for-perception system is assumed to be located in the ventral cortical stream and to be used for object recognition and to generate the conscious visual percept. Recently, Glover and Dixon suggested an alternative view by assuming that the ventral stream controls early phases of movements and therefore has an important role in movement execution (planning), while only late phases are controlled by the dorsal stream (control). A number of studies on the effects of visual illusions on grasping seemed to provide evidence for one or the other of these incompatible theories. Strong evidence in favor of Milner and Goodale's model was the finding that visual illusions affect only perception but not grasping, while evidence in favor of Glover and Dixon's model was the finding that only early phases of grasping are affected by visual illusions but not late phases. However, these findings are controversial and have led to many further studies with seemingly contradictory results. I will argue that these results are not as contradictory as it seems at first sight. Instead, I argue that the data consistently show that grasping is affected by visual illusions in a similar way as perception. This suggests that visual illusions deceive a common representation of object size that is used by perception and action. In consequence, grasping and visual illusions should not be counted as evidence for a subdivision of the visual system.
: Franz, V. H. (2006, October 27). Dual pathway hypotheses for perception and action: A critical view. (Talk presented at the ``14th Trieste Symposium on Perception and Cognition'' / Kanizsa workshop, Triest, Italy).
[ web-site ]
A number of prominent theories assume that the visual system is divided into two parallel subsystems. For example, Milner and Goodale proposed in their action-perception model two separate systems for action and perception. The vision-for-action system is assumed to be located mainly in the dorsal cortical stream and to use visual information to guide motor actions. The vision-for-perception system is assumed to be located in the ventral cortical stream and to be used for object recognition and to generate the conscious visual percept of the world. Recently, Glover and Dixon suggested an alternative view by assuming that the ventral stream controls early phases of movements and therefore has an important role in movement execution (planning phase), while only late phases are controlled by the dorsal stream (control phase). Different studies on the effects of visual illusions on grasping seemed to provide evidence for one or the other of these incompatible theories. Evidence in favor of the perception-action model was the finding that visual illusions affect only perception but not grasping, while evidence in favor of the planning-control model was the finding that only early phases of grasping are affected by visual illusions but not late phases. However, these finding are controversial and have led to many further studies with seemingly contradictory results. I will argue that these results are not as contradictory as it seems at first sight. Instead, I argue that the data consistently show that grasping is affected by visual illusions in a similar way as perception. This suggests that visual illusions deceive a common representation of object size that is used by perception and action. In consequence, grasping and visual illusions should not be counted as evidence for a subdivision of the visual system.
: Franz, V. H. (2006, August 28). Konfidenzintervalle für Quotienten von Zufallsvariablen. Universität Hamburg, Hamburg, Germany.
: Franz, V. H. (2006, July 15). Does unconscious priming exist? A new analysis based on ideal observers and information theory. (Talk presented at the workshop: ``Bridging the Gap Between Sensation and Motor Control'', Rauischholzhausen Castle, Germany).
: Franz, V. H. (2006, June 30). Does unconscious priming exist? Some skeptical views based on ideal observer analysis and information theory. (Talk presented at the workshop: ``Visual Masking and the Dynamics of Vision and Consciousness'', Hanse-Wissenschaftskolleg (HWK), Delmenhorst, Germany).
[ web-site ]
: Franz, V. H. (2006, March 10). Unbewusste Wahrnehmung, ideale Beobachter und Signalentdeckungstheorie: Eine kritische Evaluation. Technische Universität Darmstadt, Darmstadt, Germany.
: Franz, V. H. (2006, February 10). Das dichotome Gehirn: Zwei separate Pfade der Verarbeitung? Technische Universität Carolo-Wilhelmina Braunschweig, Germany.

Die kognitiven Neurowissenschaften beschrieben immer wieder Dissoziationen bei der Verarbeitung visueller Information. Derartige Dissoziationen werden allgemein als Indiz für eine getrennte Verarbeitung in unterschiedlichen Subsystemen angesehen. So schlugen Milner und Goodale (1995) aufgrund von Dissoziationen bei neuropsychologischen Patienten vor, dass visuelle Information über zwei verschiedene kortikale Pfade verarbeitet wird. Nach dieser Auffassung bildet die Verarbeitung im ventralen, kortikalen Pfad die Grundlage der bewussten Wahrnehmung und ermöglicht Leistungen wie zum Beispiel die Objekterkennung. Im dorsalen Pfad hingegen soll die visuelle Information so verarbeitet werden, dass sie direkt zur Steuerung motorischer Handlungen verwendet werden kann. Ich gebe einen Überblick über diese und ähnliche Theorien zur visuellen Informationsverarbeitung und stelle eigene Arbeiten vor, welche die Situation etwas anders erscheinen lassen, als es derzeit in Lehrbüchern vertreten wird.
: Franz, V. H. (2005, December 8). Unbewusste Wahrnehmung, ideale Beobachter und Signalentdeckungstheorie: Eine kritische Evaluation. Georg August University of Göttingen, Göttingen, Germany.
: Franz, V. H. (2005, October 18). Unbewusste Wahrnehmung, ideale Beobachter und Signalentdeckungstheorie: Eine kritische Evaluation. Leibniz Research Centre for Working Environment and Human Factors, Dortmund, Germany.
: Franz, V. H. (2003, December 5). Aktuelle Theorien zum Verhältnis von Wahrnehmung und Handlung. Universität Marburg / Institut für Neurophysik, Marburg, Germany.
: Franz, V. H. (2003, December 2). Aktuelle Theorien zum Verhältnis von Wahrnehmung und Handlung. Universität Giessen / Institut für Sportwissenschaft, Giessen, Germany.
: Franz, V. H. (2003, November 17). Aktuelle Theorien zum Verhältnis von Wahrnehmung und Handlung. Universität Bremen / SFB 517, Bremen, Germany.
: Franz, V. H. (2003, July 9). Current theories on the relationship of perception and action. (Talk at the Max Planck Institute for Psychological Research, Munich, Germany).
: Franz, V. H. (2003, May 16). The use of visual information for motor actions: Data and methodological challenges. NYU, Department of Psychology, New York, NY, USA.
: Franz, V. H. (2002, July). Neueste Ergebnisse zur Metakontrast Dissoziation. University of Bielefeld, Germany.
: Franz, V. H. (2002, June). Metakontrast bei Zeigebewegungen. University of Tübingen, Germany.
: Franz, V. H. (2002, May). Assoziationen und Dissoziationen zwischen Wahrnehmung und Handlung. University of Gießen, Germany.
: Franz, V. H. (2002, April). The visual brain in action: Do visual illusions affect grasping? University of Oldenburg, Germany.
: Franz, V. H. (2001, December 7). The visual brain in action: Do visual illusions affect grasping? MIT Laboratory for Human and Machine Haptics, Cambridge, MA, USA.
: Franz, V. H. (2001, December 6). The visual brain in action: Do visual illusions affect grasping? MIT Center for Biological and Computational Learning, Cambridge, MA, USA.
: Franz, V. H. (2001, December 5). The visual brain in action: Do visual illusions affect grasping? McGill University, Montreal, Canada.
: Franz, V. H. (2001, December 3). The visual brain in action: Do visual illusions affect grasping? University of Minnesota, Minneapolis, MN, USA.
: Franz, V. H. (2001, November 26). The visual brain in action: Do visual illusions affect grasping? UC Berkeley, Berkeley, CA, USA.
: Franz, V. H. (2001, November 11). The visual brain in action: Do visual illusions affect grasping? University of Queensland, Brisbane, Australia.
: Franz, V. H. (2001, October 26). The visual brain in action: Do visual illusions affect grasping? University of New South Wales, Sydney, Australia.
: Franz, V. H. (1999, December). Optische Illusionen: Evidenz für zwei kortikale Verarbeitungswege? University of Tübingen, Germany.
: Franz, V. H. (1997, January). Diskrete vs. kontinuierliche Modelle der Informationsverarbeitung. University of Ulm, Germany.

Conference talks

: Franz, V. H. (2010, June 2). Grasping, memory, and illusions: Can we find differences between dorsal and ventral control? (Talk presented at the workshop: ``Perception and Action'', May 30-June 4, 2010 Rauischholzhausen Castle, Germany).
: Hesse, C., & Franz, V. H. (2009). Greifen nach Zeitverzögerung: Zwei Pfade oder exponentieller Informationszerfall? (Talk presented at the ``51. Tagung experimentell arbeitender Psychologen'' (TeaP), Jena, Germany)

Die Verfügbarkeit visueller Information beeinflusst die Ausführung von Greifbewegungen. Dies wird in Aufgaben deutlich, in denen eine Zeitverzögerung zwischen Objektdarbietung und der auszuführenden Greifbewegung liegt. Die resultierende Veränderung der Bewegung kann dabei entweder auf dem Zerfall der zugrundeliegenden visuellen Information oder einer Veränderung der zugrundliegenden Verarbeitungsmechnismen (dorsal vs. ventral) beruhen. In den vorgestellten Experimenten wurde die für eine Greifbewegung verfügbare visuelle Information systematisch reduziert und damit die Anforderungen an das visuelle Gedächtnis gesteigert. Unsere Ergebnisse legen nahe, dass die zur Greifbewegung genutzte visuelle Information rasch zerfällt, was sich in einer Zunahme der maximalen Greiföffnung widerspiegelt. Es fanden sich keine Belege, die für einen qualitativen Unterschied zwischen Bewegungen zu sichtbaren bzw. erinnerten Objekten sprechen und damit unterschiedliche Verarbeitungsmechanismen befürworten. Vielmehr scheinen auch für die zur Bewegungsplanung genutzten Informationen klassische Gedächtnisprozesse, wie exponentieller Informationszerfall zu gelten.
: Kleinholdermann, U. J., Brenner, E., Franz, V. H., & Smeets, J. B. J. (2006). Grasping trapezoidal objects. Journal of Vision, 6(6), 396a. (Talk presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
The most simple view of grasping with a precision grip assumes that humans close index finger and thumb in opposite directions around the object. For circular and rectangular objects, this will result in a perpendicular approach of each digit to the surface. Thus they move in the right direction for the application of grip force after contact. Grasping trapezoidal objects at their non-parallel surfaces in this way will result in a non-perpendicular approach to the surface. However, Smeets and Brenner (1999) assume that humans try to approach an object's surface perpendicularly in order to deal with spatial inaccuracies. We used trapezoidal objects to discriminate between these views. Twenty-three participants grasped trapezoids with angles between -20 and +20 degree in steps of 5 degree. We found a tendency to approach the objects' surfaces orthogonally. This suggests that during the grasping movement participants aimed for an optimal placement accuracy of the digits rather than for an optimal direction of grip force after contact.
: Franz, V. H. (2005, August 29). Dual pathway hypotheses for perception and action. (Talk presented at the Perception & Action workshop, Rauischholzhausen Castle, Germany).
: Franz, V. H. (2005). Is there a temporal dissociation between perception and action in visual illusions? (Talk presented at the 9th European Congress of Psychology (ECP), 3.-8. July, Granada, Spain).

The planning/control model of action (e.g., Glover, Behavioral and Brain Sciences, 2004) suggests that there is a temporal dissociation between the effects of visual illusions on grasping and on perception: At the beginning of the movement the illusion should affect grasping and perception in a similar way (planning phase). Later, however, when the movement unfolds, grasping should have access to a veridical, undistorted representation of object size and therefore the illusion should be corrected (control phase). Glover and Dixon found this dynamic illusion effect in a number of studies (e.g., Journal of Experimental Psychology: Human Perception and Performance, 2001) and used it as key evidence for the planning/control model. We tested the dynamic illusion effect under full-vision as well as under no-vision conditions (i.e., participants did or did not see hand and stimuli during movement execution). Using improved methodology, we found surprisingly constant illusion effects. Also, we were able to replicate the dynamic illusion effects by allowing for two artifacts which were present in Glover and Dixon's studies: (a) inclusion of time-points at which the target object was already touched (b) erroneous statistics. We conclude that the action system never has access to a veridical representation of object size and therefore suggest to reject Glover and Dixon's planning/control model. Overall it seems that perception and action are tighter coupled than often thought.
: Franz, V. H., & Scharnowski, F. (2003). Grasp effects of visual illusions: Dynamic or stationary? Journal of Vision, 3(9), 127a. (Talk presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
In recent studies we found effects of visual illusions on the maximum grip aperture in grasping. Here, we ask whether these effects decay (or build up) during the execution of a grasp movement. Some recent studies suggest a decay (Glover & Dixon, Perception and Psychophysics, 64, 266-278, 2002), while the view of others is more consistent with a build up (Carey, Trends in Cognitive Sciences, 5, 109-113, 2001). We reanalyzed the data of different studies on the Ebbinghaus / Titchener illusion (Franz et al., Psychological Science, 11, 20-25, 2000; Franz et al., Experimental Brain Research, in press) which used very large sample sizes (26 and 52 participants). The hand aperture of each grasp movement was analysed at different, normalized time points. Special care was taken to avoid possible artefacts which might arise from the hand already touching the target object. Also, we corrected at each time point for the responsiveness of the hand aperture to a physical variation of size. Results show that the illusion effects are remarkably constant over time. This suggests that either the neuronal signals which cause the motor illusion are constant over time, or that the grasp trajectory is largely preprogrammed before the movement starts.
: Franz, V. H., Bülthoff, H. H., & Fahle, M. (2002). Are motor effects of the Titchener / Ebbinghaus illusion artifacts? Journal of Vision, 2(7), 724a. (Talk presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
PURPOSE: Previously, we reported effects of the Titchener / Ebbinghaus illusion on grasping (Franz et al., 2000). These contradict a strong version of the action versus perception hypothesis (Milner & Goodale, 1995) which states that the motor system is unaffected by visual illusions. Here, we test whether our grasp effects were artifacts (i.e. generated by non-perceptual mechanisms). This could be the case if the motor system treated the illusion inducing context elements as obstacles and tried to avoid them. To test for this possibility, we varied the distance between context elements and target. METHODS: An aluminum disc (31, 34, or 37 mm in diameter, 5 mm in height) was positioned as target on a board. Around the target either small or large context discs (10 or 58 mm in diameter) were drawn at near or far distances (24 or 31 mm midpoint to nearest point on context circles). Close to the board a monitor was mounted on which a comparison disc was displayed. In the perceptual task 52 subjects adjusted the size of the comparison stimulus to match the size of the target. In the grasping task subjects grasped the target. Subjects wore shutter glasses and could not see their hand during grasping. The grasp trajectory was recorded and the maximum preshape aperture was calculated. RESULTS: The motor illusion responded to the variation of distance between context elements and target in exactly the same way as the perceptual illusion. None of three different obstacle avoidance hypotheses can explain these results. CONCLUSIONS: Our results suggest that the same signals are responsible for the perceptual and for the motor illusion. This either indicates that the action versus hypothesis needs modification, or that the Titchener illusion is generated before the separation of the perceptual and the motor streams.
: Franz, V. H., Fahle, M., & Bülthoff, H. H. (2002). Are motor effects of visual illusions caused by different mechanisms than the perceptual illusions? Perception, 31, S144. (Talk presented at the European Conference on Visual Perception (ECVP), Glasgow, UK)

In previous studies, we found effects of the Ebbinghaus (or Titchener) illusion on grasping. This contradicts the notion that the motor system uses visual transformations which are (a) different from the perceptual transformations and (b) unaffected by visual illusions (Milner & Goodale, 1995). Here, we tested whether the grasp effects are generated independently from the perceptual illusions. This could be the case if the motor system treated the illusion-inducing context elements as obstacles and tried to avoid them. To test this hypothesis, we varied the distance between context elements and target. Aluminum discs (31, 34, or 37 mm in diameter) were surrounded by small or large context circles (10 or 58 mm in diameter) at one of two distances (24 or 31 mm midpoint target disc to nearest point on context circles). In the perceptual task, 52 participants adjusted the size of a comparison stimulus to match the size of the target disc. In the grasping task participants grasped the target disc. The trajectories were recorded and the maximum grasp apertures determined. The motor illusion responded to the variation of distance between context elements and target disc in exactly the same way as the perceptual illusion. This suggests that the same neuronal signals are responsible for the perceptual and for the motor illusion.
: Franz, V. H. (2002). Maskierte Reize beeinflussen symbolisch kodierte Zeigebewegungen. In M. Baumann, A. Keinath, & J. F. Krems (Eds.), Experimentelle Psychologie / 44. Tagung experimentell arbeitender Psychologen. Regensburg: Roderer Verlag. (Talk presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Chemnitz, Germany)

Reize, deren bewusste Wahrnehmung durch Metakontrast-Maskierung unterdrückt ist, können nicht nur einfache motorische Reaktionen beeinflussen, sondern auch Zeigebewegungen (Klotz & Neumann, 1999; Schmidt, im Druck). Wir untersuchten, ob sich diese Einflüsse auch bei fehlender räumlicher Kompatibilität zwischen imperativem Reiz und Zeigeziel nachweisen lassen. Zweiundzwanzig VPn wurde eine Abfolge von Prime und imperativem Reiz präsentiert (Dauer Prime: 31ms, ISI: 42ms, imperativer Reiz: 83ms). Prime und imperativer Reiz waren Quadrate (Orientierung: 0, 45 Grad). Entweder diskriminierten die VPn den Prime, oder reagierten so schnell wie möglich auf die Orientierung des imperativen Reizes (Tastendruck oder Zeigen rechts/links). Die VPn konnten den Prime kaum diskriminieren (d'=0.07, t(21) = 1.88, p=.07). Sie reagierten schneller (51 msec, t(21) = 8.1, p<.001) und mit anderen Trajektorien für kongruente gegenüber inkongruenten Reizen. Fast vollständig maskierte Reize haben auch dann einen Einfluß auf Zeigebewegungen, wenn keine Kompatibilität zwischen dem Ort des imperativem Reiz und dem Zeigeziel vorliegt.
: Franz, V. H. (2001). Optische Illusionen: Gibt es eine Dissoziation zwischen Wahrnehmung und Greifen? In H. H. Bülthoff, K. R. Gegenfurtner, H. A. Mallot, & R. Ulrich (Eds.), Beiträge zur 4. Tübinger Wahrnehmungskonferenz (p. 25). Kirchentellinsfurt: Knirsch. (Talk presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

Milner und Goodale (1995) schlugen vor, dass visuelle Information über zwei verschiedene kortikale Pfade verarbeitet wird. Nach dieser Auffassung bildet die Verarbeitung im ventralen, kortikalen Pfad die Grundlage der visuellen Wahrnehmung und ermöglicht Leistungen wie zum Beispiel die Objekterkennung. Im dorsalen Pfad hingegen soll die visuelle Information so verarbeiten werden, dass sie direkt zur Steuerung motorischer Bewegungen verwendet werden kann. Visuelle Information würde demnach für die Wahrnehmung qualitativ anders verarbeitet werden als zur Ansteuerung der Motorik. Als typisches Indiz wurde bisher gewertet, dass optische Täuschungen die Greifmotorik deutlich weniger beeinflussen als die Wahrnehmung. Wir kritisierten diesen Befund, da wir im Gegensatz dazu fanden, dass die Greifmotorik in ähnlicher Weise von optischen Täuschungen beeinflusst wird wie die Wahrnehmung. Es werden Ergebnisse zur Ebbinghaus Illusion, zur Müller-Lyer Illusion und zur Parallele-Linien Illusion dargestellt. In allen diesen Fällen unterlag das Greifen der Illusion. Die Illusionseffekt auf das Greifen waren nicht immer exakt gleich zu den Effekten auf die Wahrnehmung. Es konnte jedoch gezeigt werden dass die Abweichungen auf unterschiedliche Anforderungen der Greifaufgabe und der Wahrnehmungsaufgabe zurückgehen. Es wird ein Überblick über aktuelle Arbeiten zu diesem Thema gegeben. Wir kommen zu dem Schluss, dass derzeit keine überzeugende Evidenz für eine Dissoziation zwischen Wahrnehmungs- und Handlungssystem bei optischen Täuschungen besteht.
: Gegenfurtner, K. R., & Franz, V. (2001). A comparison of localization and pointing accuracy in peripheral position judgments. Investigative Ophthalmology and Visual Science, 42(4), S651. (Talk presented at the conference of the Association for Research in Vision and Ophthalmology (ARVO), Fort Lauderdale, Florida)

PURPOSE: To compare the signals that determine the perceived position of objects with the signals used by the motor system when pointing to the same objects. METHODS: A Gaussian blob was presented 6 deg to the right or left of a central fixation spot on a CRT screen. Above and below the blob vertical lines were displayed. On each trial, the blob was slightly offset to the right or left with respect to the lines. The subjects (N=11) had to judge whether the blob appeared to the right or to the left of the vertical lines. At the same time, they had to point to the center of the blob with their index finger. The trajectory of the index finger was measured using an Optotrak (TM) system. Both the blob and the vertical lines were displayed for 100 ms. Nine different offsets of the blob were used to determine psychometric functions for both perceived position and pointing. Pointing psychometric functions were calculated by determining whether the landing position of the index finger was to the right or the left of the mean landing position of those trials where the blob was aligned with the vertical lines. Accuracy is given by the slope of the psychometric functions. RESULTS: Accuracy for perceived position was significantly better than accuracy for pointing (mean 9 arc min versus 14 arc min). Performance for these two tasks correlated highly between the 11 subjects (rho = 0.72). For each subject, pointing position correlated highly with perceived position over trials and both correlated highly with the physical offset. There was also a significant correlation for each subject between the pointing error and the perceived location error, calculated by partialling out the effect of the physical offset. CONCLUSIONS: The results are in agreement with the idea that the signals determing the perceived location of an object are also used to guide the motor system in pointing towards it.
: Franz, V. H. (2000, September). Getting a grip on representational momentum. (Talk presented at the First International Workshop on Representational Momentum, Tübingen, Germany).

At Repmo2000 I will discuss a joint project that Ian M. Thornton and I are currently pursuing. The question is whether effects of representational momentum can also be found in the motor system if participants do not judge the size of an object, but act on it. We use a virtual environment with computer graphics and two robot arms (PHANToM-devices). This setup enables us to generate virtual visual stimuli and virtual haptic stimuli and to have participants grasp the stimuli. Participants will be confronted with a standard representational momentum situation and will judge the size of the stimuli as well as grasp them.
: Franz, V. H., & Fahle, M. (2000). The effect of visual illusions on perception and visually guided movements. International Journal of Psychology, 35(3/4), 283-283. (Talk presented at the International Congress of Psychology (ICP), Stockholm, Sweden)

Several reports in the literature indicate that visual size illusions may not have an influence on visually guided hand movements, i.e. that movement planning may not be subject to the illusion, implying that there exist two at least partly separated systems. One of these should deal with pure perception, the other one with perception for action. We tested the effect of the Ebbinghaus size illusion on both perception and grasping. An isolated form of the illusion (either a large circle among small circles OR vice versa) produced virtually identical results for grasping and perception, while a simultaneous version had a larger influence on perception than on grasping. The results cast some doubts on conventional ways to demonstrate the separation of pathways in normal subjects.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (2000). Größenillusionen beeinflussen das Greifen - wie auch die Wahrnehmung. In D. Vorberg, A. Fuchs, T. Futterer, A. Heinecke, U. Heinrich, U. Mattler, & S. Töllner (Eds.), Experimentelle Psychologie / 42. Tagung experimentell arbeitender Psychologen (p. 36). Lengerich: Pabst Science Publishers. (Talk presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Braunschweig, Germany)

In den letzten Jahren überprüften wir den Befund, dass visuelle Großsenillusionen auf die Greifmotorik einen deutlich geringeren Einfluss ausüben als auf die Wahrnehmung (Aglioti, DeSouza & Goodale, 1995). Dies wurde bisher als Indiz dafür gewertet, dass Informationen über visuelle Großse vom Wahrnehmungs- und vom Handlungssystem unabhängig ausgewertet werden (Action vs. Perception-Hypothese, Milner & Goodale, 1995). Es sollen unsere Ergebnisse zur Ebbinghaus Illusion, zur Müller-Lyer Illusion und zur Parallele-Linien Illusion zusammenfassend dargestellt werden. Die Hauptergebnisse sind: (a) Greifen wird von optischen Illusionen beeinflusst. (b) Der Einfluss auf das Greifen stimmt nicht immer exakt mit dem Einfluss auf die Wahrnehmung überein. Diese Unterschiede liessen sich jedoch bisher mit unterschiedlichen Anforderungen von Wahrnehmungsaufgabe und Greifaufgabe erklären. Aufgrund dieser Ergebnisse kommen wir zu dem Schluss, dass Greifen bei optischen Taüschungen keine Evidenz für eine Dissoziation zwischen Wahrnehmungs- und Handlungssystem bietet.

Conference posters

: Bannert, M. M., Franz, V. H., Bischoff, M., Blecker, C., Stark, R., Vaitl, D., Rösler, F., & Fiehler, K. (2009). Gibt es ein Kurzzeitgedächtnis für Greifbewegungen im parietalen Cortex? (Poster presented at the ``51. Tagung experimentell arbeitender Psychologen'' (TeaP), Jena, Germany)

Visuelle Kontrolle von Greifbewegungen erfordert die Anpassung der greifenden Hand an das Zielobjekt auf der Grundlage visueller Information über dessen physikalische Eigenschaften. Einzelzellableitungen an Affen zeigen, dass der anteriore intraparietale Sulcus auf die visuelle Kontrolle und kurzzeitige Speicherung von Greifbewegungen spezialisiert ist. Funktionelle Bildgebungsstudien deuten darauf hin, dass eine vergleichbare Region auch im menschlichen Gehirn existiert. Welche Rolle dieses Areal jedoch bei der kurzfristigen Speicherung visuomotorischer Repräsentationen spielt, wird allerdings kontrovers diskutiert. In der aktuellen fMRT-Studie wurden Versuchspersonen instruiert, nach einem Behaltensintervall variabler Dauer blind nach einem zuvor visuell enkodierten Objekt zu greifen. In einer Kontrollbedingung griffen Versuchspersonen unmittelbar im Anschluss an die Enkodierungsphase nach dem Objekt. Wir finden eine anhaltende Aktivierung des anterioren intraparietalen Sulcus während des Behaltensintervalls. Dies steht im Einklang zu Befunden aus Einzelzellableitungen und aktuellen Arbeitsgedächtnistheorien, denen zufolge Regionen, die für die Echtzeitverarbeitung von Informationen zuständig sind, auch zu deren kurzzeitiger Speicherung beitragen.
: Fiehler, K., Bannert, M. M., Franz, V. H., Bischoff, M., Stark, R., Blecker, C. R., Vaitl, D., & Rösler, F. (2009). The anterior intraparietal sulcus contributes to visually-guided and memory-guided grasping. (Poster presented at the Society for Neuroscience (SfN) meeting, Chicago, Ill.)

There is general agreement about the posterior parietal cortex, in particular the anterior part of the intraparietal sulcus (aIPS), being engaged in visually guided grasping. The contribution of these areas to memory-guided grasping, however, is still controversial. Electrophysiological studies in monkeys suggest a role of the aIPS in both visually guided and memory-guided grasping. However, some results from patients suggest a dissociation such that the aIPS is involved in immediate grasping while the inferior temporal cortex is involved in memory-guided grasping. Using functional magnetic resonance imaging, we investigated the neural correlates of immediate and delayed grasping in healthy humans. Participants were asked to grasp three-dimensional objects of different size and orientation with their thumb and index finger of the dominant right hand (precision grip). Vision was controlled by liquid crystal shutter goggles that were opened during object presentation but closed during grasping, thus requiring open-loop grasping. An auditory signal either presented immediately after object presentation (immediate grasping) or after a variable delay of two to twelve seconds following object presentation (delayed grasping), signalled the start of the grasp movement. We analysed cortical activity during object presentation, maintenance of object information, and immediate and delayed grasping. Object presentation activated areas along the dorsal and ventral visual streams in both hemispheres and the left sensorimotor cortex. Short-term maintenance of action-related object information revealed activation in the right aIPS and adjacent inferior parietal cortex. A similar activation was observed for delayed in contrast to immediate grasping. In line with electrophysiological monkey data, our results indicate that the aIPS does not only contribute to visually-guided grasping but also stores action-related information used for subsequent memory-guided grasping.
: Kleinholdermann, U., Gegenfurtner, K. R., & Franz, V. H. (2009). A model on human grasp point selection. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)

We present a model of where humans place index finger and thumb when grasping arbitrary objects with a precision grip. The model incorporates three weighted rules to (a) maximize force-closure, (b) optimize the position of the object's gravicenter relative to the fingers, and (c) minimize deviations from ones natural grasp angle. We determined the parameters of the model in two experiments (N=18 and N=19) with objects of simple geometry. In a third experiment (N=18) we predicted the grasp points for a set of objects with complex geometries. The results show that our simple model can surprisingly well predict human grasp point selection.
: Schum, N., Franz, V. H., Jovanovic, B., & Schwarzer, G. (2009). Prozesse der Objektverarbeitung in Wahrnehmung und Handlung im Kindes- und Erwachsenenalter. (Poster presented at the ``19. Tagung der DGPs-Fachgruppe für Entwicklungspsychologie'' (EPSY), Hildesheim, Germany)

Nach Goodale und Milner (1992) unterteilt sich unser visuelles System in zwei anatomisch und funktionell getrennte Pfade: ein ventraler Pfad zur visuellen Wahrnehmung von Objekten und ein dorsaler Pfad zur Handlungssteuerung. Die Annahme, dass der Wahrnehmungspfad Objekte mehrdimensional verarbeitet, wohingegen der Handlungspfad eindimensional handlungsrelevante Merkmale beachtet, wird kontrovers diskutiert. Wir untersuchten, ob und in welcher Weise sich solche Unterschiede in den Verarbeitungsprozessen bei Kindern und Erwachsenen zeigen. Mittels einer speeded-classification-task nach Garner (1974) überprüften wir, ob sich Kinder im Alter von 6-7 und 9-10 Jahren sowie Erwachsene (1) beim Einschätzen der Breite (Wahrnehmungsaufgabe) und (2) beim Greifen nach der Breite (Handlungsaufgabe) von rechteckigen Objekten von der Länge der Objekte beeinflussen lassen. Ein Einfluss der Objektlänge auf die Wahrnehmung oder das Greifen nach der Objektbreite, würde auf eine mehrdimensionale Objektverarbeitung hinweisen. Beim Einschätzen der Breite zeigte sich im Erwachsenen- und Kindesalter eine mehrdimensionale Verarbeitung von Breite und Länge. Die Beurteilung der Breite der Objekte wurde somit von der Länge der Objekte beeinflusst. Im Gegensatz dazu beachteten Erwachsene beim Greifen nur die Breite der Objekte. Die Objektlänge hatte keinen Einfluss auf das Greifverhalten. Im Kindesalter deuten erste Ergebnisse darauf hin, dass sich für beide Altersgruppen eine mehrdimensionale Verarbeitung beim Greifen darstellt. Dieses Ergebnismuster würde dafür sprechen, dass die Wahrnehmung von Objektmerkmalen bei Kindern und Erwachsenen mehrdimensional ist, wohingegen die Verarbeitung von Objektmerkmalen beim Handeln im Kindesalter mehrdimensional, im Erwachsenenalter aber eindimensional ist.
: Franz, V. H., & Bruno, N. (2008). Visually guided grasping and the Müller-Lyer illusion: As for pointing, the data look contradictory but in fact they are not. Journal of Vision, 8(6), 298. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
Some models of human vision propose a functional division of labor between vision-for-perception and vision-for-action (Milner & Goodale, 1995, The visual brain in action). This proposal is supported by neuropsychological, brain-imaging, and psychophysical studies. However, it has remained controversial in its prediction that actions are not affected by visual illusions. Here we re-analyze 16 studies on grasping the Müller-Lyer illusion (see Bruno, Bernardis & Gentilucci, in press, Neuroscience & Biobehavioral Reviews, for a similar meta-analysis on pointing). We find that median percent effects across studies are indeed slightly larger for perceptual than for grasping measures. However, all grasping effects are larger than zero and the two distributions show substantial overlap, with grasping effects showing a substantial variability. After detailed examination of methodological differences between and within the perceptual and grasping measures, we show that, as for pointing, critical roles in explaining such variability are played by the number of trials/condition (a learning-attentional effect), by the availability of visual feedback during movement, and by conditions at the programming phase of the action. We discuss to which degree these can explain differences between illusory effects on perception, grasping, and pointing, as well as their implications for the perception-action model.
: Franz, V. H., Hesse, C., & Kollath, S. (2008). Gedächtniseffekte im Greifen: Gibt es einen Wechsel von dorsaler zu ventraler Kontrolle? (Poster presented at the ``50. Tagung experimentell arbeitender Psychologen'' (TeaP), Marburg, Germany)

Nach Milner und Goodale (1995) soll Greifen nur dann vom dorsalen, kortikalen Pfad gesteuert werden, wenn visuelle Information über das Zielobjekt zeitnah verfügbar ist. Dies wird durch Studien gestützt, die eine Zunahme der Effekte optischer Illusionen auf das Greifen nach Zeitverzögerung berichten. Es wird angenommen, dass ohne Zeitverzögerung die Bewegung von dem dorsalen Pfad (immun gegen Illusionen) gesteuert wird, während nach der Zeitverzögerung der ventrale Pfad die Kontrolle übernimmt (beeinflusst von Illusionen). In Einklang mit anderen Studien fanden wir für die Müller-Lyer Illusion eine Zunahme der Illusion nach einer Zeitverzögerung von 5 Sekunden zwischen Stimuluspräsentation und Greifen. Diese Zunahme konnte jedoch durch eine Konfundierung erklärt werden: In den Bedingungen ohne Zeitverzögerung war auch visuelles Feedback während der Bewegung verfügbar, welches eine Online-Korrektur der Illusion ermöglicht. Die Zunahme der Illusionseffekte im Greifen nach Zeitverzögerung sprechen damit nicht für einen Wechsel der Kontrolle von dorsalem zu ventralem Pfad.
: Hesse, C., & Franz, V. H. (2008). Adaptive grasping: Corrective processes after perturbations of object size. Journal of Vision, 8(6), 300. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
When grasping an object the pre-shaping of the hand is a highly stable motor pattern which is largely pre-determined by object-related visual input (Jeannerod, 1984). If the object size changes during movement execution, the initially planned motor program has to be adjusted. How these adjustments are accomplished is still a matter of debate. We investigated the corrective responses using a size perturbation paradigm. Participants grasped real objects of different sizes which were visually presented using a mirror setup. In 25 visually presented object changed its size and became 1 cm larger or smaller, matching the size of the real object to be grasped. The perturbation could occur at two moments in time: (1) early: as soon as the hand left the starting position, (2) late: after 2/3 of the movement distance. In Experiment 1 participants could see their hand during grasping whereas in Experiment 2 vision of the hand was prevented. By combining the size perturbation paradigm with the presence or absence of visual information about the hand we were able to determine the relative contribution of feedback and feed-forward processes to on-line corrections of the grip. Results indicate that the availability of visual information about the hand influenced grasping kinematics (e.g., larger MGA if the hand was not seen) but had only little effect on the corrections to the new object size. In both experiments maximum grip aperture was perfectly adapted to the new object size after an early perturbation, whereas this correction was not accomplished after a late perturbation. The adaptation of the grip resulted from smooth changes in the aperture over time. These findings suggest that small changes in object size are corrected by a smooth amendment of the initially planned motor program using feed-forward mechanisms.
: Hesse, C., & Franz, V. H. (2008). Smooth adjustments of grasping movements after perturbations of object size. (Poster presented at the ``50. Tagung experimentell arbeitender Psychologen'' (TeaP), Marburg, Germany)

We investigated the mechanisms underlying the on-line corrections of grasping using a size-perturbation paradigm. Participants grasped real objects of different sizes which were visually presented using a mirror setup. In 25% of the trials the visually presented object changed its size by 1 cm, matching the size of the real object to be grasped. The perturbation could occur at two moments in time (early/ late). Furthermore we varied whether participants could see their hand during movement execution. Results indicate that maximum grip aperture is perfectly adapted to the new size of the object after an early perturbation, but not after a late perturbation. Corrective processes were similar in both experiments indicating that seeing the hand is not necessary to correct the movement successfully. Moreover, the adaptation was achieved by smooth changes in the aperture suggesting that the initially planned motor program is amended.
: Duemmler, T., Franz, V. H., Jovanovic, B., & Schwarzer, G. (2007). Effects of the Ebbinghaus illusion on perception and grasping during childhood. (Poster presented at the Society for Research in Child Development meeting (SRCD), Boston, Massachusetts, USA)

Based on the hypothesis of two different streams processing visual information, one for perception and one for motor performance, it was presumed that contextual visual illusions such as the Ebbinghaus illusion might only affect perception but not grasping (Agliotti, DeSouza & Goodale, 1995). Nevertheless, later findings in adults showed both perception and grasping are affected by the Ebbinghaus illusion to about the same extent and the same direction (Franz, Gegenfurtner, Buelthoff & Fahle, 2000). In the Ebbinghaus illusion, a target disk is perceived smaller when it is surrounded by larger circles than when it is surrounded by smaller circles. When adults grasp for the target disk they accordingly use a larger grip aperture for the disk which is perceived larger as for the one which is perceived smaller. From a developmental perspective, it has been reported that the illusion effect in perception increases from the age of 4 years on (Kaldy and Kovac 2003) and, more strikingly, that the illusion effect on grasping reverses: Hanisch, Konczak and Dohle, (2001) found that children of 7 years used a wider grip aperture for disks which they judge to be smaller and vice versa. By varying distance between target disk and surrounding circles we examined whether this negative illusion effect might be due to an adjustment of grip aperture relative to the overall size of the illusion configuration. Further, we were interested in the general developmental trend concerning the strength of the Ebbinghaus illusion on perception. To this end, 6- to 7- and 9- to 11-year-olds performed a perceptual and a grasping task, 4-year-olds only the perceptual task. In the perceptual task participants performed a discrimination task, comparing the size of the target disk with an isolated comparison disk. In the grasping task, participants grasped the target disc and their grip apertures were recorded. We construed four different illusion conditions varying size of context circles (small-large) and distance between context circles and target disc (near-far) independently. In the perceptual task a significant effect of illusion context was observed. Children of all three age groups judged the target disk larger when it was surrounded by small circles, whereas they judged it smaller when it was surrounded by large circles. Largest illusion effects appeared for the small-near and the large-far conditions. This corresponds to previous results for adults. 4- to 5-year-olds and 6- to 7-year-olds were equally deceived by the illusion, whereas 9- to 11-year-olds were deceived a little less. Thus, the previous finding of an increase in illusion strength was not supported. First analyses of grip apertures showed no evidence for a negative illusion effect. Rather, as in adults, perception and grasping seem to be deceived by the illusion in the same direction. These results would be consistent with the notion that perception and action are deceived to a similar extent by visual illusions. However, the result needs further corroboration, especially with respect to the discrepancy to the results of Hanisch et al. (2001). Possible reasons for this discrepancy are discussed.
: Duemmler, T., Franz, V. H., Jovanovic, B., & Schwarzer, G. (2007). Täuschungseffekte der Ebbinghaus-Illusion auf Wahrnehmung und Handlung im Kindesalter. (Poster presented at the ``18. Tagung der DGPs-Fachgruppe für Entwicklungspsychologie'' (EPSY), Heidelberg, Germany)

Eine Studie von Hanisch, Konczak & Dohle (2001) zeigte, dass die Ebbinghaus-Illusion Wahrnehmung und Handlung im Kindesalter unterschiedlich beeinflusst. Während in der Wahrnehmung der klassische Täuschungseffekt beobachtet wurde, zeigte sich ein negativer Handlungseffekt: Kinder griffen mit einer größeren Griffapertur nach illusorisch kleiner wahrgenommenen Scheiben und umgekehrt. Da bei den verwendeten Stimuli die Größe der Kontextkreise mit dem Abstand zwischen Kontextkreisen und zentraler Scheibe konfundiert waren, kann dieser negative Greifeffekt dadurch zu erklären sein, dass Kinder ihre Griffapertur an die Spaltgröße zwischen innerer Scheibe und Kontextelementen, oder an die Gesamtgröße einer Ebbinghausfigur anpassten. Die vorliegende Studie untersuchte, ob bei 5-7-Jährigen und 9-11-Jährigen einer dieser Mechanismen den negativen Greifeffekt erklären kann. Als Stimuli dienten Illusionsfiguren, in welchen die Größe der Kontextkreise sowie der Abstand zwischen Kontextkreisen und zentraler Scheibe unabhängig variiert waren. Zur Bestimmung des Wahrnehmungseffekts verglichen Kinder im Rahmen eines Staircasedesigns die zentrale Scheibe mit einer isoliert präsentierten Vergleichsscheibe. Für die Bestimmung des Greifeffekts griffen sie wiederholt nach der zentralen Scheibe, wobei die Griffapertur gemessen wurde. Unsere Ergebnisse zeigten keine Belege für einen negativen Greifeffekt, stattdessen wurden in beiden Altersgruppen Wahrnehmung und Handlung qualitativ gleich getäuscht: größer wahrgenommene Scheiben wurden mit einer größeren Apertur gegriffen und umgekehrt. Während die Täuschungseffekte bei 9-11-Jährigen auch quantitativ gleich groß waren, war der Greifeffekt bei 5-7-Jährigen nur gering und somit schwächer als deren Wahrnehmungseffekt. Es wird diskutiert, ob die Unterschiede zwischen Wahrnehmungs- und Greifeffekten bei 5-7-Jährigen durch eine größere Varianz der kindlichen Greifmotorik oder durch unterschiedliche Verarbeitungswege der Täuschung im visuellen System zu erklären sind.
: Franz, V. H., Hesse, C., & Kollath, S. (2007). Grasping after a delay: More ventral than dorsal? Journal of Vision, 7(9), 157a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
It is often assumed that grasping is only controlled by the dorsal cortical stream if visual information about the target object is easily available. After a short delay between stimulus presentation and grasping the dorsal information should be decayed and the action should be guided by the ventral stream. Accordingly, grasping should not be affected by certain visual illusions under full-vision conditions, but it should be affected after a delay (because only the ventral stream is assumed to be deceived by these illusions). We tested this for the Müller-Lyer illusion. In experiment 1 (N=16), we investigated grasp illusion and perceptual illusion for full-vision and 5sec delay conditions. The perceptual illusion was independent of delay (p=.23), while grasping showed a strong increase of the illusion effects (p<.001). This replicates the increase of the motor illusion found in the literature. In experiment 2 (N=8), we tested whether the delay causes this increase by comparing open loop grasping (shutter goggles close when movement starts) with the 5sec delay condition. Illusion effects on grasping were constant (p=.90), suggesting that delay is not the critical factor. In experiment 3 (N=20), we systematically decreased the amount of visual feedback available during the grasping movement using the conditions: full vision, vision suppressed when fingers had traveled 2/3 or 1/3 of the way to the target object, open loop (goggles close when movement starts), and open loop after go-signal (goggles close with auditory go-signal). Illusion effects decreased the more visual feedback was provided (p=.001). This suggests that the critical factor is visual feedback and not different memory systems in dorsal and ventral streams.
: Hesse, C., de Grave, D. D. J., Franz, V. H., Brenner, E., & Smeets, J. B. J. (2007). Planning movements well in advance. Journal of Vision, 7(9), 160a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
[ web-site ]
It has been suggested that the precise metrics of grasping movements directed to visible objects are computed in real-time and are therefore unaffected by previous experience or any earlier programming (Cant, et al., 2005, Neuropsychologia, 43(2), 216-226). We have tested whether the properties of a visually presented distractor object can influence the kinematics of a subsequent grasping movement performed under full vision. Ten participants grasped a target object after viewing a small or large elliptical distractor object in a certain orientation (0 degree or 30 degree). The target object appeared at the same location and could be either the same object in the same orientation as the distractor, or a circular object, which is neutral in orientation. When grasping the circular target object, grip orientation was affected in the direction of the previously presented distractor object. That is, at the time of maximum grip aperture grip orientation was 23.3 degree +/- 2.2 degree after having seen the distractor in 0 degree orientation and 28.9 degree +/- 1.6 degree after having seen the distractor in 30 degree orientation. Moreover, as in classical visuomotor priming, grasping movements were initiated 30ms +/- 4ms faster when distractor and target were identical. Our study provides evidence that the planning of fully visually guided movements is influenced by prior perceptual experience. This planning is reflected in a change of movement parameters, in particular grip orientation, by the properties of the previously perceived object. This finding challenges the notion that grasping movements are controlled on the basis of real-time information alone.
: Hesse, C., & Franz, V. H. (2007). Die Verwendung visueller Informationen zur Online-Kontrolle von Greifbewegungen. In K. F. Wender, S. Mecklenbräuker, G. D. Rey, & T. Wehr (Eds.), Experimentelle Psychologie / Beiträge zur 49. Tagung experimentell arbeitender Psychologen (p. 288). Lengerich: Pabst Verlag. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Trier, Germany)

Veränderungen der Objektgröße während Greifbewegungen und die daraus resultierenden Korrekturprozesse ermöglichen Aussagen über die Verwendung visuellen Feedbacks während motorischer Handlungen. Bisherige Untersuchungen beschränkten sich auf relativ große Objektgrößenveränderungen (z.B. Paulignan 1991: 1.5 vs. 6 cm), welche zu Bewegungsbeginn erfolgten. Wir untersuchten die Auswirkung kleinerer Objektgrößenveränderungen (1 cm größer/kleiner) zu verschiedenen Bewegungszeitpunkten (früh: 30 cm vs. spät: 12 cm vor Objekt). Acht Probanden griffen Stäbchen (Präzisionsgriff) verschiedener Länge (45, 55, 65 mm), die mittels eines Spiegelsetups dargeboten wurden und in 40% der Durchgänge ihre Größe veränderten. Das projizierte Abbild des Objekts stimmte am Ende der Bewegung visuell mit dem zu greifenden Objekt überein. Wir fanden bei frühen Veränderungen eine perfekte Anpassung der Größe der maximalen Greiföffnung an die neue Objektgröße, während diese Anpassung bei späten Veränderungen nicht mehr erfolgte. Die Ergebnisse legen nahe, dass kleinere Veränderungen zu Bewegungsbeginn schnell und kontinuierlich in eine Greifbewegung integriert werden können.
: de Grave, D. D. J., Hesse, C., Brouwer, A. M., & Franz, V. H. (2006). Fixation locations when grasping occluded objects. (Poster presented at the European Conference on Visual Perception (ECVP), St Petersburg, Russia).

When grasping objects with a precision grip and both contact positions of the digits are visible, subjects look in the direction of the index finger (top of the object) or at the contact position that requires most accuracy (Brouwer et al., submitted). However, contact positions of the digits are not always visible due to occlusion by other objects. Where do subjects fixate when one or both of these contact positions on the object are occluded by another object? Subjects were asked to grasp flat shapes with a precision grip at predefined contact positions (index finger on top and thumb at bottom of shape). Either the contact position of the thumb or that of the finger or both were occluded during grasping. The first saccades showed a general tendency to land near an intersection of the occluder and a visible part of the shape. When one digit was occluded second saccades tended to go in the direction of the finger or to the digit that required more accuracy. When both digits were occluded this tendency was only found when object information could be extrapolated. Thus an occluder affects fixation positions but it does not prevent fixating occluded object parts.
: Hesse, C., & Franz, V. H. (2006). Grasping: A stereotyped visuomotor pattern? (Poster presented at the European Conference on Visual Perception (ECVP), St Petersburg, Russia).

When grasping objects with a precision grip, index finger and thumb open gradually to a maximum grip aperture (MGA) which occurs in the second half of the movement and is influenced by several factors such as object size and target visibility (Jeannerod, 1984; Hu et al., 1999). However, we still have little knowledge about how grasping is controlled by the nervous system. We therefore investigated the characteristics of the grip aperture under different conditions. Experiment 1 examined the effect of object size (39, 41, 43 mm) on aperture in different visibility conditions reducing the amount of visual feedback available during grasping: full vision, full vision until movement initiation, full vision until start signal, and 5s delay (visual occlusion for 5s before movement initiation). In Experiment 2, participants grasped a variety of object sizes (1-10 cm) under full vision. Results show that MGA was determined by object size and the availability of visual feedback as predicted. Nevertheless, the aperture profiles of the different conditions showed similar characteristics. We suggest a very simple model of grip aperture which may account for most of the observed changes in aperture and evaluate the appropriateness of the model.
: Hesse, C., & Franz, V. H. (2006). Visuelles Gedächtnis bei Greifbewegungen. In H. Hecht, S. Berti, G. Meinhardt, & M. Gamer (Eds.), Experimentelle Psychologie / Beiträge zur 48. Tagung experimentell arbeitender Psychologen (p. 268). Lengerich: Pabst Verlag. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Mainz, Germany)

Wir untersuchten inwiefern sich die Greifbewegungskinematik bei reduzierter visueller Information verändert. 48 Probanden griffen Stäbchen verschiedener Länge (39, 41, 43 mm) im Präzisionsgriff nach einer Präsentationszeit von 1 sec und Darbietung eines Startsignals. Dabei schränkten verschiedene Sichtbedingungen die verfügbare visuelle Information systematisch ein: (1) volle Sicht, (2) volle Sicht bis Bewegungsbeginn (Reaktionszeit), (3) volle Sicht bis Startsignal, (4) keine Sicht und Verzögerung des Startsignals um 5 sec. Wir fanden von (1) bis (4), korrespondierend zur Abnahme der verfügbaren visuellen Information, eine Zunahme der maximalen Greiföffnung. Der größte Anstieg zeigte sich zwischen den Bedingungen (1) und (2). Zudem war die Zunahme der Greiföffnung zwischen den Bedingungen (2-3) und (3-4), trotz der stark unterschiedlichen Verkürzung der Objektsichtbarkeitsdauer (350 ms Reaktionszeit vs. 5 sec Verzögerung) vergleichbar groß. Dies spricht entweder für einen exponentiellen Zerfall der Gedächtnisspur oder aber für die besondere Bedeutsamkeit der Zeitspanne zwischen Startsignal und Bewegungsbeginn für die Bewegungsplanung.
: Hesse, C., & Franz, V. H. (2006). The use of visual memory for grasping. In H. H. Bülthoff, S. Gillner, H. A. Mallot, & R. Ulrich (Eds.), Beiträge zur 9. Tübinger Wahrnehmungskonferenz (p. 84). Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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We tested which components of visual memory are most important for grasping by varying the amount of visual feedback available to the participants during a grasping movement. Furthermore we examined the effects of visual memory on grasping kinematics. 48 participants had to grasp bars of different lengths (39 mm, 41 mm, 43 mm) but constant width (8 mm) and depth (5 mm) after a preview period of 1 s and in response to an auditory cue. There were four different visual conditions, which were designed to increase memory load successively: (1) full vision of hand and target during grasping, (2) full vision until movement initiation, (3) full vision until start signal, (4) no vision and 5s-delay (visual occlusion for a period of 5 s before the start signal was given and movement initiation started). Note that the main difference between the conditions (2) and (3) was the target visibility during the reaction time interval. For each movement the grip aperture between index finger and thumb was analyzed as a function of time. In accordance with earlier studies [1,2,3], we found that maximum grip aperture was consistently larger for the longer targets (F(1.7, 78.3) = 68.7, p < .001) and increased with higher memory load (F(2.1, 96.5) = 168.5, p < .001). The most prominent increase in grip aperture was found between condition (1) and (2). In addition, the increase in grip aperture between condition (2) and (3) was equally large as the increase of grip aperture between condition (3) and (4), although in the first case the hold time in memory was increased by only 350 ms whereas in the second case it was increased by 5 s. This suggests either an exponential decrease of the memory trace or that the critical programming of the movement takes place during the time period between go-signal and movement beginning.
: Franz, V. H. (2005). Metacontrast masking: Effects of barely visible stimuli on pointing movements. Journal of Vision, 5(8), 356a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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Stimuli which are masked by metacontrast can nevertheless affect motoric responses. In a typical experiment, two squares are presented sequentially (both rotated by either 0 or 45 degree). The first square (the prime) is masked by the second square (the target), such that perceptual discrimination of the orientation of the prime is very low. However, if subjects are asked to respond quickly to the orientation of the target by pointing left or right, the prime still affects the trajectory: If the orientation of the prime is inconsistent with the orientation of the target, pointing first goes in the wrong direction. This result might be interpreted as a dissociation between perception (we cannot discriminate the prime's orientation) and action (the prime's orientation nevertheless affects pointing). However, it is very difficult to get discrimination performance really to zero, such that this dissociation might not be very convincing. Here, I tested whether the effect on action breaks down after taking very serious measures to suppress discrimination performance. For this, I used stimuli which are known to produce a good metacontrast suppression, reduced the contrast of the prime to very low values, and also presented the stimuli at unpredictable positions circular around the fixation point (metacontrast is known to be stronger if the stimuli are not fixated). Results show that with this procedure discrimination performance is almost zero, but the cost of this perfect suppression in perception is that the effect on action also breaks down. These results are consistent with the notion that the effects on action under normal conditions (with imperfect suppression of discrimination performance) are generated by similar sources as the residual discrimination performance.
: de Grave, D. D., Franz, V. H., & Gegenfurtner, K. R. (2005). The influence of the Brentano illusion on saccades and pointing movements. Perception, 34, S242. (Poster presented at the European Conference on Visual Perception (ECVP), A Coruna, Spain)
: de Grave, D. D., Franz, V. H., & Gegenfurtner, K. R. (2005). The coding of combined pointing movements and saccades in a length illusion. In H. H. Bülthoff, H. A. Mallot, R. Ulrich, & F. A. Wichmann (Eds.), Beiträge zur 8. Tübinger Wahrnehmungskonferenz. Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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When making an eye or hand movement to a visual target, different sources of information can be used. Either visual judgments of direction and distance (or length) of the required displacement can be used (vector coding), or the final position (position coding), or a combination of both. In an earlier study (de Grave et al., 2004) it was shown that pointing movements mainly use vector coding.To find out which source of information is used for combined eye and hand movements, we used the Brentano illusion, a version of the Müller-Lyer illusion. This illusion primarily influences judgements of length, but not the position information. Thus, a task will only be influenced by this illusion if the task requires a visual estimate of length. In this study we investigated the influence of the Brentano illusion on pointing and saccadic eye movements when these movements are performed in the same trial. Ten subjects fixated in the middle of the touchscreen and also started the pointing movement with their index finger at this position. After the stimulus was presented for 200 milliseconds they made saccades and pointing movements in four directions (up, down, left, right). Movements were always from an outer vertex of the Brentano illusion toward the middle vertex. Pointing movements as well as saccades showed an effect of the illusion, indicating that length information was used. A trial-by-trial correlation for pointing and saccades (rho = 0.05 +/- 0.03) was non-significant. This might either be due to small between trial variability of the illusion relative to the noise in each response or to an independent, parallel generation of the illusion effects for the two responses. In both tasks an effect of the illusion is found (pointing: 26.19% +/- 2.72%; saccades: 20.14% +/- 2.51%), indicating that length is used (vector coded). There is no significant difference in illusion effect between the pointing task and the saccadic eye movement task. We conclude that the results favour the interpretation of using the same information in eye and hand movements.
: Brouwer, A., Franz, V. H., Kerzel, D., & Gegenfurtner, K. R. (2005). Looking during grasping. In H. H. Bülthoff, H. A. Mallot, R. Ulrich, & F. A. Wichmann (Eds.), Beiträge zur 8. Tübinger Wahrnehmungskonferenz. Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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: Brouwer, A., Franz, V. H., Kerzel, D., & Gegenfurtner, K. R. (2005). Fixating for grasping. Journal of Vision, 5(8), 117a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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In a grasping task, Johansson et al. (2001) found that subjects look at the position to which the finger tips are guided. However, in their experiment, only the contact position of the thumb was visible. We investigated what happens if the contact positions of both finger and thumb are visible. We recorded eye and finger movements. In a first experiment, subjects always grasped with the index finger at the top and the thumb at the bottom of a flat shape that was mounted on a horizontal bar. In order to see whether a salient feature of a shape would affect the fixation positions, we presented an (asymmetric) cross in 4 orientations (with the crossing of the bars representing the salient feature). In order to see whether gaze is attracted to the position where the finger has to be guided relatively precisely, we presented a triangle in two orientations that subjects had to contact at the base and at the pointed top (i.e., a higher required precision to contact the top than the base). We found that the crossing of the bars cross attracted the gaze whereas the top of a triangle did not. A prominent result was that subjects fixated above the center of the shape. In order to distinguish between subjects fixating the upper part of the shape versus being attracted by the index finger, we mounted a square and a triangle in two orientations on a vertical bar. We asked subjects to grasp first with one hand and then with the other so that the shape remained constant but the contact positions of the index finger and thumb were reversed. Subjects still looked above the center. In addition, the gaze was attracted to the index finger for the triangle but to the thumb for the square. We conclude that both features of the shape and the grasp affect gaze location. The exact location depends on the specific circumstances.
: de Grave, D. D., Franz, V. H., & Gegenfurtner, K. R. (2005). The coding of combined pointing movements and saccades in the Brentano illusion. Journal of Vision, 5(8), 207a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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For movements to a visual target either visual judgments of direction and distance (or length) of the required displacement can be used (vector coding), or the final position (position coding), or a combination of both. Earlier studies using the Brentano illusion (de Grave et al., VSS 2002) showed that pointing movements and saccades both use vector coding, however not to the same extent. The saccades relied more strongly on vector coding than the hand (pointing), leading to the conclusion that the eyes (saccades) and the hand (pointing) use different information for their movement. However, these studies do not rule out the possibility that eye and hand use the same information. In the pointing study correction saccades could have been made by the time the pointing movement was finished. This could explain the smaller effect on pointing if eye position information is used in pointing. In this study we tested whether combined saccades and pointing movements are influenced by the Brentano illusion to the same extent when the stimulus is only presented for 200 ms, so subjects could not make a corrective saccade. Subjects started with their index finger in the middle of a touchscreen and made saccades and pointing movements in four directions. Movements were always from an outer vertex of the Brentano illusion toward the middle vertex. We found an illusion effect of about 25 task. This favours the interpretation that the same information is used in eye and hand movements.
: Franz, V. H. (2004). The dynamic illusion effect: An interesting artifact. Journal of Vision, 4(8), 840a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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A number of studies suggested that the effects of visual illusions on grasping are large at the start of the movement and then gradually decay while the movement unfolds. This effect was named dynamic illusion effect and led to intensive theorizing about the processes guiding visuo-motor behavior (e.g., Glover & Dixon, Perception and Psychophysics, 2002; Glover, Behavioral and Brain Sciences, in press). Using improved methodology, I tested whether the dynamic illusion effect exists. Participants grasped disks which were exposed to the Ebbinghaus / Titchener illusion. The hand aperture of each grasp was analyzed at different, normalized time points. At each time point a correction for the responsiveness of the hand aperture to a physical variation of size was performed. Special care was taken to avoid possible artifacts which might arise from the hand already touching the target object. Different to our previous study (Franz & Scharnowski, VSS-2003), participants could see their hand and the stimuli all the time while grasping (closed-loop condition). Results show that the illusion effects are remarkably constant over time and that the dynamic illusion effect which was found by previous studies was most likely due to a contamination of the data by the hand already touching the target object.
: Franz, V. H., & Kollath, S. (2004). Sind die Effekte optischer Illusionen auf das Greifen zeitabhängig? In D. Kerzel, V. H. Franz, & K. R. Gegenfurtner (Eds.), Experimentelle Psychologie / 46. Tagung experimentell arbeitender Psychologen (p. 80). Lengerich: Pabst Verlag. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Gießen, Germany)

Neuere Modelle der Sensomotorik gehen davon aus, dass die Effekte optischer Täuschungen auf Bewegungen zeitlich variabel sind [Glover, Trends in Cognitive Sciences, 6, 288-292 (2002); Glover and Dixon, Perception and Psychophysics, 64, 266-278 (2002)]. So soll die Ebbinghaus Illusion zu Beginn einer Greifbewegung stärker wirken wie am Ende. Glover interpretiert dies als Indiz für zwei Prozesse zur Steuerung von Bewegungen: Einen frühen Planungs Prozess, welcher der Illusion unterliegt und dem ventralen, kortikalen Pfad zugeordnet wird und einen späten Kontroll Prozess, welcher der Illusion nicht unterliegt und dem dorsalen Pfad zugeordnet wird. Wir überprüften dieses Modell unter verbesserten methodischen Bedingungen. Versuchspersonen griffen Scheiben innerhalb einer Ebbinghaus Figur, wobei sie während der gesamten Bewegung ihre Hand und die Scheibe sehen konnten. Der zeitliche Verlauf der Handöffnung wurde bestimmt, und es wurde sichergestellt, dass das Objekt noch nicht berührt war. Es zeigten sich zeitlich erstaunlich konstante Illusionseffekte. Dies steht in Widerspruch zu dem Planungs-Kontroll Modell.
: Stockmeier, K., Gegenfurtner, K. R., Bülthoff, H. H., & Franz, V. H. (2004). Greifen isoluminanter Stimuli. In D. Kerzel, V. H. Franz, & K. R. Gegenfurtner (Eds.), Experimentelle Psychologie / 46. Tagung experimentell arbeitender Psychologen (p. 258). Lengerich: Pabst Verlag. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Gießen, Germany)

Visuelle Information wird in anatomisch unterschiedlichen kortikalen Pfaden verarbeitet, deren genaue Funktion jedoch noch umstritten ist. Goodale und Milner [TiNS,15,97-112(1992)] ordnen dem ventralen Pfad Aufgaben der Objekterkennung zu, während der dorsale Pfad visuelle Reize zur Ausführung von motorischen Handlungen verarbeiten soll. Auch die Verarbeitung von Farbinformation wird häufig dem ventralen Pfad zugeordnet. Man könnte also annehmen, dass das Greifen von Objekten, die ausschließlich über ihre Farbe definiert sind, beeinträchtigt ist. Wir untersuchten Greifbewegungen nach im Vergleich zum Hintergrund isoluminanten (grün, X=0.2856, Y=0.6020) vs. über ihren Helligkeitskontrast definierte Scheiben verschiedener Größe (30, 35 und 40 mm). Diese wurden visuell über einen Spiegel dargestellt, haptisch verwendeten wir reale Scheiben unter dem Spiegel. In einer Wahrnehmungsaufgabe stellten die Versuchsteilnehmer die Größe eines Vergleichsreizes ein. Wir fanden keine Beeinträchtigung der Greifbewegung oder Größenwahrnehmung im isoluminanten Fall. Reine Farbinformation kann zur Ausführung der Greifbewegung genutzt werden.
: Franz, V. H., & Gegenfurtner, K. R. (2004). Is there a dynamic illusion effect in grasping? In H. H. Bülthoff, H. A. Mallot, R. Ulrich, & F. A. Wichmann (Eds.), Beiträge zur 7. Tübinger Wahrnehmungskonferenz (p. 83). Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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Glover and Dixon [1] suggested that the effects of visual illusions on grasping vary over the time course of the grasp movement. For example, the Ebbinghaus illusion should exert a large effect at the beginning of the movement, while the effect should decay at the end of the movement. Glover [2] interprets this result as evidence for two different processes which guide movements: An early planning process which should reside in the ventral cortical pathway and a late control process in the dorsal cortical pathway. In an number of experiments I tested these claims. Participants grasped disc surrounded by Ebbinghaus figures and the temporal dynamics of the grasp trajectories was determined. Grasping was performed with and without visual feedback. Special care was taken to determine the time point at which participants touched the target object, because from this time on the measured illusion effects will be contaminated. I found surprisingly constant illusion effects over time. This result challenges the planning-control model. References: [1] Glover, S. and Dixon, P. (2002): Perc. and Psychophysics, 64, 266-278. [2] Glover, S. (2002): Trends in Cogn. Sci., 6, 288-292.
: Stockmeier, K., Karnath, H.-O., Franz, V. H., & Himmelbach, M. (2004). The role of the posterior parietal cortex in the on-line control of grasping movements. In H. H. Bülthoff, H. A. Mallot, R. Ulrich, & F. A. Wichmann (Eds.), Beiträge zur 7. Tübinger Wahrnehmungskonferenz (p. 125). Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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Lesions of the posterior parietal cortex (PPC) in humans cause severe visuomotor deficits. These patients demonstrate large deviations of pointing and reaching movements to peripheral targets and an inability to adjust their grip aperture to different object sizes. An additional deficit to adjust goal-directed movements to perturbations of target positions during movement execution has been recently shown. However, it is unclear whether such a deficit of an on-line correction mechanism also affects the distal component of grasping movements, i.e. whether patients with lesions of the PPC can adjust their grip aperture to perturbations of object size during movement execution. We compared the performance of a patient with bilateral lesions of the PPC to the performance of healthy controls in a virtual grasping task. A virtual disc (36 or 44 mm) was rendered using stereo computer graphics. Virtual, haptic feedback was given using two robot arms (PHANToM TM). In half of the trials, the virtual disc either increased to a size of 52 mm or decreased to a size of 28 mm. Otherwise the objects size was stable during the trial. The patients performance towards the unperturbed discs was not impaired compared to the grasping kinematics of the healthy controls. In contrast, her grasping movements towards the perturbed objects seemed to be more prone to error than the movements of the healthy controls. This finding supports the previously suggested crucial role of the PPC in the online control of visuomotor actions and shows, that it is also involved in the online control of the distal component of grasping movements.
: Brouwer, A., Franz, V. H., & Thornton, I. M. (2003). Grasping and representational momentum. Journal of Vision, 3(9), 126a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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Observers tend to misremember the stopping point of a change in the direction of the change. We investigated whether this representational momentum effect is reflected in grasping. To do this, we presented 14 subjects with a sequence of 3 still images in which a pair of household pliers was seen to open or close in 1 cm steps. In a visual task, subjects indicated whether a fourth, probe image differed from the stopping point of the sequence. In a grasping task, subjects reached out and closed a virtual version of the third pair of pliers, just after the image disappeared. The subjects' thumb and index finger were attached to robot arms which allowed us to provide haptic feedback and to measure the movement of the digits. When grasping identical versions of the third pliers, subjects opened their digits wider if the width of the pliers had been increasing compared to when the width had been decreasing. This is consistent with representational momentum. The direction of change (decreasing or increasing pliers width) had an effect equivalent to 4 mm physical width variation in grasping. In the visual task, the pliers that were perceived as equal to the third ones tended to have a larger opening width when the width had been increasing than when it had been decreasing. This is also consistent with representational momentum. However, the visual effect was only significant for subjects who did the visual task first (p<.01). For these subjects, the size of the effect was 1 mm pliers width. Although subjects were asked to grasp the third pliers and got the appropriate haptic feedback, they apparently extrapolated the opening or closing of the pliers. This grasping effect does not appear to be directly related to the visual representational momentum effect, because the impact of direction was larger and more reliable in grasping than in perception and the effects were not correlated between subjects.
: Stockmeier, K., Bülthoff, H. H., & Franz, V. H. (2003). How do we grasp (virtual) objects in three-dimensional space? Journal of Vision, 3(9), 383a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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Jeannerod (1981,1984) studied extensively the relationships between object size and grasping parameters, which has been influential for the interpretation of grasping data. The maximum grip aperture (MGA) scales linearly with object size, but the slope is less than 1 (app. 0.82, cf. Smeets & Brenner 99). Here, we investigated if the location of the object in three-dimensional space influences the MGA. As well we addressed the question if the grasping of virtual objects shows the same characteristics as natural prehension. Virtual environments could enable experimenters to easily vary objects after the movement onset and therefore to explore the mechanisms of online control in visually guided movements. A virtual disc (36, 40, or 44 mm in diameter) was rendered using stereo computer graphics in 27 positions in different heights and locations relative to the observer. Virtual, haptic feedback was given using two robot arms (PHANToM TM). One robot arm was connected to the index finger, one to the thumb. Ten participants grasped the discs and transported them to a goal area, where they dropped the discs. The stereoscopically rendered discs were viewed through a mirror, such that the visual and haptic feedback matched. The position of the finger tips was measured using the two robot arms and an Optotrak (TM), in order to test for the accuracy of the PHANToM devices. The MGA was dependent on the distance of the object with respect to the observers body but not on the height of the disc. Participants scaled their MGA according to the size of the virtual disc, but with a slightly smaller slope (0.64+/-0.06) compared to natural environments. This could indicate that tactile feedback (in addition to haptic feedback) is needed to perform natural grasping movements.
: Stockmeier, K., Bülthoff, H. H., & Franz, V. H. (2003). Wie real ist eine virtuelle Scheibe? In H. H. Bülthoff, K. R. Gegenfurtner, H. A. Mallot, R. Ulrich, & F. A. Wichmann (Eds.), Beiträge zur 6. Tübinger Wahrnehmungskonferenz (p. 86). Knirsch Verlag Kirchentellisfurt. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)
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Virtuelle Aufbauten bieten die Möglichkeit die visuelle und haptische Informationen von Greifobjekten unabhängig zu variieren. Aber gelten beim Greifen virtueller Objekte die gleichen Gesetze, wie bei realen Gegenständen? Wir variierten in diesem Experiment die Position und Grösse einer virtuellen Scheibe. Wir betrachteten, ob die maximale Handöffnung der Versuchsteilnehmer der Veränderung der Scheibengrösse angepasst wird und welchen Einfluss die Position der Scheibe im Raum auf die maximale Griffweite hat. Eine virtuell gerenderte Scheibe mit einem Durchmesser von 36, 40 oder 44 mm und einer Dicke von 15 mm wurde in 27 Positionen im Raum dargestellt. Virtuelles haptisches Feedback wurde mit zwei Roboterarmen (PHANToM TM) simuliert. Einer der beiden Roboterarme war mit dem Daumen verbunden, der zweite mit dem Zeigefinger, um den Versuchsteilnehmern ein haptisches Feedback der Scheibe bei der Durchführung eines Präzisionsgriffs zu vermitteln. Die Aufgabe der 10 Teilnehmer war die Scheibe zu greifen, zu einem Ziel zu transportieren und dort die Scheibe fallenzulassen. Die virtuellen Scheiben wurden durch einen Spiegel betrachtet, so dass das visuelle und haptische Feedback räumlich übereinstimmten. Wir fanden, dass die Griffweite grösser war, wenn die Scheibe weiter entfernt vom Körper des Beobachters lag. Die Höhe der Scheibe beeinflusste die maximale Griffweite nicht. In der Literatur findet man meist keinen Einfluss der Entfernung auf die Griffweite (vgl. Meulenbroek et al, Experimental Brain Research, 138, 219-234, 2001). Au?erdem reagierten die Teilnehmer etwas anders als erwartet auf eine Änderung der Scheibengrösse, sie skalierten ihren Griff nur mit einer Steigung von 0.64+/-0.06 im Verhältnis zur wirklichen Änderung der Scheibengrösse (in realen Umgebungen erwartet man eine Steigung von ca. 0.82, vgl. Smeets & Brenner, Motor Control, 3, 237-271, 1999). Ein Grund für diese leicht abweichenden Ergebnisse könnte sein, dass zur Durchführung von natürlichen Greifbewegungen nicht nur haptisches Feedback über die Objektgeometrie benötigt wird, sondern auch taktiles Feedback der Objektoberfläche (welches von uns in diesem Aufbau nicht simuliert wurde).
: Brouwer, A., Franz, V. H., Thornton, I. M., & Bülthoff, H. H. (2003). Anticipating translating versus transforming objects: Visual perception and grasping. Perception, 32, S62. (Poster presented at the European Conference on Visual Perception (ECVP), Paris, France)
: Stockmeier, K., Bülthoff, H. H., & Franz, V. H. (2002). Effects of the Ebbinghaus Illusion on grasping in a virtual environment. Perception, 31, S86. (Poster presented at the European Conference on Visual Perception (ECVP), Glasgow, UK)

It is an open question, whether the Ebbinghaus (or Titchener) illusion affects perception more than grasping. Evidence for a stronger effect on perception has often been based on a perceptual task called manual estimation. We compared manual estimation to a standard perceptual measure as well as to grasping. Virtual target discs (diameter: 38, 40, or 42mm), surrounded by small or large discs (diameter: 10 or 58mm) were displayed stereoscopically on a monitor, generating the Ebbinghaus illusion. In the grasping task, ten participants grasped the target. Haptic feedback was simulated by two robot arms (PHANToM TM). In the manual estimation task participants indicated the size of the target using index finger and thumb (without seeing their fingers). In the standard perceptual task they adjusted a comparison to match the target. We found illusion effects on manual estimation (3.0mm, SE 0.65mm) which were larger than both, the grasp effects (2.2mm, SE 0.41mm), as well as the effects on the standard perceptual measure (0.96mm, SE 0.17mm). This suggests that manual estimation shows relatively large illusion effects.
: Praeg, E., Neumann, O., Klotz, W., Bülthoff, H. H., Fahle, M., Franz, V. H., & Heumann, M. (2002). Der Einfluss maskierter Reize auf Zeigebewegungen. In H. H. Bülthoff, K. R. Gegenfurtner, H. A. Mallot, & R. Ulrich (Eds.), Beiträge zur 5. Tübinger Wahrnehmungskonferenz (p. 141). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

Priming von motorischen Reaktionen durch nicht-bewusst wahrnehmbare Reize wurde bereits vielfach demonstriert (z.B. Klotz & Neumann, 1999). Einige Studien beschäftigten sich auch schon mit dem Einfluss auf andere Formen von Verhalten wie der Latenz einer vokalen Reaktion (Ansorge, Klotz & Neumann, 1998) oder großmotorischen Reaktionen (z.B. Sprünge; Kibele, 2000). In einer Untersuchung von Schmidt (im Druck) wurde erstmals auch der Einfluss maskierter Reize auf Zeigebewegungen untersucht, wobei die Versuchspersonen (VPen) direkt auf geprimte Zielreize zeigen sollten. Hierdurch ergab sich jedoch eine direkte Kompatibilität zwischen Ort des Zielreizes und dem Ziel der Zeigebewegung. Wir versuchten diese Kompatibilität auszuschließen, indem die Zielreize lediglich die Richtung angaben in welche gezeigt werden sollte. Sieben VPen wurde eine Abfolge von Prime und Zielreiz so präsentiert, dass der Zielreiz den Prime vollständig maskierte (Dauer Prime: 26 msec, Zielreiz: 91 msec, Inter Stimulus Intervall: 39 msec). Der Zielreiz war ein Quadrat in einer von zwei möglichen Orientierungen (0 oder 45 Grad). Der Prime war entweder ebenfalls ein Quadrat oder ein Ring (neutraler Reiz). Bei kongruenten Abfolgen hatten Prime und Zielreiz dieselbe Orientierung, bei inkongruenten Abfolgen unterschieden sich die Orientierungen um 45 Grad. In der Wahrnehmungsaufgabe sollten die VPen entscheiden, ob der Prime ein Quadrat oder ein Ring war. In zwei weiteren Aufgaben sollten sie so schnell wie möglich auf die Orientierung des Zielreizes reagieren (Reaktionszeitaufgabe: Tastendruck rechts oder links, Zeigeaufgabe: Zeigen zu einem Zielpunkt rechts oder links). Die Zeigebewegungen wurden mit Hilfe von Infrarot-Markern auf dem Zeigefinger der VP aufgezeichnet (Optotrak System). Die VPen konnten den Prime in der Wahrnehmungsaufgabe nicht diskriminieren (d'=0.1, t(6) = 1.18, p=.28). In der Reaktionszeitaufgabe zeigte sich ein Vorteil für kongruente Reize gegenüber inkongruenten Reizen (57 msec, t(6) = 8.2, p<.001). Dieser Vorteil zeigte sich ebenfalls in unterschiedlichen Trajektorien in der Zeigeaufgabe. Es scheint, daß vollständig maskierte Reize auch dann einen Einfluß auf Zeigebewegungen haben, wenn keine Kompatibilität zwischen dem Ort des Zielreizes und dem Ziel der Zeigebewegung vorliegt.
: Franz, V. H., Bülthoff, H. H., & Fahle, M. (2002). Grasp effects of visual illusions: Simply artifacts? In H. H. Bülthoff, K. R. Gegenfurtner, H. A. Mallot, & R. Ulrich (Eds.), Beiträge zur 5. Tübinger Wahrnehmungskonferenz (p. 138). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

It is an open question whether visual illusions affect motor responses to the same extent as perceptual responses. In previous studies (e.g.: Franz, Gegenfurtner, B?lthoff, & Fahle 2000) we found similar effects of the Ebbinghaus illusion on perception and on grasping. This finding contradicts a strong version of the action versus perception hypothesis (Milner & Goodale, 1995) which states that the motor system is unaffected by visual illusions. Here, we tested whether our grasp effects might have been artifacts. This could be the case if the motor system treated the illusion inducing context elements as obstacles and tried to avoid them. To test for this possibility, we varied the distance between context elements and target. An aluminum disc (31, 34, or 37 mm in diameter, 5 mm in height) was positioned as target on a board. Around the target either small or large context discs (10 or 58 mm in diameter) were drawn at near or far distances (24 or 31 mm midpoint to nearest point on context circles). Close to the board a monitor was mounted on which a comparison disc was displayed. In the perceptual task 52 subjects adjusted the size of the comparison stimulus to match the size of the target. In the grasping task subjects grasped the target. Subjects wore shutter glasses and could not see their hand during grasping. The grasp trajectory was recorded and the maximum preshape aperture was calculated. The motor illusion responded to the variation of distance between context elements and target in exactly the same way as the perceptual illusion. None of three different obstacle avoidance hypotheses can explain these results. Our results suggest that the same signals are responsible for the perceptual and for the motor illusion. This either indicates that the action versus hypothesis needs modification, or that the Ebbinghaus illusion is generated before the separation of the perceptual and the motor streams.
: Praeg, E., Heumann, M., Fahle, M., Bülthoff, H. H., & Franz, V. H. (2002). Effects of backward masked stimuli on pointing movements. Perception, 31, S87. (Poster presented at the European Conference on Visual Perception (ECVP), Glasgow, UK)

Stimuli which are completely masked for conscious perception can still show robust effects on motor responses. Schmidt (Psychological Science, 2002) found that masked stimuli affected pointing movements. However, there was a direct compatibility between the position of the imperative stimulus and the goal of the pointing movement. Here, we eliminated this compatibility by using imperative stimuli that only symbolically coded the direction of the pointing movements. Successions of primes and imperative stimuli were shown (duration prime: 31 ms, ISI: 42 ms, target: 83 ms). Primes and imperative stimuli were squares, oriented at 0 or 45 degrees. Twenty-two participants (a) discriminated amongst the primes, (b) responded to the orientation of the imperative stimulus by pointing to a left or right target, and (c) responded by pushing a left or right button. Participants were not able to discriminate amongst the primes (d'=0.07, p=.10). Nevertheless, the reaction times differed between congruent and incongruent conditions (51 ms, p<.001), as did the pointing trajectories. Completely masked stimuli can affect pointing movements even if there is no compatibility between the location of the imperative stimulus and the goal of the pointing movement
: Gegenfurtner, K. R., & Franz, V. H. (2001). Ein Vergleich von Wahrnehmung und Handlung bei der peripheren Lokalisation. In Experimentelle Psychologie / 43. Tagung experimentell arbeitender Psychologen. Lengerich: Pabst Science Publishers. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Regensburg, Germany)

Ziel dieser Experimente war ein Vergleich der Genauigkeit mit der die Position von peripher dargebotenen Objekten eingeschätzt wird mit der Genauigkeit von Zeigebewegungen zu diesen Objekten hin. Die Vpn mussten dabei in jedem Durchgang die Position eines scheibenförmigen Reizes relativ zu einer konstanten Markierungslinie angeben. Gleichzeitig mussten die Vpn mit dem Zeigefinger auf die Scheibe zeigen. Die Zeigebewegungen wurden mit einem Optotrak-System aufgezeichnet. Dabei war die perzeptuelle Genauigkeit deutlich höher als die Genauigkeit der Zeigebewegungen (9 versus 14 Winkelminuten) Für die einzelnen Versuchspersonen (N=11) ergab sich jedoch eine hohe Korrelation (rho=0.72) zwischen den beiden Aufgaben. Des weiteren korrelierten die Lokalisationsfehler bei beiden Aufgaben für alle Vpn über die einzelnen Durchgänge hinweg. Die Ergebnisse unterstützen die Hypothese dass die Signale, die die wahrgenommene Position eines Objekts bestimmen auch benutzt werden um das motorische System bei Zeigebewegungen zu führen.
: Franz, V. H., Bülthoff, H. H., Fahle, M., & Thornton, I. M. (2001). Grasping and representational momentum. Perception, 30, S87. (Poster presented at the European Conference on Visual Perception (ECVP), Kusadasi, Turkey)

If a moving object suddenly disappears its last seen position is usually perceived as being further forward along the path of motion. This representational momentum effect can also be found in objects which change size. We tested whether representational momentum affects the action system. Cubes of different sizes were presented to ten participants on a monitor. In each trial, three cubes were presented for 20 msec with increasing or decreasing sizes (steps of 1 cm width difference). In the perceptual task participants compared the last cube to a comparison cube. In the motor task they grasped the last cube using a virtual haptic setup (two robot arms attached to index finger and thumb). In grasping we found a normal representational momentum effect: Participants opened their fingers wider if a cube was preceded by smaller cubes than if it was preceded by larger cubes. In the perceptual task, however, the effect was reversed. The perceptual effect and the grasping effect correlated between observers. This suggests that the motor effect is related to the perceptual effect.
: Hartung, B., Franz, V. H., Kersten, D., & Bülthoff, H. H. (2001). Is the motor system affected by the hollow face illusion? Journal of Vision, 1(3), 256a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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When viewing the inside of a mask or mold of a human face, the face is frequently perceived as being a normal (convex) face, instead of the veridical, hollow (concave) face (the hollow face illusion). Thus, familiarity with the shape of faces dominates perception, even when in conflict with stereo depth cues. It has been suggested that visuomotor tasks are not affected by illusions that fool perception (Aglioti et al, 1995). In a previous experiment, we showed that reaching is affected by the hollow face illusion. However, the stimuli used lacked haptic feedback, which may produce fundamentally different (pantomimed) reaches which are driven by perception, not the usual visuomotor processes (Goodale et al, 1994). In work described here, we investigated whether the hollow face illusion holds for visuomotor tasks when haptic feedback is present. Computer images of normal and hollow faces were presented in stereo, such that stereo and familiarity depth cues were consistent or in conflict. In the visuomotor task, participants reached to either the nose or cheek. At the end of the reach, subjects received haptic feedback at the tip of the reaching finger. The maximum distance reached was used as an estimate of target position. In the perceptual task, subjects gave a numerical estimate of the distance to either the nose or cheek in arbitrary units, chosen by each subject. The perceptual and visuomotor distance estimates were similar. Both were dominated by object familiarity, shown by the nose estimates being closer to the subject than cheek estimates. However, hollow faces were estimated to be flatter than normal faces. This suggests that the visual system combines stereo and familiarity cues resulting in a 'flattening' of the hollow face. These results are consistent with the previous results from stimuli that did not include haptic feedback. Supported in part by: NIH R01 EY11507, DFG grant Fa119/15-3.
: Berndt, I., Wascher, E., Franz, V. H., Götz, K., & Bülthoff, H. H. (2001). The effect of mirrored visual feedback on the EEG correlates of pointing direction. Journal of Vision, 1(3), 318a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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Purpose: Looking through laterally mirroring prisms produces at least two changes in the phenomenal appearance of the world: When stretching your right arm, for example, visual feedback will indicate that it is your left arm that is moving. But not only will the 'wrong' limb seem to be moving, it will also move in the diametrically opposite direction. Usually output and feedback of an action 'fit' (i.e., go to and come from the same limb). But when looking through mirroring prisms, visual feedback comes from the opposite arm and opposite direction. In order to behave properly under these circumstances, some kind of recalibration has to occur. The contralateral hemisphere is more strongly involved in controlling these arm movements. It is possible that this recalibration alters the lateralization of the neural activity that controls these movements. To test for this, we recorded event-related potentials (ERPs) and event-related lateralizations (ERLs) of the EEG during pointing movements with and without laterally mirrored vision. Targets were presented either centrally or laterally. Results: We found effects of mirrored vision on the lateralization of neural activity. The relative involvement of the hemisphere ipsilateral to the SEEN target position (objective position is reversed with mirrored feedback) increased, especially around 300-400ms after stimulus onset. Additionally, differences in the ERPs around the same time after target onset were evident. Both effects were maximal around the parietal and parieto-occipital sites, suggesting modified stimulus processing.
: Franz, V. H., Thornton, I. M., Fahle, M., & Bülthoff, H. H. (2001). Representational momentum in the motor system? Journal of Vision, 1(3), 253a. (Poster presented at the Vision Sciences Society conference (VSS), Sarasota, Florida)
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PURPOSE: If presented with a moving object which suddenly disappears observers usually misjudge the object's last seen position as being further forward along the path of motion. This effect, called representational momentum, can also be seen in objects that change size or shape. It has been argued that the effect is due to perceptual anticipation. We tested whether a similar effect is present in the motor system. METHODS: Using stereo computer graphics we presented cubes of different sizes on a CRT monitor. In each trial three cubes were successively presented for 200 msec with increasing or decreasing size (steps of 1 cm width difference). Ten participants either compared the last cube to a comparison cube (perceptual task) or grasped the cube using a virtual haptic setup (motor task). The setup consisted of two robot arms (Phantom TM) attached to index finger and thumb. The robot arms were controlled to create forces equivalent to the forces created by real objects. The CRT monitor was viewed via a mirror such that the visual position of the cubes matched the position of the virtual haptic objects. RESULTS: In the motor task participants opened their fingers by 1.1+/-0.4 mm wider if they grasped a cube that was preceded by smaller cubes than if they grasped a cube that was preceded by larger cubes. This is the well-known representational momentum effect. In the perceptual task the effect was reversed (-2.2+/-0.4 mm). The effects correlated between observers (r=.71, p=.02). CONCLUSIONS: It seems that a representational momentum occurs also in grasping tasks. The correlation between observers suggests that the motor effect is related to the perceptual effect. However, our perceptual task showed a reversed effect. Reasons for this discrepancy will be discussed.
: Berndt, I., Wascher, E., Franz, V. H., Götz, K., & Bülthoff, H. H. (2001). Lateralisierung der hirnelektrischen Aktivität während Zeigebewegungen mit gespiegeltem Blickfeld. In H. H. Bülthoff, K. R. Gegenfurtner, H. A. Mallot, & R. Ulrich (Eds.), Beiträge zur 4. Tübinger Wahrnehmungskonferenz (p. 147). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

FRAGESTELLUNG: Schaut man durch eine rechts-links spiegelnde Brille, so beobachtet man zwei Phänomene: Zeigt man z.B. mit dem rechten Arm, dann sieht es so aus, als führe der linke Arm diese Bewegung aus. Zudem scheint die Bewegung in die entgegengesetzte Richtung zu verlaufen. Befehl und Rückmeldung stimmen also nicht mehr überein, sind gegenlaufig. Ein effizientes Verhalten mit gespiegeltem Feedback erfordert eine Umkodierung der visuomotorischen Koordination. Diese sollte sich in einer Veränderung der neuronalen Aktivität im EEG niederschlagen. Wir fanden in einer vorangegangenen Studie, dass sich die verschiedenen Anteile einer Zeigebewegung in Lateralisierungen hirnelektrischer Potentiale im EEG (event-related lateralizations = ERLs) abbilden: Auswahl des Effektors, Lokalisation des Zielreizes, Bewegungsrichtung und Kontrolle der räumlich gerichteten Bewegung. Diese Lateralisierungen des EEG während der Zeigebewegung sollten sich auch durch die Spiegelung der visuellen Rückmeldung spezifisch verändern. METHODE: Um dies zu untersuchen wurden EEG-Messungen während Zeigebewegungen mit und ohne Spiegelung des Gesichtsfeldes durchgeführt. Der Zielreiz wurde dabei entweder zentral oder lateralisiert (+/- 1,7 grad) dargeboten. ERGEBNISSE: Es zeigte sich ein Effekt der Spiegelung auf die Lateralisierung des EEGs. Dieser bestand aus einer höheren Aktivierung der zum gesehenen Zielreiz ipsilateralen Hemisphäre im Vergleich zur ungespiegelten Bedingung. (Zu beachten ist, dass sich die objektive Position bei Spiegelung umkehrt.) Dieser Effekt trat ca. 300-400ms nach Stimulus Onset auf und war maximal in parietalen und parieto-occipitalen Regionen. SCHLUSSFOLGERUNG: Die Spiegelung verursachte eine räumlich und zeitlich eingrenzbare Veränderung der Lateralisierung neuronaler Aktivität. Es liegt nahe, dass dies eine Modifikation der Zielreiz-Verarbeitung darstellt und durch die Umkodierung der visuomotorischen Koordination verursacht wird.
: Berndt, I., Wascher, E., Franz, V. H., & Bülthoff, H. H. (2000, October). A psychophysical and psychophysiological investigation of processing effort in manual pointing movements. (Poster presented at the Symposium on Neural Control of Movement Synergy, Ohlstadt, Germany).
: Franz, V. H., Fahle, M., Gegenfurtner, K. R., & Bülthoff, H. H. (2000). Effects of visual illusions on grasping: The Parallel-Lines illusion. Investigative Ophthalmology and Visual Science, 41(4), S43. (Poster presented at the conference of the Association for Research in Vision and Ophthalmology (ARVO), Fort Lauderdale, Florida)

PURPOSE: Visually guided motor behavior is assumed to be rather unreceptive to size illusions, indicating two different cortical processing streams for the purposes of perception and action (e.g., Aglioti, DeSouza, & Goodale, 1995; Current Biology 5, 679-685). To the contrary, we showed that grasping is influenced by the Ebbinghaus / Titchener illusion (Franz, Gegenfurtner, Bülthoff, & Fahle, 2000; Psychological Science 11, 20-25) and by the Müller-Lyer illusion (ARVO99). In the present study, we investigated the Parallel-Lines illusion. METHODS: Plastic bars (40, 43, 46 and 49 mm long, 5 mm wide) were presented to twenty-six participants. The bars were accompanied by two parallel lines that had a distance of 11 mm to the main axis of the target bar and were either 100 mm long (enlarging version of the illusion) or 22 mm long (shrinking version of the illusion). In the grasping task, participants grasped the bars and the maximal aperture between thumb and index finger was measured using an Optotrak (TM) system. In the visual perception task, participants adjusted the length of a comparison bar on a computer monitor to match the length of the plastic bars. RESULTS: We found clear effects of the Parallel-Lines illusion on grasping as well as on visual perception. The overall effect on grasping (1.2+-0.32 mm) was smaller than on perception (2.3+-0.26 mm). The individual effects were highly correlated across participants (rho = .61, p<.001). That is, a participant showing a large perceptual effect also showed a large motor effect. CONCLUSIONS: The Parallel-Lines illusion is the first illusion in which we consistently found a smaller effect of the illusion on grasping than on perception. However, there is a strong across-participants correlation of the illusion effects, suggesting that the illusion is generated by the same neuronal source. Reasons for the smaller motor illusion are discussed.
: Franz, V. H., Fahle, M., Gegenfurtner, K. R., & Bülthoff, H. H. (2000). Der Einfluß optischer Täuschungen auf das Greifen: Die Parallele-Linien Täuschung. In H. H. Bülthoff, M. Fahle, K. R. Gegenfurtner, & H. A. Mallot (Eds.), Beiträge zur 3. Tübinger Wahrnehmungskonferenz (p. 154). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

FRAGESTELLUNG: Motorische Handlungen sollen von optischen Täuschungen kaum beeinflußt werden. Dies wird als Indiz dafür gewertet, dass visuelle Information über qualitativ unterschiedliche Prozesse für die Zwecke von Wahrnehmung und Handlung verarbeitet wird (z.B. Aglioti, DeSouza, & Goodale, 1995, Current Biology 5, 679-685). Im Gegensatz zu dieser Auffassung konnten wir zeigen, dass Greifen von der Ebbinghaus / Titchener Illusion in ähnlicher Weise wie die Wahrnehmung beeinflußt wird (Franz, Gegenfurtner, Bülthoff, & Fahle, 2000; Psychological Science 11, 20-25). In der vorliegenden Studie sollten diese Ergebnisse auf die Parallele-Linien Illusion verallgemeinert werden. METHODEN: Sechsundzwanzig Versuchspersonen wurden Plastik-Stäbchen (40, 43, 46 und 49 mm lang, 5 mm breit) präsentiert. Parallel zu den Stäbchen wurden in einem Abstand von 11 mm zwei Linien präsentiert, die entweder 100 mm lang waren (illusionär vergrößernde Figur) oder 22 mm lang waren (illusionär verkleinernde Figur). In der Greifaufgabe sollten die Versuchspersonen die Stäbchen greifen und die maximale Handöffnung zwischen Zeigefinger und Daumen wurde mittels eines Optotrak (TM) Systems gemessen. In der Wahrnehmungsaufgabe stellten die Versuchspersonen einen Vergleichsreiz auf einem Computermonitor so ein, dass er ihnen gleich lang erschien wie das Plastik-Stäbchen. ERGEBNISSE: Die Ergebnisse zeigen klare Effekte der Parallelen-Linien Illusion sowohl auf das Greifen wie auch auf die Wahrnehmung. Der Greifeffekt (1,2 +/- 0,32 mm) war jedoch kleiner als der Wahrnehmungseffekt (2,3 +/- 0,26 mm). Die individuelle Größe der Effekte war korreliert zwischen den Versuchspersonen (rho = ,61, p<,001). Das heißt, eine Versuchsperson, die einen starken Wahrnehmungseffekt hatte, zeigte ebenfalls einen großen Effekt auf das Greifen. SCHLUSSFOLGERUNGEN: Die Parallele Linien Illusion ist die erste Illusion, bei der wir einen konsistent kleineren Illusionseffekt für das Greifen als für die Wahrnehmung gefunden haben. Wir fanden jedoch ebenfalls eine starke Korrelation der Effekte zwischen den Versuchspersonen. Dies legt nahe, dass der Greifeffekt und der Wahrnehmungseffekt den gleichen Ursprung haben. Gründe für die absolut kleinere Größe des Greifeffekts werden diskutiert.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (1999). Der Einfluß von Größenillusionen auf das Greifen: Die Müller-Lyer Illusion. In E. Schröger, A. Mecklinger, & A. Widmann (Eds.), Experimentelle Psychologie / 41. Tagung experimentell arbeitender Psychologen (p. 96). Lengerich: Pabst Science Publishers. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Leipzig, Germany)

Auf der TeaP98 hatten wir einen Einfluss der Ebbinghaus / Titchener Illusion sowohl auf die Wahrnehmung als auch auf die Greifmotorik gezeigt. Diese Daten widersprechen der verbreiteten Auffassung, dass das motorische System nur in geringem Masse visuellen Großsenillusionen unterliegt (Aglioti, DeSouza & Goodale, 1995). Damit wird die Theorie in Frage gestellt, dass im Handlungsund im Wahrnehmungssystem visuelle Information qualitativ unterschiedlich verarbeitet wird (Milner & Goodale, 1995). In dem vorliegenden Experiment wurde untersucht, inwieweit sich die Ergebnisse zur Ebbinghaus Illusion auf die Müller-Lyer Taüschung generalisieren lassen. Zwölf Versuchspersonen (VPn) wurden Plastikstäbchen (40, 43, 46 und 49 mm lang, 5 mm breit) auf der Oberfläche eines flach liegenden Bildschirmes dargeboten. Auf dem Bildschirm wurden entweder nach aussen oder nach innen gerichtete Pfeilspitzen gezeigt, so dass sich aus Stäbchen und Pfeilspitzen die Müller-Lyer Taüschung ergab. Die VPn griffen die Stäbchen und die maximale Handöffnung vor Berührung der Stäbchen wurde mittles eines Optotrak (TM) Systems ermittelt. In einer zweiten Aufgabe wurde der Einfluss der Illusion auf die Wahrnehmung mittels einer Einstell-Prozedur ermittelt. Die Ergebnisse zeigen starke Einflußse der Illusion sowohl auf die Wahrnehmung als auch auf das Greifen. Auch diese Ergebnis widerspricht der Auffassung, dass das Greifen nur in geringem Masse visuellen Illusionen unterliege. Interessanterweise ist in unserem Experiment der Einfluss auf das Greifen sogar großser als auf die Wahrnehmung. Gründe hierfür werden diskutiert.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (1999). Optische Täuschungen: Wird die Hand weniger getäuscht als das Auge? In H. H. Bülthoff, M. Fahle, K. R. Gegenfurtner, & H. A. Mallot (Eds.), Beiträge zur 2. Tübinger Wahrnehmungskonferenz (p. 104). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

Die prominente ``Action vs. Perception'' Hypothese von Milner & Goodale (1995) nimmt an, dass visuelle Information für die Zwecke von Wahrnehmung und Handlung unterschiedlich verarbeitet wird. Als starke Evidenz wurden bisher Befunde an optischen Taüschungen gewertet. So berichteten zum Beispiel Aglioti, DeSouza & Goodale (1995), dass Greifen durch optische Taüschungen kaum beeinflusst werde. Im Gegensatz zu diesen Befunden hatten wir einen Einfluss der Ebbinghaus / Titchener Illusion sowohl auf die Wahrnehmung als auch auf die Greifmotorik gezeigt. In dem vorliegenden Experiment wurde untersucht, inwieweit dies ebenfalls auf die Müller-Lyer Taüschung zutrifft. Zwölf Versuchspersonen (VPn) wurden Plastikstäbchen (40, 43, 46 und 49 mm lang, 5 mm breit) auf der Oberfläche eines flach liegenden Bildschirmes dargeboten. Auf dem Bildschirm wurden entweder nach aussen oder nach innen gerichtete Pfeilspitzen gezeigt, so dass sich aus Stäbchen und Pfeilspitzen die Müller-Lyer Taüschung ergab. Die VPn griffen die Stäbchen und die maximale Handöffnung vor Berührung der Stäbchen wurde mittels eines Optotrak (TM) Systems ermittelt. In einer zweiten Aufgabe wurde der Einfluss der Illusion auf die Wahrnehmung mittels eines Herstellungsverfahrens ermittelt. Die Ergebnisse zeigen starke Einflußse der Illusion sowohl auf die Wahrnehmung (2.1+-0.3 mm) als auch auf das Greifen (3.5+-0.5 mm). Dieses Ergebnis widerspricht der Auffassung, dass das Greifen nur in geringem Masse visuellen Illusionen unterliege. Interessanterweise ist in unserem Experiment der Einfluss auf das Greifen sogar großser als auf die Wahrnehmung.
: Franz, V. H., Fahle, M., Gegenfurtner, K. R., & Bülthoff, H. H. (1999). Grasping visual illusions: No difference between perception and action? Investigative Ophthalmology and Visual Science, 40(4), S413. (Poster presented at the conference of the Association for Research in Vision and Ophthalmology (ARVO), Fort Lauderdale, Florida)

PURPOSE: Visually guided motor behavior is assumed to be rather unreceptive to size illusions, indicating two different cortical processing streams for the purposes of perception and action (e.g., Aglioti, DeSouza & Goodale, 1995; Current Biology 5, 679-685). Having shown - in contrary - that grasping and perception are equally influenced by the Ebbinghaus / Titchener Illusion (ECVP 98), we tested whether this is also true for the Mueller-Lyer Illusion. METHODS: Plastic bars (40, 43, 46 and 49 mm long, 5 mm wide) were positioned on top of a horizontally oriented monitor. Fins were presented on the monitor being directed either outwards or inwards, such that the fins and the bars resulted in the Mueller-Lyer Illusion. In the grasping task, twelve subjects grasped the bars and the maximal aperture between thumb and index finger was measured using an Optotrak (TM) system. In the visual perception task, the subjects adjusted the length of a comparison bar on the screen to match the length of the plastic bars. RESULTS: There were strong effects of the Mueller-Lyer Illusion on grasping as well as on visual perception. The effect on grasping (3.5+-0.5 mm) was even larger than on perception (2.1+-0.3 mm). CONCLUSIONS: Our results show that grasping is influenced by visual illusions, indicating that the motor system is receptive to visual illusions.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (1998). Täuschen Größenillusionen sowohl die Hand wie das Auge? In H. Lachnit, A. Jacobs, & F. Rösler (Eds.), Experimentelle Psychologie / 40. Tagung experimentell arbeitender Psychologen (p. 80). Lengerich: Pabst Science Publishers. (Poster presented at the ``Tagung experimentell arbeitender Psychologen'' (TeaP), Marburg, Germany)

Visuelle Größenillusionen sollen auf die Greifmotorik einen deutlich geringeren Einluss ausüben als auf die Wahrnehmung (Aglioti, DeSouza & Goodale, 1995). Dies wird als Indiz dafür gewertet, dass Information über visuelle Großse vom Wahrnehmungs- und vom Handlungssystem unabhängig ausgewertet werden. In dem vorliegenden Experiment sollte diese Hypthese überprüft werden. Insbesondere sollten mögliche Artefakte durch motorisches Lernen vermieden werden. Sechzehn Versuchspersonen (VPn) wurden Scheiben mittels Bildschirm und Stereobrille virtuell dargeboten. Eine zentrale Scheibe war von fünf grossen bzw. kleinen Kontext-Scheiben umgeben, so dass sich die Ebbinghaus Illusion ergab: Bei grossen Kontext-Scheiben wird die zentrale Scheibe kleiner, bei kleinen Kontext-Scheiben größer wahrgenommen. Der Durchmesser der zentrale Scheibe variierte von 27 bis 37 mm, in 2 mm Schritten. Die Wahrnehmung der virtuellen zentrale Scheibe deckte sich in der räumlichen Ausdehnung mit einer realen Scheibe, die jedoch hinter einem Spiegel lag und daher von den VPn nicht gesehen wurde. Die VP führte eine Greifbewegung nach der virtuellen Scheibe durch und erhielt haptisches Feedback durch die reale Scheibe. Mittels eines Optotrak (TM) - Systems wurde die maximale Handöffnung vor Berührung der Scheibe gemessen. Der Einfluß der Illusion auf die Wahrnehmung wurde über eine Einstell-Prozedur ermittelt. Im Widerspruch zu den Ergebnissen von Aglioti et. al. fanden wir sowohl für die wahrgenommene Größe als auch für die maximale Griffgröße einen Einfluß der Illusion. Dies legt nahe, dass die gleichen Signale sowohl für Handlung als auch für die Wahrnehmung benutzt werden. Verschiedene Erklärungen für diesen Widerspruch werden anhand von Kontrollexperimenten diskutiert.
: Franz, V. H., Fahle, M., Gegenfurtner, K. R., & Bülthoff, H. H. (1998). Size-contrast illusions deceive grasping as well as perception. Perception, 27, S140. (Poster presented at the European Conference on Visual Perception (ECVP), Oxford, UK)

Size contrast illusions are assumed to exert a smaller effect on human motor behavior than on perception, indicating different cortical pathways for perception and action (e.g., Aglioti, DeSouza & Goodale, 1995 Current Biology 5 679-685). We tried to replicate these findings for the Ebbinghaus Illusion. Special effort was taken to minimize effects of motor learning and on assessment of the size of the perceptual illusion. An aluminum disc (28, 31, 34 or 37 mm in diameter, 5 mm in height) was positioned as target on a board. Around the target either small or large context discs were drawn (10 or 58 mm in diameter). Close to the board a monitor was mounted on which a comparison disc was displayed. In a visual task twelve subjects adjusted the size of the comparison disc to match the size of the target. In a grasping task subjects grasped the target. Subjects wore shutter glasses and could not see their hand during grasping (open loop condition). The grasp trajectory was recorded and the maximum preshape aperture was calculated. Preshape aperture and adjusted size showed strong and similar linear relationships to the size of the target. The mean perceptual effect of the illusion was 1.4 mm (SE = 0.1 mm) while the effect of the illusion on preshape aperture was 1.5 mm (SE = 0.4 mm). Thus, grasping was just as much influenced by the illusion as perception. Possible reasons for this discrepancy to previous studies are discussed.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (1998). Greifen als Test für die Unterscheidung von Wahrnehmung und Handlung. In H. H. Bülthoff, M. Fahle, K. R. Gegenfurtner, & H. A. Mallot (Eds.), Beiträge zur 1. Tübinger Wahrnehmungskonferenz (p. 162). Kirchentellinsfurt: Knirsch. (Poster presented at the ``Tübinger Wahrnehmungskonferenz'' (TWK), Tübingen, Germany)

In der Literatur wird angenommen, dass die Verarbeitung visueller Information für die Zwecke von Wahrnehmung und Handlung unterschiedlich verlaüft. Diese Unterscheidung soll sich bei gesunden Personen darin zeigen, dass Großsenillusionen auf die Greifmotorik einen deutlich geringeren Einluss ausüben als auf die Wahrnehmung (Aglioti, DeSouza & Goodale, 1995; Brenner & Smeets, 1996). Zur Überprüfung der Hypothese wurde eine virtuelle Untersuchungsapparatur verwendet, die eine weitgehende Manipulation der visuellen Information gestattet. Mit dieser Apparatur liessen sich Störvariablen besser als in vorherigen Studien kontrollieren. Sechzehn Versuchspersonen (VPn) wurden Scheiben mittels Bildschirm und Stereobrille virtuell dargeboten. Eine zentrale Scheibe war von fünf grossen bzw. kleinen Kontext-Scheiben umgeben, so dass sich die Ebbinghaus Illusion ergab: Bei grossen Kontext-Scheiben wird die zentrale Scheibe kleiner, bei kleinen Kontext-Scheiben großser wahrgenommen. Der Durchmesser der zentrale Scheibe variierte von 27 bis 37 mm, in 2 mm Schritten. Die Wahrnehmung der virtuellen zentrale Scheibe deckte sich in der raümlichen Ausdehnung mit einer realen Scheibe, die jedoch hinter einem Spiegel lag und daher von den VPn nicht gesehen wurde. Die VP führte eine Greifbewegung nach der virtuellen Scheibe durch und erhielt haptisches Feedback durch die reale Scheibe. Mittels eines Optotrak - Systems wurde die maximale Handöffnung vor Berührung der Scheibe gemessen. Der Einfluß der Illusion auf die Wahrnehmung wurde über eine Einstell - Prozedur ermittelt. Wir fanden sowohl für die wahrgenommene Großse als auch für die maximale Griffgroßse einen Einfluß der Illusion. Dieses Ergebnis steht im Widerspruch zu den Ergebnissen in der Literatur. Gründe für diese Diskrepanz werden diskutiert.
: Franz, V. H., Gegenfurtner, K. R., Bülthoff, H. H., & Fahle, M. (1998). Grasping isoluminant objects. Investigative Ophthalmology and Visual Science, 39(4), S1095. (Poster presented at the conference of the Association for Research in Vision and Ophthalmology (ARVO), Fort Lauderdale, Florida)

PURPOSE: It is frequently assumed that dorsal and ventral cortical streams process different aspects of the visual world. In particular, the signals used for motor control seem to be generated mainly by the dorsal system (e.g., Milner and Goodale, 1995). However, the sensitivity of the dorsal system to stimuli defined exclusively by color seems to be rather poor. We investigated whether grasping is impaired for such isoluminant objects. METHODS: Virtual discs were displayed on a computer screen. Sixteen subjects (Ss) viewed the screen via a mirror. Real discs (25, 30, 35 and 40 mm in diameter), mounted behind the mirror, matched the size of the virtual discs. Ss had to grasp the virtual discs, receiving haptic feedback from the real discs. The grasp trajectory was recorded using an Optotrak (TM) system and the preshape aperture was calculated. In a second task, Ss had to adjust the displayed size of a comparison disc to match the target disc. Half of the target discs were isoluminant, that is, there was no difference in luminance between object and background. The discs were either red or green, the background was grey. The other half of the target discs were grey and either lighter or darker than the background. The comparison disc in the perceptual task was always lighter than the background. RESULTS: We found similar linear relationships between object size and preshape aperture for both grey and isoluminant stimuli. Grasping was not impaired at isoluminance. In the perceptual task the size of the isoluminant stimuli was slightly overestimated. CONCLUSIONS. These results suggest that the same color information which is used for the perception of size, is used by the motor system to control grip aperture. This information could be provided to the motor system via inputs from the ventral stream, or by a residual sensitivity of the dorsal system to color.

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