Neural representation of form-contingent color filling-in in the early visual cortex

Abstract

Perceptual filling-in exemplifies the constructive nature of visual processing. Color, a prominent surface property of visual objects, can appear to spread to neighboring areas that lack any color. We investigated cortical responses to a color filling-in illusion that effectively dissociates perceived color from the retinal input (van Lier, Vergeer, & Anstis, 2009). Observers adapted to a star-shaped stimulus with alternating red- and cyan-colored points to elicit a complementary afterimage. By presenting an achromatic outline that enclosed one of the two afterimage colors, perceptual filling-in of that color was induced in the unadapted central region. Visual cortical activity was monitored with fMRI, and analyzed using multivariate pattern analysis. Activity patterns in early visual areas (V1-V4) reliably distinguished between the two color-induced filled-in conditions, but only higher extrastriate visual areas showed the predicted correspondence with color perception. Activity patterns allowed for reliable generalization between filled-in colors and physical presentations of perceptually matched colors in areas V3 and V4, but not in earlier visual areas. These findings suggest that the perception of filled-in surface color likely requires more extensive processing by extrastriate visual areas, in order for the neural representation of surface color to become aligned with perceptually matched real colors.

Figure 1

Experimental design, stimuli, and reported color values during perceptual filling-in. (A) The color filling-in experiment consisted of two possible configurations for the adapting stimulus, which led to a negative color afterimage in local regions receiving direct color stimulation. After adaptation, either a reddish or cyanish filled-in percept could be induced in the central achromatic region by presenting the appropriate test stimulus. The dashed circle region depicts the visual-field location of interest, which was functionally localized in separate fMRI runs. (B) The control experiment measured cortical responses in the central region to physically presented colors. Test stimuli were uniformly colored with matched chromaticity values obtained for the two possible filling-in percepts. (C) Group-averaged values of perceived chromaticity during filling-in, shown in cone-contrast color space (error bars were smaller than the symbols). The two outer points indicate the colors in the adapting stimulus; the arrows indicate the direction of color shift in the central region induced by color filling-in.

Figure 2

Classification accuracy with multivoxel pattern analysis (gray bars) and with decoding of univariate mean BOLD amplitudes (white bars) for filling-in condition (A), control condition (B), and generalized classification performance between the two conditions (C). Error bars represent ±1 SEM with six observers. Dashed line represents chance-level performance in classification. (*p < 0.05, **p < 0.01.)