Matteo Toscani | Justus-Liebig-University of Giessen (original) (raw)

Papers by Matteo Toscani

Journal of vision, 2015

The anchoring theory of lightness perception proposes that the perception of white is linked to t... more The anchoring theory of lightness perception proposes that the perception of white is linked to the brightest surface in a scene. A lot of evidence for some form of anchoring has been amassed, but mainly under photopic illumination conditions. There is hardly any research on lightness perception under scotopic conditions, when only rod photoreceptors are active. We tested whether anchoring also works under these conditions. In other words: are all cats really gray at night, even white ones? We printed 10 chips, such that they were equally spaced on the L* scale of the CIELUV color-space (under a D65 illuminant). 6 naive observers first viewed the maximally and minimally reflecting chips at 277 cd/m^2. They were instructed to memorize their percepts as 100% white and 0% white. Observers adapted to three light levels (1.2x10^(-4) (scotopic), 28, and 277 cd/m^2; counter-balanced) and viewed our chips, one at a time, in two, randomly-ordered blocks. For scotopic adaptation, observers wo...

Journal of Vision, 2015

Lightness of a surface depends on luminance sampled trough fixations (Toscani et al.,2013). This ... more Lightness of a surface depends on luminance sampled trough fixations (Toscani et al.,2013). This information changes when the surface moves across a light field and so do lightness matches (Zdravkovic, 2008). We studied luminance integration over time and space in a lightness matching task. A pendulum was presented in a virtual scene with clearly visible decreasing illumination from left to right. Observers either freely looked at the scene or had to fixate a spot on a dark or light region on the pendulum for the whole duration of each trial. The pendulum either moved across the whole scene starting from the light or the dark side or was kept motionless in these two regions of interest. The pendulum was matched lighter when observers were forced to fixate a light region than a dark one, showing that fixation positions do influence perceived lightness. Observers produced lighter matches when the pendulum was on the lighter side of the scene than when it was on the darker side. Lighter matches were also obtained when the pendulum moved from dark to light, as compared to the other way around. When eye movements were not constrained, this difference was less pronounced. In free viewing, observers tended to adjust their fixation positions as to counteract the illumination difference, preferring lighter regions on the pendulum when it was on the darker side and vice versa.Eye movements seem to play a compensatory role for illumination changes due to object motion. Our results emphasize the importance of fixation positions for achieving perceptual stability. Meeting abstract presented at VSS 2015.

It is currently debated whether the perception of gloss is linked to the statistical parameters o... more 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 lumi-nance 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.

In four experiments we investigated the perception of numerosity in the peripheral visual field. ... more 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 peripheral perceived numerosity, implying that numerosity could be partly estimated through the individuation of the elements populating the array.

The variable resolution and limited processing capacity of the human visual system requires us to... more 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. lightness constancy | lightness perception | visual perception | attention J udging the lightness of visual stimuli has been studied for centuries , since the original investigations by Weber (1) and Fechner (2). The light reaching the eye is the product of the illumination and the reflectance of the object, and also depends on the scene geometry (3). However, only the proportion of reflected light is an invariant property of the object and thus of great importance for vision. There are several well-established factors that support lightness constancy in the face of these challenges. On the one hand, lateral inhibition between retinal neurons filters out shallow intensity gradients, which are mostly caused by illumination effects (4, 5). On the other hand, more complex factors also have an effect on lightness perception, such as object shape (6–9) or the interpretation of transparent surfaces (10, 11). However, eye movements have been almost completely neglected so far, even though a general influence of viewing behavior has been shown for some color constancy tasks (12–15). This finding is surprising because the visual system needs to sample the local properties of objects and this is accomplished by moving the eyes and the focus of spatial attention around. Because visual acuity, luminance sensitivity, contrast sensitivity, and color sensitivity change with retinal eccentricity (16–18), our visual system has to stitch together its representation of the world from many small samples to analyze the visual scene in detail. Peripheral vision is not only characterized by poor resolution, but also the appearance of basic visual features—like spatial frequency, luminance, or chromatic saturation—is distorted in the periphery of the visual field (19–22). Eye movements may then be used to select relevant information, even for stimuli that are above threshold in peripheral vision. We investigated whether the distribution of fixations on an object has an effect on its apparent lightness. For surfaces made of a single material, the reflected light varies with the illu-minant and its interactions with the surface's geometry, whereas the reflectance is a property of the material. To judge the lightness of an object, defined as its apparent reflectance (23), the visual system has to select a single value from a whole distribution of local luminance values across the whole object. We therefore tested the hypothesis of a link between the local information sampled from individual fixations and the apparent lightness of an object. First, we show that observers tend to take heavily into account the brighter parts of objects when they are asked to match the color or lightness of these objects. Second, we show that observers tend to fixate on the brighter parts of the objects as they make their match. Third, we show that this link between fixations and lightness perception is causal. When we forced the observers to look at particular points on the objects, their lightness impressions changed according to the luminance of the fixated regions. Fourth, we show that eye fixations and attention both contribute to this effect. Fifth, we show that the brighter parts of objects are particularly diagnostic of the object's reflectance. Results Observers had to adjust the color of a small patch of light to match the color of one of several real objects presented to them, as illustrated in Fig. 1 A and B. The luminance adjusted by the observers was significantly higher than the mean luminance of the light being reflected from the object into the eye (t 5 = 11.6084, P < 0.001). In fact, the matches closely correspond to the brightest parts of the objects. This finding indicates that the brighter regions of the objects are weighted more heavily. Fig. 2 shows the ob-servers' average lightness matches together with samples of the same object (a paper cone) under different illumination conditions. The match is quite similar to the brightest parts of the cone and a piece of paper cut out from the cone, such that it is oriented perpendicular to the light source, maximizing its luminance. The piece of paper cut out from the cone, mounted at the location of the matching box, appears much darker than the cone, because the surface of the computer monitor was oriented nearly parallel to the light source. Observers are known to be far from perfect in taking such geometrical aspects into account (24). The match is also much brighter than the mean luminance across the whole object, which raises the question as to why observers match the brightest parts of the objects, and whether this strategy is of advantage. Role of Eye Fixations. We explored a possible role of eye movements by measuring the fixation locations on the objects (Fig. 3A). Fig. 3B shows histograms of the luminance distributions across the objects together with a luminance histogram of the fixated regions. Even though the object luminance histograms are typically skewed toward darker values, the luminance distributions associated with fixations are centered on values higher than the median. Binomial tests performed for each object revealed that the proportions of fixations on brighter points than the median were significantly higher than chance (all Ps < 0.001). To investigate the relationship

There exist large interindividual differences in the amount of chromatic induction [Vis. Res. 49,... more 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.

Facial expressions play a key role in affective and social behavior. However, the temporal dynami... more 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 are traditionally considered a passive consequence of the lack of stimulation of sens... more 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 eVect, probably due to a functional role of alpha oscillation in agreement with the 'gating function' theory.

Visual acuity, luminance sensitivity, contrast sensitivity, and color sensitivity are maximal in ... more 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.

We have recently shown that eye movements have an effect on lightness estimation of real objects.... more 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.

Journal of vision, 2015

Journal of Vision, 2015