Separating color from color contrast.

Visual objects can be described by their color and by their color contrast. For example, a red disk in front of a white background appears "red with high color contrast," whereas a red disk in front of a slightly less-saturated red background will appear "red with low color contrast." This paper examines the visual response to color contrast in a cone-based color space. The stimulus consists of two disks whose chromaticity and/or luminance modulate in time along a line in a DKL color space; the chromaticity and luminance levels of the two disks are always identical. One disk is surrounded by a static ring whose color is at one end of the color line, and the other disk is surrounded by a static ring whose color is at the opposite end of the color line. The disks appear to modulate in antiphase (following the contrast information), yet they can also appear to be approximately the same color (following the chromatic/luminance information). The observers' task was to adjust the color angle of modulating disks until the antiphase appearance was eliminated-creating a contrast null. Observers set contrast nulls at a color angle approximately 90 deg away from the line connecting the colors of the surround rings; this result occurred in both chromoluminant and equiluminant color planes, although two observers showed a flattening near equiluminance in the chromoluminance planes. To account for the data, I present a model that contains one pathway for color and another pathway for color contrast. I show that (1) the model correctly predicts orthogonal directions in color space for the contrast nulling task; (2) the response of the contrast pathway appears to be faster than the response of the color pathway; (3) the response of the contrast pathway may mediate detection thresholds under some conditions (a finding that can account for some of the effects of surround luminance on temporal sensitivity); (4) the asynchronous modulation can be seen even when the stimulus is blurred; and (5) the asynchrony does not require a disk-ring configuration.