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Dr. Matteo Toscani  University of Gießen 

Otto-Behaghel-Strasse 10F, 35394 Giessen 
Building F1, Room 307
 
Phone: +49(0)641 / 99-26110
 

Toscani@uni-giessen.de 

Highlights


 


 

 

 

 


Journal Articles:

Lightness perception for matte and glossy complex shapes. Toscani M., Valsecchi M., Gegenfurtner K.R. (2017).  Vision Research . Humans are able to estimate the reflective properties of the surface (albedo) of an object despite the large variability in the reflected light due to shading, illumination and specular reflection. Here we first used a physically based rendering simulation to study how different statistics (i.e, percentiles) based on the luminance distributions of matte and glossy objects predict the overall surface albedo. We found that the brightest parts of matte surfaces are good predictors of the surface albedo. As expected, the brightest parts led to poor performance in glossy surfaces. We then asked human observers to sort four (2 matte and 2 glossy) objects in a virtual scene in terms of their albedo. The brightest parts of matte surfaces highly correlated with human judgments, whereas in glossy surfaces, the highest correlation was achieved by percentiles within the darker half of the objects' luminance distributions. Furthermore, glossy surfaces tend to appear darker than matte ones, and observers are less precise in judging their lightness. We then manipulated different bands of the virtual objects' luminance distributions separately for glossy and matte surfaces. Modulating the brightest parts of the luminance distributions of the glossy surfaces had a limited impact on lightness perception, whereas it clearly influenced the perceived lightness of the matte objects. Our results demonstrate that human observers effectively ignore specular reflections while evaluating the lightness of glossy objects, which results in a bias to perceive glossy objects as darker.

Lightness perception for surfaces moving through different illumination levels. Toscani M.,  Zdravković, S., Gegenfurtner, K.R (2016) Journal of Vision. Lightness perception has mainly been studied with static scenes so far. This study presents four experiments investigating lightness perception under dynamic illumination conditions. We asked participants for lightness matches of a virtual three-dimensional target moving through a light field while their eye movements were recorded. We found that the target appeared differently, depending on the direction of motion in the light field and its precise position in the light field. Lightness was also strongly affected by the choice of fixation positions with the spatiotemporal image sequence. Overall, lightness constancy was improved when observers could freely view the object, over when they were forced to fixate certain regions. Our results show that dynamic scenes and nonuniform light fields are particularly challenging for our visual system. Eye movements in such scenarios are chosen to improve lightness constancy.

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.


Conference Articles:

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.


Contributions in conferences:

 


 

 

 

 

 

 

Research experience & Education

2013-now. Postdoctoral Researcher. University of Giessen.

2013 -Phd (Doctor Rerum Naturalium). University of Giessen. Thesis: “the effect of fixation positions on lightness perception”(supervision of professor Karl Gegenfurtner, Ph.D).

2010 - European Visual Neuroscience Summer School  "From Spikes to Awareness".

2009- M.Sc Experimental Psychology. University of Florence, Italy. Master thesis on “the functional role of alpha waves in visual suppression” (supervision of Stefano Baldassi, Ph.D; PisaVisionLab). 

2007 - B.Sc General and Experimental Psychology. University of Florence, Italy. 

     

 

 

 

 

 

 

 

 

 

Research interests:

My research has mostly concentrated on the perception of surfaces lightness and the role that eye movements play for it. I showed that a single lightness impression of a surface uniform in albedo depends on the fixated luminances (Toscani et al, 2013a; Toscani et al, 2013b). The local properties sampled at high resolution by foveation affect the global percept of the surface properties: also when whole luminance distribution is sampled through peripheral vision, the information seen in central view has a higher weight in perceiving lightness.

I'm generally intetested on how central and peripheral view interact in constructing the appeareance of the world, and on how we achieve a stable percept despite eye movements.

    I am also interested in how geometry and light interact in determining a global impression of the properties of surfaces. For instance, despite shading, human observers are capable to form a single impression on the lightness of a surface uniform in albedo, and I am interested in the underlying mechanisms.

    Since many tridimensional models of objects and surfaces are made available by the computer graphics community, it is possible to combine them with physically based rendering systems in order to achieve physically plausible simulations of reflection from different geometries and illumination conditions (Toscani et al, 2013a). Psychophysical results could be compared with the simulation results in order to establish how optimally people exploit the information about the surface properties. For instance the brightest parts of a matte surface uniform in albedo are particularly informative about its diffuse reflectance whereas in a glossy surface those parts include a large specular component which mostly depends on the intensity of the illumination (Toscani et al, 2013a, Toscani et al, 2013c, Toscani et al, 2013d). I therefore find interesting to determine whether people judge glossy and matte surfaces using different strategies.
    Given a certain interaction between the shape of a surface and its reflective properties I also find interesting to study where in the brain this interaction occurs and what kind of neural mechanism are responsible for that. For instance we know that the color perception of a shape usually associated with a certain color, like a banana, is biased towards that color (Hansen, T., Olkkonen, M., Walter, S., & Gegenfurtner, K. R. (2006). Memory modulates color appearance. Nature neuroscience, 9(11), 1367-1368.).
 

Visual Neuroscience Summer School

Karl Gegenfurtner

Stefano Baldassi

 

University of Florence