Configural and perceptual factors influencing the perception of color transparency

The mechanisms by which the brain represents colors are largely unknown. In addition, the large number of color phenomena in the natural world has made understanding color rather difficult. Color transparency perception, which is studied in this thesis, is precisely one of these interesting phenomena: when a surface is seen both in plain view and through a transparent overlay, the visual system still identifies it as a single surface. Processes of the visual system have widely inspired researchers in many domains such as neurosciences, psychology, as well as computer vision. The progress of digital imaging technologies requires research engineers to deal with issues that demand knowledge of human visual processing. To humans, an image is not a random collection of pixels, but a meaningful arrangement of regions and objects. One thus can be inspired by the human visual system to investigate color representation and its applicability to digital image processing. Finding a model of perception is still a challenging matter for researchers among multidisciplinary fields. This thesis discusses the problem of defining an accurate model of transparency perception. Despite the large number of studies on this topic, the underlying mechanisms are still not well understood. Investigating perceptual transparency is challenging due to its interactions with different visual phenomena, but the most intensively studied conditions for perceptual transparency are those involving achromatic luminance and chromatic constraints. Although these models differ in many aspects, a broad distinction can be drawn between models of additive and subtractive transparency. The General Convergence Model (GCM) combines both additive and subtractive color mixtures in showing that systematic chromatic changes in a linear color space, such as translation and convergence (or a combination of both), lead to perceptual transparency. However, while this model seems to be a necessary condition, it is not a sufficient one for transparency perception. A first motivation of this thesis was to evaluate and define situations more general than the GCM. Several chromatic changes consistent or not with the GCM were generated. Additional parameters, such as configural complexity, luminance level, magnitude of the chromatic change and shift direction were tested. The main results showed that observers' responses are influenced by each of the above cited parameters. Convergences appear significantly more transparent when motion is added for bipartite configurations, or when they are generated in a checkerboard configuration. Translations are influenced by both configuration and motion. Shears are described as opaque, except when short vector lengths are combined with motion: the overlay tends to be transparent. Divergences are strongly affected by motion and vector lengths, and rotations by a combination of checkerboard configuration with luminance level and vector length. These results question the generality of the GCM. We also investigated the effects of shadows on the perception of a transparent filter. An attempt to extend these models to handle transparency perception in complex scenes involving surfaces varying in shape and depth, change in conditions of illumination and shadow, is described. A lightness-matching task was performed to evaluate how much constancy is shown by the subject among six experimental conditions, in which shadow position, shadow blur, shadow and filter blending values were varied. The results showed that lightness constancy is very high even if surfaces were seen under both filter and shadow. A systematic deviation from perfect constancy in a manner consistent with a perceived additive shift was also observed. Because the GCM includes additive mixture and is related to color and lightness constancy, these results are promising and may be explained ultimately by this model

Study of Systematic Chromatic Changes in Color Space to Model Color Transparency

Several studies have suggested that translation and convergence in a linear trichromatic color space lead to the perception of transparency, but other transformations, such as shear and rotation,do not. We have designed a psychophysical experiment to study thelimits of such systemic chromatic changes, adding categories such as different luminance levels and vector lengths. The number of our stimuli and the number of observations provide strong statistical support for D'Zmura's model. Our main results show that for vectors exceeding a minimal length, convergence and translation (except in the equiluminant plane) lead to the perception of transparency, while shear and divergence do not. However, our results reveal that small shears and divergences also appear transparent. We also found that large translations in the equiluminant plane tend to be less often judged as transparent

Spatial selectivity of the watercolor effect

International audienceThe spatial selectivity of the watercolor effect (WCE) was assessed by measuring its strength as a function of the luminance contrast of its inducing contours for different spatial configurations, using a maximum likelihood scaling procedure. The approach has previously been demonstrated to provide an efficient method for investigating the WCE as well as other perceptual dimensions. We show that the strength is narrowly tuned to the width of the contour, that it is optimal when its pair of inducing contours are of equal width, and that the strength can be increased by varying the overall size of the stimulus when the width of the inducing contour is not optimal. The results support a neural substrate that has characteristics not unlike double-opponent, color-luminance cells observed in cortical area V1

Increasing Attentional Load Boosts Saccadic adaptation

International audiencePurpose: Visual exploration relies on saccadic eye movements and attention processes. Saccadic adaptation mechanisms, which calibrate the oculomotor commands to continuously maintain the accuracy of saccades, have been suggested to act at downstream (motor) and upstream (visuoattentional) levels of visuomotor transformation. Conversely, whether attention can directly affect saccadic adaptation remains unknown. To answer this question, we manipulated the level of attention engaged in a visual discrimination task performed during saccadic adaptation.Methods: Participants performed low or high attention demanding orientation discrimination tasks on largely or faintly oriented Gabor patches, respectively, which served as targets for reactive saccades. Gabor patches systematically jumped backward during eye motion to elicit an adaptive shortening of saccades, and replaced 50 msec later (100 msec in two subjects) by a mask. Subjects judged whether Gabors’ orientation was “nearly horizontal” versus “nearly vertical” (low attention demanding) or “slightly left” versus “slightly right” (high attention demanding), or made no discrimination (control task).Results: We found that the build-up and the retention of adaptation of reactive saccades were larger in the “high attention demanding” condition than in the “low attention demanding” and the no-discrimination control conditions.Conclusions: These results indicate that increasing the level of attention to the perceptual processing of otherwise identical targets boosts saccadic adaptation, and suggest that saccadic adaptation mechanisms and attentional load effects may functionally share common neural substrates

Functional activation of the cerebral cortex related to sensorimotor adaptation of reactive and voluntary saccades

International audiencePotentially dangerous events in the environment evoke automatic ocular responses, called reactive saccades. Adaptation processes, which maintain saccade accuracy against various events (e.g. growth, aging, neuro-muscular lesions), are to date mostly relayed to cerebellar activity. Here we demonstrate that adaptation of reactive saccades also involves cerebral cortical areas. Moreover, we provide the first identification of the neural substrates of adaptation of voluntary saccades, representing the complement to reactive saccades for the active exploration of our environment. An fMRI approach was designed to isolate adaptation from sac-cade production: an adaptation condition in which the visual target stepped backward 50 ms after saccade termination was compared to a control condition where the same target backstep occurred 500 ms after sac-cade termination. Subjects were tested for reactive and voluntary saccades in separate sessions. Multi-voxel pattern analyses of fMRI data from previously-defined regions of interests (ROIs) significantly discriminated between adaptation and control conditions for several ROIs. Some of these areas were revealed for adaptation of both saccade categories (cerebellum, frontal cortex), whereas others were specifically related to reactive saccades (temporo-parietal junction, hMT +/V5) or to voluntary saccades (medial and posterior areas of intra-parietal sulcus). These findings critically extend our knowledge on brain motor plasticity by showing that saccadic adaptation relies on a hitherto unknown contribution of the cerebral cortex

Quantifying the watercolor effect: from stimulus properties to neural models

International audienceVisual illusions are perceptions that violate our expectations with respect to what weunderstand about the physical stimulus, for example, surfaces of identical spectralcomposition that appear to be of different colors. Such phenomena are thought to revealmechanisms, biases, priors or strategies that the brain uses to interpret the visual environment.In natural viewing, we perceive all surfaces within a context of surrounding light and nearbyobjects, and this context affects their appearance. Change in color appearance due to thesurrounding light is called color induction and can be of type contrast, when appearance of atest region shifts in chromaticity away from that of the surround and assimilation when theshift is toward that of the surround.Pinna et al. (Pinna, 1987; Pinna et al., 2001) demonstrated a long-range, colorassimilation phenomenon called the Watercolor Effect (WCE) that provides an interestingexample for studying such processes. The WCE occurs when a wavy, dark, chromaticcontour delineating a figure is flanked on the inside by a lighter chromatic contour on a brightbackground. The lighter color spreads into the entire enclosed area so that the interior surfaceis perceived as filled in with a uniform color. Previous studies reported also a weakcoloration effect exterior to the contour (Cao et al., 2011; Devinck et al., 2005). The WCE isdistinguished from other assimilation illusions due by to its spatial extent; the phenomenonhas been reported to be observed over distances of up to 45 deg (Pinna et al., 2001). Here, wereview and discuss stimulus configurations, based on different procedures, that induce theWCE and their implications for a neural model of this phenomenon