"Brain-reading" of perceived colors reveals a feature mixing mechanism underlying perceptual filling-in in cortical area V1.

Visual filling-in occurs when a retinally stabilized object undergoes perceptual fading. As the term "filling-in" implies, it is commonly believed that information about the apparently vanished object is lost and replaced solely by information arising from the surrounding background. Here we report multivoxel pattern analysis fMRI data that challenge this long-held belief. When subjects view blue disks on a red background while fixating, the stimulus and background appear to turn a uniform purple upon perceptual fading, suggesting that a feature mixing mechanism may underlie color filling-in. We find that ensemble fMRI signals in retinotopic visual areas reliably predict (i) which of three colors a subject reports seeing; (ii) whether a subject is in a perceptually filled-in state or not; and (iii) furthermore, while subjects are in the perceptual state of filling-in, the BOLD signal activation pattern in the sub-areas of V1 corresponding to the location of the blue disks behaves as if subjects are in fact viewing a perceptually mixed color (purple), rather than the color of the disks (blue) or the color of the background (red). These results imply that the mechanism of filling-in in stimuli in which figure and background surfaces are equated is a process of "feature mixing", not "feature replacement". These data indicate that feature mixing may involve cortical areas as early as V1.