Dr. Matteo Toscani University of Giessen
Otto-Behaghel-Strasse 10F, 35394 Giessen
Building F1, Room 307
Phone: +49(0)641 / 99-26110
Lightness perception for surfaces moving through different illumination levels. Toscani M., Zdravković, S., Gegenfurtner, K.R (2016) Journal of Vision.
The many colours of "the dress". Gegenfurtner, K. R., Bloj, M., & Toscani, M. (2015). Current Biology. There has been an intense discussion among the public about the colour of a dress, shown in a picture posted originally on Tumblr (http://swiked.tumblr.com/post/112073818575/guys-please-help-me-is-this-dress-white-and; accessed on 10:56 am GMT on Tue 24 Mar 2015). Some people argue that they see a white dress with golden lace, while others describe the dress as blue with black lace. Here we show that the question "what colour is the dress?" has more than two answers. In fact, there is a continuum of colour percepts across different observers. We measured colour matches on a calibrated screen for two groups of observers who had reported different percepts of the dress. Surprisingly, differences between the two groups arose mainly from differences in lightness, rather than chromaticity of the colours they adjusted to match the dress. We speculate that the ambiguity arises in the case of this particular image because the distribution of colours within the dress closely matches the distribution of natural daylights. This makes it more difficult to disambiguate illumination changes from those in reflectance.
Statistical correlates of perceived gloss in natural images. Wiebel, C. B., Toscani, M., & Gegenfurtner, K. R. (2015). Vision research. It is currently debated whether the perception of gloss is linked to the statistical parameters of the retinal image. In particular, it has been suggested that gloss is highly correlated with the skewness of the luminance histogram. However, other psychophysical work with artificial stimuli has shown that skewness alone is not enough to induce the perception of gloss. Here, we analyzed many images of natural surfaces to search for potential statistical correlates of perceived gloss. We found that skewness indeed correlates with gloss when using rendered stimuli, but that the standard deviation, a measure of contrast, correlates better with perceived gloss when using photographs of natural surfaces. We verified the important role of contrast by manipulating skewness and contrast within images. Changing the contrast in images significantly modulates perceived gloss, but manipulating the skewness of the luminance histogram had only a small effect.
Perceived numerosity is reduced in peripheral vision. Valsecchi M, Toscani M, Gegenfurtner KR. 2013. Journal of vision. In four experiments we investigated the perception of numerosity in the peripheral visual field. We found that the perceived numerosity of a peripheral cloud of dots was judged to be inferior to the one of a central cloud of dots, particularly when the dots were highly clustered. Blurring the stimuli accordingly to peripheral spatial frequency sensitivity did not abolish the effect and had little impact on numerosity judgments. In a dedicated control experiment we ruled out that the reduction in peripheral perceived numerosity is secondary to a reduction of perceived stimulus size. We suggest that visual crowding might be at the origin of the observed reduction in p.eripheral perceived numerosity, implying that numerosity could be partly estimated through the individuation of the elements populating the array.
Selection of visual information for lightness judgments by eye movements. Toscani M, Valsecchi M, Gegenfurtner KR. 2013b. Phil Trans R Soc B. When judging the lightness of objects, the visual system has to take into account many factors such as shading, scene geometry, occlusions or transparency. The problem then is to estimate global lightness based on a number of local samples that differ in luminance. Here we show that eye fixations play a prominent role in this selection process. We explored a special case of transparency for which the visual system separates surface reflectance from interfering conditions to generate a layered image representation. Eye movements were recorded while the observers matched the lightness of the layered stimulus. We found that observers did focus their fixations on the target layer, and this sampling strategy affected their lightness perception. The effect of image segmentation on perceived lightness was highly correlated with the fixation strategy and was strongly affected when we manipulated it using a gaze contingent display. Finally, we disrupted the segmentation process showing that it causally drives the selection strategy. Selection through eye fixations can such serve as a simple heuristic to estimate the target reflectance.
Optimal sampling of visual information for lightness judgments. Valsecchi M, Toscani M, Gegenfurtner KR. 2013a PNAS 2013. The variable resolution and limited processing capacity of the human visual system requires us to sample the world with eye movements and attentive processes. Here we show that where observers look can strongly modulate their reports of simple surface attributes, such as lightness. When observers matched the color of natural objects they based their judgments on the brightest parts of the objects; at the same time, they tended to fixate points with above-average luminance. When we forced participants to fixate a specific point on the object using a gaze-contingent display setup, the matched lightness was higher when observers fixated bright regions. This finding indicates a causal link between the luminance of the fixated region and the lightness match for the whole object. Simulations with rendered physical lighting show that higher values in an object's luminance distribution are particularly informative about reflectance. This sampling strategy is an efficient and simple heuristic for the visual system to achieve accurate and invariant judgments of lightness.
Role of eye movements in chromatic induction. Granzier JJG, Toscani M, and Gegenfurtner KR. 2012. JOSA 2012. There exist large interindividual differences in the amount of chromatic induction [Vis. Res. 49, 2261 (2009)]. One possible reason for these differences between subjects could be differences in subjects' eye movements. In experiment 1, subjects either had to look exclusively at the background or at the adjustable disk while they set the disk to a neutral gray as their eye position was being recorded. We found a significant difference in the amount of induction between the two viewing conditions. In a second experiment, subjects were freely looking at the display. We found no correlation between subjects' eye movements and the amount of induction. We conclude that eye movements only play a role under artificial (forced looking) viewing conditions and that eye movements do not seem to play a large role for chromatic induction under natural viewing conditions.
Fearful expressions enhance recognition memory: Electrophysiological evidence. Righi S, Marzi T, Toscani M, Baldassi S, Ottonello S, Viggiano MP. 2012. Acta Psychologica. Facial expressions play a key role in affective and social behavior. However, the temporal dynamics of the brain responses to emotional faces remain still unclear, in particular an open question is at what stage of face processing expressions might influence encoding and recognition memory. To try and answer this question we recorded the event-related potentials (ERPs) elicited in an old/new recognition task. A novel aspect of the present design was that whereas faces were presented during the study phase with either a happy, fearful or neutral expression, they were always neutral during the memory retrieval task. The ERP results showed three main findings: An enhanced early fronto-central positivity for faces encoded as fearful, both during the study and the retrieval phase. During encoding subsequent memory (Dm effect) was influenced by emotion. At retrieval the early components P100 and N170 were modulated by the emotional expression of the face at the encoding phase. Finally, the later ERP components related to recognition memory were modulated by the previously encoded facial expressions. Overall, these results suggest that face recognition is modulated by top-down influences from brain areas associated with emotional memory, enhancing encoding and retrieval in particular for fearful emotional expressions.
Alpha waves: a neural signature of visual suppression. Toscani M, Marzi T, Righi S, Viggiano MP, Baldassi S. 2010. Exp Brain Research. Alpha waves are traditionally considered a passive consequence of the lack of stimulation of sensory areas. However, recent results have challenged this view by showing a modulation of alpha activity in cortical areas representing unattended information during active tasks. These data have led us to think that alpha waves would support a 'gating function' on sensorial stimulation that actively inhibits unattended information in attentional tasks. Visual suppression occurring during a saccade and blink entails an inhibition of incoming visual information, and it seems to occur at an early processing stage. In this study, we hypothesized that the neural mechanism through which the visual system exerts this inhibition is the active imposition of alpha oscillations in the occipital cortex, which in turn predicts an increment of alpha amplitude during a visual suppression phenomena. We measured visual suppression occurring during short closures of the eyelids, a situation well suited for EEG recordings and stimulated the retinae with an intra-oral light administered through the palate. In the behavioral experiment, detection thresholds were measured with eyes steady open and steady closed, showing a reduction of sensitivity in the latter case. In the EEG recordings performed under identical conditions we found stronger alpha activity with closed eyes. Since the stimulation does not depend on whether the eyes were open or closed, we reasoned that this should be a central effect, probably due to a functional role of alpha oscillation in agreement with the 'gating function' theory.
Effect of fixation positions on perception of lightness. Toscani M, Valsecchi M, Gegenfurtner KR. 2015. In IS&T/SPIE Electronic Imaging (pp. 93940R-93940R). International Society for Optics and Photonics. Visual acuity, luminance sensitivity, contrast sensitivity, and color sensitivity are maximal in the fovea and decrease with retinal eccentricity. Therefore every scene is perceived by integrating the small, high resolution samples collected by moving the eyes around. Moreover, when viewing ambiguous figures the fixated position influences the dominance of the possible percepts. Therefore fixations could serve as a selection mechanism whose function is not confined to finely resolve the selected detail of the scene. Here this hypothesis is tested in the lightness perception domain. In a first series of experiments we demonstrated that when observers matched the color of natural objects they based their lightness judgments on objects' brightest parts. During this task the observers tended to fixate points with above average luminance, suggesting a relationship between perception and fixations that we causally proved using a gaze contingent display in a subsequent experiment. Simulations with rendered physical lighting show that higher values in an object's luminance distribution are particularly informative about reflectance. In a second series of experiments we considered a high level strategy that the visual system uses to segment the visual scene in a layered representation. We demonstrated that eye movement sampling mediates between the layer segregation and its effects on lightness perception. Together these studies show that eye fixations are partially responsible for the selection of information from a scene that allows the visual system to estimate the reflectance of a surface.
The Optimal Estimator of Objects' Lightness Predicting Perceptions. Toscani M, Valsecchi M, Gegenfurtner KR. 2012. Proceedings of the 3rd International Conference on Appearance. We have recently shown that eye movements have an effect on lightness estimation of real objects. Observers tended to fixate points with above-average luminance and they overestimated the objects' lightness. The matched lightness was higher when observers were forced to fixate a bright region of the object than when they fixated a darker region. In the present work we performed a simulation with a physically based rendering system, showing that this is an efficient and simple heuristic for the visual system to arrive at accurate and invariant judgments of lightness.
ECVP August 2016. Barcelona (Spain): Oral presentation. "Giessen's hyperspectral images of fruits and vegetables database (GHIFVD)". (Ennis R, Toscani M, Schiller F, Hansen T, Gegenfurtner KR).
ECVP August 2016. Barcelona (Spain): Oral presentation. "Object selective influence of fixated luminance on brightness perception in the periphery of the visual field". (Toscani M, Valsecchi M, Gegenfurtner KR).
VSS May 2016. St. Petersburg (Florida). Poster presentation."Probing the illumination on #The Dress". (Toscani M, Doerschner K, Gegenfurtner KR).
ECVP August 2015. Liverpool (UK): Oral presentation. "Scotopic lightness perception" (Ennis RJ, Toscani M, Gegenfurtner KR).
VSS May 2015. St. Petersburg (Florida). Poster presentation."At night even white cats are gray: scotopic lightness perception". (Ennis RJ, Toscani M, Gegenfurtner KR).
VSS May 2015. St. Petersburg (Florida). Poster presentation."Viewing strategies that aid lightness constancy in dynamic scenes". (Toscani M, Valsecchi M, Gegenfurtner KR).
SPIE (IS&T/SPIE Electronic Imaging)
February 2015. San Francisco (California). Oral presentation:
"Effect of fixation positions on perception of lightness".
(Toscani M, Valsecchi M, Gegenfurtner KR)
(Toscani M, Valsecchi M, Gegenfurtner KR)
ECVP August 2014. Belgrade (Serbia): Oral presentation: "Perceived lightness over space and time" (Toscani M, Zdravković S, Gegenfurtner KR)
TeaP March 2014. Giessen (Germany). "Predicting lightness judgments from image statistics of matte and glossy virtual objects" (Toscani M, Valsecchi M, Gegenfurtner KR)
VSS May 2014. Naples (Florida): Poster presentation: "Predicting lightness rankings from image statistics of matte and glossy surfaces". (Toscani M, Valsecchi M, Gegenfurtner KR) at the "Vision Science Society Annual Meeting".
ECVP August 2013. Bremen (Germany): Oral presentation: "Perceived numerosity in the peripheral visual field" (Valsecchi M, Toscani M, Gegenfurtner KR)
ECVP August 2013. Bremen (Germany): Oral presentation: " Predicting lightness judgments from luminance distributions of matte and glossy virtual objects " (Toscani M, Valsecchi M, Dilger MD, Zirbes A, Gegenfurtner KR)
VSS May 2013. Naples (Florida): Poster presentation: " Lightness perception for glossy objects". (Toscani M, Valsecchi M, Gegenfurtner KR) at the "Vision Science Society Annual Meeting".
ECVP September 2012. Alghero (Italy): Poster presentation: " Spatial frequency overestimation with retinal eccentricity for isoluminant chromatic gratings " (Toscani M, Valsecchi M, Gegenfurtner KR)
ECVP September 2012. Alghero (Italy): Poster presentation: " Perceived numerosity in the peripheral visual ﬁeld" (Valsecchi M, Toscani M, Gegenfurtner KR)
VSS May 2012. Naples (Florida): Poster presentation: "The optimal estimator of lightness".(Toscani M, Valsecchi M, Gegenfurtner KR) at the "Vision Science Society Annual Meeting".
Predicting Perception. April 2012. Edinburgh (UK): Oral presentation: " The Optimal Estimator of Objects' Lightness" (Toscani M, Valsecchi M, Gegenfurtner KR) at the "Predicting Perception" 3rd Conference on Appearance.
ECVP September 2011. Toulouse (France): Oral presentation: " A functional link between image segmentation, lightness perception and eye movements? " (Toscani M, Gegenfurtner KR).
ECVP September 2011. Toulouse (France): Poster presentation: " Do individual differences in eye movements explain differences in chromatic induction between subjects? " (Granzier JM, Toscani M, Gegenfurtner KR).
IBRO. July 2011. Florence (Italy): Poster presentation: "The role of eye movements in lightness perception" (Toscani M, Valsecchi M, Gegenfurtner KR) at the 8th IBRO World Congress of Neuroscience.
VSS May 2011. Naples
(Florida): Poster presentation: "Where we look determines what we see"
(Toscani M, Valsecchi M, Gegenfurtner KR) at the "Vision Science Society