Abstract
Filling-in phenomena have been assumed to occur retinotopically based on luminance edge signals (e.g. Cornsweet Illusion, Troxler Fading, etc). In contrast to this view, we have previously reported that a prolonged fixation at a color gradient interrupted by sharp luminance edges, results in a color-spreading beyond the luminance edges (Shimojo, Wu, & Kanai, 2002, Perception 31, suppl.).
One way to account for this illusion is that color-spreading occurs along perceptual surfaces, rather than retinotopically. Here, we tested this idea using transparent motion in which two surfaces are moving in opposite directions, each having a different color gradient from center to periphery. In a typical stimulus, the color of dots gradually changed from red to green as the eccentricity increased on one surface, and on the other surface, the color of dots gradually shifted from green to red. Thus, at an intermediate eccentricity, there arises a yellow area where the colors of two surfaces are close to each other. The width of this area was varied by changing the steepness of the gradient and the effect of occlusion on this area was tested. Our findings include, 1) The color spreading occurred selectively for each surface, supporting the surface-based account, 2) A shallow gradient did not necessarily result in an immediate color-spreading, as opposed to the classical edge-based filling-in phenomena.
Although the final percept of the color for each surface was perceptually similar between the shallow and the steep gradients, two separate effects seem to be involved. First, the color information from the fovea dominates over that of the periphery and spreads in this direction, as in our previous report. Second, when the peripheral colors consist of the colors same as in the fovea, but combined with the opposite motion direction, these colors are actively used to facilitate a percept of homogeneously colored surface via a motion-color misbinding process.