Karen K. Valois

A multi-stage color model.

The first stage of our model has three cone types, with L:M:S cones in ratios of 10:5:1. In the second stage, retinal connectivity leads to three pairs of cone-opponent, and one pair of cone-nonopponent systems. At a third (cortical) stage of color processing, the S-opponent cells are added to or subtracted from the L- and M-opponent units to split and rotate the one effective parvo geniculate response axis into separate RG and YB color axes, and separate luminance from color. We also discuss changes with eccentricity, and connectivity based on correlated neural activity.

Motion-reversal reveals two motion mechanisms functioning in scotopic vision.

We studied scotopic motion mechanisms, using a two-frame sinusoidal grating separated by various ISIs equated for mean luminance level. Perceived direction of displacement varied with both ISI and luminance. As luminance decreased, apparent motion reversal disappeared. This is predicted by a first-order motion model if the underlying temporal impulse response function varies from biphasic under photopic conditions to monophasic under scotopic conditions. Performance at long (but not short) ISIs depends upon stimulus contrast, suggesting there is also a scotopic feature-tracking mechanism. With isoluminant and high spatial frequency gratings, where the temporal impulse response function is monophasic, no motion reversal was observed.

Motion contrast and motion integration

When a moving aperture contains a drifting grating, the perception of aperture movement is strongly affected by the grating movement. We have studied this interaction, using a moving circular patch of sinusoidal grating matched to the background in mean luminance. The circular window, or aperture, could be defined either by an abrupt transition from a full-contrast grating to the background (hard aperture) or by a two-dimensional Gaussian fall-off in contrast (soft aperture). The grating movement could be controlled independently of the aperture motion. Subjects judged the direction of the aperture movement (i.e. the movement of the patch as a whole). We find that an illusory motion of a stationary aperture can be induced depending on the direction of the grating drift. A hard aperture presented in the fovea appears to move in the direction opposite the grating movement, demonstrating simultaneous motion contrast. However, a soft aperture presented in the periphery appears to move in the same direction as the drifting grating, demonstrating motion integration (assimilation). These results are discussed in the context of interactions between short-range and long-range motion mechanisms and with respect to the significance of boundaries in determining the figure-ground relationship of motion signals.

The role of color in the motion system

We have examined the ability of observers to determine the direction of movement of a variety of colored plaid patterns. When the two plaid components are of unequal spatial frequency or of unequal luminance or chromatic contrast, observers judge the direction of movement incorrectly. These errors are correlated with a misjudgement of the speeds of the two components. Our results provide support for an initial decomposition into oriented components followed by a subsequent component-to-pattern recombination of moving equiluminant and colored plaids. At equal multiples of threshold contrast a moving luminance grating is about 8 times more powerful than a moving equiluminant grating in determining the apparent direction of motion of a plaid. When both are present, luminance and color do not interact linearly. Color and motion must be processed in parallel in at least partially separate pathways.

Modulation of perceived contrast by a moving surround

The apparent contrast of a center pattern depends on the contrast of its surround. To examine the suprathreshold perception of moving patterns, we measured the perceived contrast of a moving grating while the direction and speed of the surround patterns varied. Subjects matched the apparent contrast of a center patch embedded in surround patches to that of a patch with no surround pattern. Temporal frequency, Michelson contrast and movement direction of both center and surround patterns varied systematically. We found that: (1) contrast reduction is most prominent when the center and surround have the same velocity (velocity selectivity); (2) contrast enhancement occurs when the surround moves at a higher speed than the center, if the difference in temporal frequencies of center and surround exceeds 10-20, independent of the directional relationship between center and surround; (3) contrast reduction is stronger for higher surround contrasts with lower center contrasts; and (4) contrast enhancement is relatively unaffected by center and surround contrasts. We conclude that the contrast perception of moving patterns is influenced by directionally-selective mechanisms except at high temporal frequencies. Our results further suggest that there is not only the lateral inhibition often assumed to influence contrast gain control, but also an excitatory connection between motion encoding units.

Light adaptation in motion direction judgments

We examined the time course of light adaptation in the visual motion system. Subjects judged the direction of a two-frame apparent-motion display, with the two frames separated by a 50-ms interstimulus interval of the same mean luminance. The phase of the first frame was randomly determined on each trial. The grating presented in the second frame was phase shifted either leftward or rightward by pi/2 with respect to the grating in the first frame. At some variable point during the first frame, the mean luminance of the pattern increased or decreased by 1-3 log units. Mean luminance levels varied from scotopic or low mesopic to photopic levels. We found that the perceived direction of motion depended jointly on the luminance level of the first frame grating and the time at which the shift in average luminance occurs. When the average luminance increases from scotopic or mesopic to photopic levels at least 0.5 s before the offset of the first frame, motion in the 3pi/2 direction is perceived. When average luminance decreases to low mesopic or scotopic levels, motion in the pi/2 direction is perceived if the change occurs 1.0 s or more before first frame offset, depending on the size of the luminance step. Thus light adaptation in the visual motion system is essentially complete within 1 s. This suggests a rapid change in the shape (biphasic or monophasic) of the temporal impulse response functions that feed into a first-order motion mechanism.

Color-selective analysis of luminance-varying stimuli

Alternating adaptation to red and green luminance-varying gratings of different spatial frequencies simultaneously induces opposing color-selective size aftereffects (Blakemore & Sutton, 1969) in the same retinal locus. With single-color adaptation, the aftereffect is larger and affects test patterns of both colors, though not equally. The color-insensitive portion of the effect shows very substantial interocular and cross-orientation transfer. The color-selective aftereffect, which accounts for about 1/3 of the total effect, is highly selective for both orientation and eye of origin. Thus, both color-selective and color-insensitive mechanisms participate in determining the perceptual characteristics of luminance-varying patterns.

Spatial vision based upon color differences

The role of color vision is not limited to the acquisition and appreciation of information about the spectral composition of stimulus patches. its historical realm. Rather, color vision allows one to use information about stimulus spectral parameters to determine other interesting and relevant object characteristics. To understand the role of color in spatial vision, it is necessary to examine both the extent to which spatial discriminations can be based solely upon color differences and the interaction between color and luminance variations when they are simultaneously present. The well-known differences in the spatial and temporal contrast sensitivity functions for color and luminance and the apparently impoverished input from the color mechanisms to certain higher functions obscure the fact that spatial discriminations based solely upon color differences are quite good. For example, spatial frequency discriminations between high-contrast patterns at isoluminance are only slightly poorer than for comparable luminance patterns. averaging about 5-6% of the base frequency. Similarly, orientation differences of about 1 deg between isoluminant patterns can be reliably discriminated at high contrasts, even for stimuli that lie along a tritanopic confusion axis5. Similar comparisons from several tasks will be reviewed, as will tasks (e.g., masking and adaptation) involving color-luminance interactions. These provide information about the target behavior that must ultimately be explained if the physiological basis of color vision is to be understood.To understand the role of color in spatial vision, it is necessary to examine both the extent to which spatial discriminations can be based solely upon color differences and the interaction between color and luminance variations when they are simultaneously present. The well- known differences in the spatial and temporal contrast sensitivity functions for color and luminance and the apparently impoverished input from the color mechanisms to certain higher functions obscure the fact that spatial discriminations based solely upon color differences are quite good. For example, spatial frequency discriminations between high-contrast patterns at isoluminance are only slightly poorer than for comparable luminance patterns, averaging about 5% to 6% of the base frequency. Similarly, orientation differences of about 1 deg between isoluminant patterns can be reliably discriminated at high contrasts, even for stimuli that lie along a tritanopic confusion axis. Similar comparisons from several tasks are reviewed, as are tasks involving color-luminance interactions. These provide information about the target behavior that must ultimately be explained if the physiological basis of color vision is to be understood.