The appearance of a patch of color or its contrast depends not only on the stimulus itself but also on the surrounding stimuli (induction effects-simultaneous contrast). A comprehensive computational physiological model is presented to describe chromatic adaptation of the first (retinal) and second (cortical) orders, and to predict the different chromatic induction effects. We propose that the chromatic induction of the first order that yields perceived complementary colors can be predicted by retinal adaptation mechanisms, contrary to previous suggestions. The second order of the proposed adaptation mechanism succeeds to predict the automatic perceived inhibition or facilitation of the central contrast of a texture stimulus, depending on the surrounding contrast. Furthermore, contrary to other models, this model is able to also predict the effect of variegated surrounding on the central perceived color.
In classical assimilation effects, intermediate luminance patches appear lighter when their immediate surround is comprised of white patches and appear darker when their immediate surround is comprised of dark patches. With patches either darker or lighter than both inducing patches, the direction of the brightness effect is reversed and termed as "inverted assimilation effect." Several explanations and models have been suggested, some are relevant to specific stimulus geometry, anchoring theory, and models that involve high level cortical processing (such as scission, etc.). None of these studies predicted the various types of assimilation effects and their inverted effects. We suggest here a compound brightness model, which is based on contrast-contrast induction (second-order adaptation mechanism). The suggested model predicts the various types of brightness assimilation effects and their inverted effects. The model is composed of three main stages: (1) composing post-retinal second-order opponent receptive fields, (2) calculations of local and remote contrast, and (3) adaptation of the second-order (contrast-contrast induction). We also utilize a variation of the Jacobi iteration process to enable elegant edge integration in order to evaluate the model is performance.
The human visual system faces many challenges, among them the need to overcome the imperfections of its optics, which degrade the retinal image. One of the most dominant limitations is longitudinal chromatic aberration (LCA), which causes short wavelengths (blue light) to be focused in front of the retina with consequent blurring of the retinal chromatic image. The perceived visual appearance, however, does not display such chromatic distortions. The intriguing question, therefore, is how the perceived visual appearance of a sharp and clear chromatic image is achieved despite the imperfections of the ocular optics. To address this issue, we propose a neural mechanism and computational model, based on the unique properties of the S-cone pathway. The model suggests that the visual system overcomes LCA through two known properties of the S channel: (1) omitting the contribution of the S channel from the high-spatial resolution pathway (utilizing only the L and M channels). (b) Having large and coextensive receptive fields that correspond to the small bistratified cells. Here, we use computational simulations of our model on real images to show how integrating these two basic principles can provide a significant compensation for LCA. Further support for the proposed neuronal mechanism is given by the ability of the model to predict an enigmatic visual phenomenon of large color shifts as part of the assimilation effect.
The color dove illusion is a type of a positive afterimage. The color dove illusion can be obtained with different colors and spatial surrounding regions. In this illusion the disappearance of a colored background causes an empty shape to obtain the hue of the disappeared background. The effect can be obtained even when the surrounding background area is relatively thin and when it is presented for a very short duration. This illusion occurs only when the contour of the empty shape remains after the background disappearance. Thus the color dove illusion is a positive aftereffect, which is different from classical positive aftereffects. The parameters of the color dove illusion as well as variants, such as the flickering variation, are discussed. Related concepts such as simultaneous contrast and spatiotemporal edge are discussed
The appearance of a visual stimulus depends not only on the stimulus itself but also on other surrounding and remote stimuli. Induction (simultaneous contrast) and contrast induction are among the important appearance phenomena related to the spatial surrounding effects. Induction is the psychophysical phenomenon of the change in the appearance of a color (or an achromatic stimulus) caused by the presence of a surrounding stimulus. Color contrast induction is the modulating effect of the surrounding contrast on the perceived contrast of the central area (1). Color contrast is the distance between colors on a perceptual uniform color space, such as that of Demngton et al 1984, (2). The "intermixed effect, i.e. the smaller perceived effect of a variegated surround vs. a homogeneous surround while the surround is composed of the same average chromaticity and intensity at both surround areas has also been studied (3). These color and color contrast effects have been tested by many groups. However, their physiological mechanism and computational aspects are in dispute or not known. Color induction (first order of color adaptation) is widely accepted to be associated with the very well known phenomenon of color constancy (a psychophysical phenomenon in which the system can partly discount illumination chromaticity). In this study we attempt to claim and demonstrate that color induction is an epi-phenomenon of color constancy, since the same mechanism cause both effects. Contrast induction (adaptation of the second order) is suggested as a mechanism for enhancing the differences in contrast surfaces or objects. Thus, both mechanisms serve for enhancing the differences between the stimulus and its surrounding area.We'suggest a comprehensive computational model based on the retinal (opponent) and cortical (double-opponent) color-coded receptive fields (RF) and on first order (retinal) (43) and second order (cortical) adaptation mechanisms (6). As far as we know, there is no previous suggested comprehensive model in the literature which describes all these induction effects and no previous biological model has succeeded in predicting color constancy and color enhancement or the color contrast of real images (4,6). 2.The Model 2.1. Responses of three types of On-center color-coded cells, the first order from the cones through several processing layers. These cells have a color-opponent receptive field with a centersurround spatial structure. (A receptive field (RF) is that region in the visual field from which a visual stimulus will elicit a response.) The cells considered here belong to the three most common color-coded types in the retina, they are labeled L+M', M'L-and S+(L+M)-, with L, M and S standing for long, medium and short wavelength sensitivity.The spectral composition of the light reaching the retina when an illumination falls on the surfaces of objects and is reflected from them is the input to the cones level. The quantum catch of each of the three cone types, L-$, &and S , , . , is expressed by an inner produ...
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