Sunčica Zdravković

Anchoring versus spatial filtering accounts of simultaneous lightness contrast.

The oldest lightness illusion is called simultaneous contrast. A gray square placed on a black background appears lighter than an identical gray square placed on a white background. For over a hundred years, this illusion has been generally attributed to lateral inhibition or spatial filtering. Receptor cells stimulated by the gray square on the white background are strongly inhibited by nearby cells stimulated by the bright white background. Recently, a new explanation for this illusion was proposed as part of a larger theory of lightness called anchoring theory. The lightness of each target square is computed relative to the highest luminance in its local framework (consisting of only the target and its surrounding background) and relative to the highest luminance in the entire display. For each target, perceived lightness is held to depend on a weighted average of these two computations. According to this story, the contrast illusion stems mostly from the tendency of the gray square on the black background to rise toward white, its computed value in its local framework. We report six experiments in which these two theories of simultaneous contrast are pitted against each other. In each case, the results favor the anchoring model. The difficulty of deriving predictions from the spatial filtering models is discussed, along with the ease of deriving highly specific predictions from the anchoring model.

Lightness of an Object under Two Illumination Levels

Anchoring theory (Gilchrist et al, 1999 Psychological Review 106 795 – 834) predicts a wide range of lightness errors, including failures of constancy in multi-illumination scenes and a long list of well-known lightness illusions seen under homogeneous illumination. Lightness values are computed both locally and globally and then averaged together. Local values are computed within a given region of homogeneous illumination. Thus, for an object that extends through two different illumination levels, anchoring theory produces two values, one for the patch in brighter illumination and one for the patch in dimmer illumination. Observers can give matches for these patches separately, but they can also give a single match for the whole object. Anchoring theory in its current form is unable to predict these object matches. We report eight experiments in which we studied the relationship between patch matches and object matches. The results show that the object match represents a compromise between the match for the patch in the field of highest illumination and the patch in the largest field of illumination. These two principles are parallel to the rules found for anchoring lightness: highest luminance rule and area rule.

Effects of proficiency and age of language acquisition on working memory performance in bilinguals

This study examined language proficiency and age of language acquisition influences on working memory performance in bilinguals. Bilingual subjects were administered reading span task in parallel versions for their first and second language. In Experiment 1, language proficiency effect was tested by examination of low and highly proficient second language speakers. In Experiment 2, age of language acquisition was examined by comparing the performance of proficient second language speakers who acquired second language either early or later in their lives. Both proficiency and age of language acquisition were found to affect bilingual working memory performance, and the proficiency effect was observed even at very high levels of language competence. The results support the notion of working memory as a domain that is influenced both by a general pool of resources and certain domain specific factors.

Grouping Illumination Frameworks

According to Koffka (1935), the lightness of a target surface is determined by the relationship between the target and the illumination frame of reference to which it belongs. However, each scene contains numerous illumination frames, and judging each one separately would lead to an enormous amount of computing. Grouping those frames that are in the same illumination would simplify the computation. We report a series of experiments demonstrating that nonadjacent regions of the visual field under the same illumination level are perceptually grouped together and function, to some extent, as a single framework. A small coplanar group of patches under its own illumination exhibits compression of perceived range of gray shades. We obtained the reduction in compression in the presence of an identically illuminated 25-patch Mondrian tableau mounted nearby the coplanar group. The influence of the Mondrian display was reduced when it was (a) moved laterally away from the test display, (b) moved farther back in depth from the test display, or (c) rotated to a different orientation.

Side flankers produce less crowding, but only for letters

Identification of isolated and crowded letter (B, D, F, G, K, N, L, S, T) and symbol stimuli (%, /, ?, @, }, <, £, §, μ) was examined across the visual field in a two-alternative forced-choice match-to-sample task (2AFC-MTS). During isolated presentation, identification accuracy did not differ between the two stimulus types. Identification rates for the central characters within the three-character strings were higher for letters than for symbols at the horizontal and vertical meridian (Experiment 1), and at diagonal locations (Experiment 2). However, this reduction of parafoveal letter crowding was present in horizontally but not in vertically oriented strings of stimuli. The same pattern of results was replicated in the periphery of the visual field (Experiment 3). The obtained results are in agreement with the proposition that the receptive fields of letter detectors are modified during reading acquisition, in order to support efficient letter identification (Tydgat & Grainger, 2009). However, the pervasive presence of the effect across the visual field suggests that it could originate from a non-retinotopic stage of visual processing.

Lightness perception for surfaces moving through different illumination levels

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.

Information extraction from shadowed regions in images: an eye movement study.

Natural scenes often contain variations in local luminance as a result of cast shadows and illumination from different directions. When making judgments about such scenes, it may be hypothesized that darker regions (with lower relative contrast due to a lack of illumination) are avoided as they may provide less detailed information than well-illuminated areas. We here test this hypothesis, first by presenting participants images of faces that were digitally modified to simulate the effect of a shadow over half of the image, and second by presenting photographs of faces taken with side illumination, also resulting in the appearance of a shadow across half of the face. While participants viewed these images, they were asked to perform different tasks on the images, to allow for the presentation of the different versions of each image (left shadow, right shadow, no shadow), and to distract the observers from the contrast and illumination manipulations. The results confirm our hypothesis and demonstrate that observers fixate the better illuminated regions of the images.

Grouping Factors and the Reverse Contrast Illusion

In simultaneous lightness contrast, two identical gray target squares lying on backgrounds of different intensities appear different in lightness. Traditionally, this illusion was explained by lateral inhibitory mechanisms operating retinotopically. More recently, spatial filtering models have been preferred. We report tests of an anchoring theory account in which the illusion is attributed to grouping rules used by the visual system to compute lightness. We parametrically varied the belongingness of two gray target bars to their respective backgrounds so that they either appeared to group with a set of bars flanking them, or they appeared to group with their respective backgrounds. In all variations, the retinal adjacency of the gray squares and their backgrounds was essentially unchanged. We report data from seven experiments showing that manipulation of the grouping rules governs the size and direction of the simultaneous lightness contrast illusion. These results support the idea that simultaneous lightness contrast is the product of anchoring within perceptual groups.

Color influences identification of the moving objects more than shape

When people track moving objects, they concentrate on different characteristics. Recent results show that people more often concentrate on spatiotemporal than featural properties of the objects. In other words, location and direction of motion seem to be more informative properties than the stable featural characteristics. This finding contradicts some of our knowledge about cognitive system. Current research was done in attempt to specify the effect of featural characteristics, especially color and shape. In Experiment 1, subjects were asked to track four mobile targets presented with another four moving objects. After the motion has stopped, they had to mark the initial four targets. Our results have shown that participants pay more attention to the featural properties than to spatiotemporal characteristics. Since our task was more difficult than the tasks typically reported in the literature, the results might be interpreted as if the subjects relied mostly on attentional processes. The task in Experiment 2 was made even more difficult: the subjects were asked to direct attention on identity of every target. Consequently, the task demanded more complex cognitive processes and emphasizing effects of featural properties. Results suggest that color and shape does not have the same influences on multiple object tracking, but that color has more significant effect.

The roles of image decomposition and edge curvature in the 'snake' lightness illusion.

The snake illusion is an effect in which the lightness of target patches is strongly affected by the luminance of remote patches. One explanation is that such images are decomposed into a pattern of illumination and a pattern of reflectance, involving a classification of luminance edges into illumination and reflectance edges. Based on this decomposition, perceived reflectance is determined by discounting the illumination. A problem for this account is that image decomposition is not unique, and that different decompositions may lead to different lightness predictions. One way to rule out alternative decompositions and ensure correct predictions is to postulate that the visual system tends to classify curved luminance edges as reflectance edges rather than illumination edges. We have constructed several variations of the basic snake display in order to test the proposed curvature constraint and the more general image decomposition hypothesis. Although the results from some displays have confirmed previous findings of the effect of curvature, the general pattern of data questions the relevance of the shape of luminance edges for the determination of lightness in this class of displays. The data also argue against an image decomposition mechanism as an explanation of this effect. As an alternative, a tentative neurally based account is sketched.