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Sang Wook Hong, Frank Tong; Neural Representation of Form-Based Color Filling-In in Early Visual Cortex. Journal of Vision 2011;11(11):379. doi: 10.1167/11.11.379.
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Perceptual filling-in exemplifies the constructive nature of visual processing. Under special viewing conditions, surface properties such as color can readily spread to neighboring retinal locations in the absence of direct input. Recent psychophysical studies have shown that interactions between negative color images and achromatic contours can lead to multiple possible percepts of filled-in color (van Lier et al., 2009). If observers adapt to a bi-colored starburst pattern, with each spoke of the star alternately colored in red or green, then the achromatic central portion of the star can appear reddish or greenish depending on whether achromatic contours are subsequently presented to reinforce the red or green regions of the negative color afterimage. We investigated the neural bases of this form-based color filling-in by using functional magnetic resonanance imaging (fMRI) in conjunction with multivariate pattern classification. In experimental runs, observers viewed filling-in displays that induced the perception of red or green in the central portion of the starburst pattern. In control runs, observers viewed weakly saturated physical colors in the central region, matching in hue to the reported filled-in percepts of red or green. We found that activity patterns throughout early visual areas V1 to V4 could reliably discriminate between the filling-in conditions that elicited an impression of red or green, even though no reliable color difference was present in the stimulus. More important, a linear classifier trained on real colors could reliably classify the filled-in color, but only in higher visual areas (areas V3 and V4). These results suggest that cortical filling-in of surface color may be accomplished at relatively later stages of visual processing, and that subjective color experiences may more closely reflect the activity found in higher extrastriate visual areas.
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