Qasim Zaidi

Mechanisms of simultaneous color induction

A new method of measuring simultaneous contrast, or chromatic induction, is introduced and used to test the hypotheses that induction results from either multiplicative or subtractive interaction of either like receptors or like second-stage, opponent mechanisms. Predictions derived from these hypotheses do not predict the outcome of the experiments as well as the traditional notion that induced colors are in the direction complementary to the inducing color with respect to the test color. We conclude that simultaneous contrast is a consequence of interaction within higher-level chromatic mechanisms.

Interactions between color and luminance in the perception of orientation

At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.

Lateral interactions within color mechanism in simultaneous induced contrast

The perceived color of a region of visual space is a function not only of the spectral composition of the light incident from it, but also depends on the light incident from surrounding regions. The color contrast induced into a region is a result of lateral interactions between neural mechanisms. These interactions were studied by measuring the induced effect of circularly symmetric spatial sine-waves on a circular central test region. The phase of the surrounding sine-waves was changed uniformly in time, inducing a modulation in the appearance of the test. Observers adjusted the amplitude of real sinusoidal modulation in the test in order to null the induced modulation, and the nulling modulation was used as a measure of the induced effect. Spatial additivity was tested by using pairs of sine-waves of distinct spatial frequencies. The results showed that brightness induction can be characterized as a linear spatial process, i.e. the effects of parts of the surround at different distances from the test are summed, after the effect of each part is weighted by a negative exponential as a function of distance from the test. The magnitude of pure chromatic induction, however, is a result of nonlinear spatial interactions. Thus, these results have implications for the connections between visual mechanisms that process brightness and chromatic contrast.

Visual Mechanisms that Signal the Direction of Color Changes

Evidence is presented for a visual capacity specialized to sense the chromatic direction of change in colors over time. Discrimination thresholds were measured between pairs of suprathreshold color changes presented in consecutive intervals. In one interval, the color of a spatially uniform disk was changed at a constant speed along the circumference of a circle in an equiluminant color plane. In the other, an instantaneous change, which can be described as a vector in the equiluminant plane, was added to the circular color modulation. Averaging across conditions showed that the threshold for discriminating between a pair of purely temporal color changes was approximately proportional to the cosine of the color angle between them. The model that is presented to account for these results is based on parallel directional-color mechanisms that are tuned to different directions in color-space and are responsive to change in one color direction but not its opposite.

Colour constancy in context: roles for local adaptation and levels of reference.

By determining the locations of boundaries between colour categories, we measured changes in the colour appearance of test-reflectances as a function of the simulated illumination. Test-reflectances were displayed against a variegated background of reflectance samples. Under prolonged adaptation to each illuminant, observers demonstrated a high degree of appearance-based colour constancy. By using backgrounds that consisted of chromatically biased sets of reflectances, we tested whether this stability depends on estimates of the illuminants cone-coordinates based on simple scene statistics. The chromatic bias of the background had only a small effect on the classification of test materials. To compare the roles of spatially local and spatially extended estimation processes, we then (unknown to the observer) simulated different illuminants on the test and on the background. Observers continued to demonstrate reasonable colour constancy. To examine the relative roles of automatic adaptation and perceptual strategies, we reduced the duration of exposure to the test compared to exposure to the background (under the conflicting illuminant). The results suggest that mechanisms that preserve information across successive test-presentations (e.g. spatially local adaptation with a time course of a few seconds, and perceptual adjustments to levels of reference) are key determinants of the stability of colour appearance.

Adaptive orthogonalization of opponent-color signals

This paper concerns the processing of the outputs of the two opponent-color mechanisms in the human visual system. We present experimental evidence that opponent-color signals interact after joint modulation even though they are essentially independent under neutral steady adaptation and after exclusive modulation of each mechanism. In addition, prolonged modulation linearizes the response function of each mechanism. The changes in interaction serve to orthogonalize opponent signals with respect to the adapting modulation, and the changes in response functions serve to equalize the relative frequencies of different levels of response to the adapting modulation. Adaptive orthogonalization reduces sensitivity to the adapting color direction, improves sensitivity to the orthogonal direction, and predicts shifts in color appearance. Response equalization enhances effective contrast and explains the difference between the effects of adaptation to uniform versus temporally or spatially modulated stimuli.

Color constancy in variegated scenes: role of low-level mechanisms in discounting illumination changes

For a visual system to possess color constancy across varying illumination, chromatic signals from a scene must remain constant at some neural stage. We found that photoreceptor and opponent-color signals from a large sample of natural and man-made objects under one kind of natural daylight were almost perfectly correlated with the signals from those objects under every other spectrally different phase of daylight. Consequently, in scenes consisting of many objects, the effect of illumination changes on specific color mechanisms can be simulated by shifting all chromaticities by an additive or multiplicative constant along a theoretical axis. When the effect of the illuminant change was restricted to specific color mechanisms, thresholds for detecting a change in the colors in a scene were significantly elevated in the presence of spatial variations along the same chromatic axis as the simulated chromaticity shift. In a variegated scene, correlations between spatially local chromatic signals across illuminants, and the desensitization caused by eye movements across spatial variations, help the visual system to attenuate the perceptual effects that are due to changes in illumination.

The effect of adaptation on the differential sensitivity of the S-cone color system

This paper presents a psychophysical dissection of the S-cone color system. Experiments were guided by a skeletal model that assumed a first stage consisting of S-, M- and L-cones, and a second stage of the opponent combination of the S and L+M signals. The response of the S-cone system was isolated by measuring difference thresholds between lights that were equiluminant tritanopic confusion pairs and thus differed only in S-cone excitation. Two types of mechanisms that control sensitivity in the S-cone system were identified: (i) static mechanisms that have a restricted range and thus limit discrimination to a small range of inputs; and (ii) adaptive mechanisms that change the state of the system in response to changes in steady illumination, so that the system is sensitive to small changes from the adapting light. These mechanisms were localized by lights that stimulated the S-cone system while keeping the signal constant at either the S, the L+M, or the post-opponent stage. The response function of the static mechanism was estimated by measuring difference thresholds at judgment points other than the steady adapting light. This procedure was repeated at a number of adaptation lights to examine the properties of adaptive mechanisms. The data were consistent with an elaborated model that included identical multiplicative gain control mechanisms in the S and L+M pre-opponent branches, and a post-opponent static sigmoidal nonlinearity with different amounts of compression for positive and negative opponent inputs.

Latency characteristics of the short-wavelength-sensitive cones and their associated pathways

There are many distinct types of retinal ganglion and LGN cells that have opponent cone inputs and which may carry chromatic information. Of interest are the asymmetries in those LGN cells that carry S-cone signals: in S-ON cells, S+ signals are opposed by (L + M) whereas, in many S-OFF cells, L+ signals are opposed by (S + M), giving -S + L - M (C. Tailby, S. G. Solomon, & P. Lennie, 2008). However, the S-opponent pathway is traditionally modeled as +/-[S - (L + M)]. A phase lag of the S-cone signal has been inferred from psychophysical thresholds for discriminating combinations of simultaneous sinusoidal modulations along +/-[L - M] and +/-[S - (L + M)] directions (C. F. Stromeyer, R. T. Eskew, R. E. Kronauer, & L. Spillmann, 1991). We extend this experiment, measuring discrimination thresholds as a function of the phase delay between pairs of orthogonal component modulations. When one of the components isolates the tritan axis, there are phase delays at which discrimination is impossible; when neither component is aligned with the tritan axis, discrimination is possible at all delays. The data imply that the S-cone signal is delayed by approximately 12 ms relative to (L - M) responses. Given that post-receptoral mechanisms show diverse tuning around the tritan axis, we suggest that the delay arises before the S-opponent channels are constructed, possibly in the S-cones themselves.

Induced effects of backgrounds and foregrounds in three-dimensional configurations: the role of T-junctions.

In three-dimensional configurations, and two-dimensional pictures of such configurations, simultaneous contrast induction from proximate backgrounds affects perceived brightness, color, and internal contrast to a greater extent than induction from coplanar or occluding surrounds or from more distant backgrounds. In the projected image the presence of occluding flanks or retinally adjacent distant backgrounds is indicated by T-junctions. However, the presence of T-junctions inhibits induced contrast irrespective of the three-dimensional percept. The configurations in this paper refute the notions that perceived coplanarity or perceptual belonging necessarily enhance induced contrast.