Abstract
The McCollough Effect (ME) is a color afterimage produced by exposure to colored, oriented patterns. For example, following viewing of alternating vertical red and horizontal green stripes, vertical black and white patterns appear greenish, while horizontal black and white patterns appear reddish. The physiological locus of the effect has yet to be determined conclusively. We used fMRI and multivariate pattern analysis to identify neural loci producing the ME. We induced the ME with an augmented-reality-based method, wherein participants were exposed to a “McCollough World” of narrowband orientation-filtered video that appeared as black/green horizontal and black/red vertical stripes alternating every 2 sec. This caused a powerful ME amenable to study with neuroimaging. fMRI stimuli consisted of an array of circular square-wave grating patches that were either horizontal/vertical or diagonally rotated (45 deg/135 deg). Separate blocks displayed achromatic or red/green gratings for each orientation and color combination. Participants were scanned twice; once in a pre-adaptation scan, and once following two hours of adaptation in the “McCollough World.” Following adaptation, achromatic horizontal/vertical gratings appeared colorful. Distributed patterns of activation across voxels in early visual cortex were used to train a classifier to distinguish achromatic vs. red/green blocks in the pre-test, and this classifier was then used on the post-adaptation data. Classification results were above chance in the pre-adaptation scans, indicating that patterns of activity differed reliably between red/green and achromatic conditions. Classification of the post-adaptation data differed reliably from the pre-adaptation results, and followed more closely the perceived, not the veridical, color of the gratings. That is, following induction the physically black and white gratings were classified more often as red/green than as achromatic, matching observers’ percepts. This provides evidence that the ME arises from changes in color- and orientation-selective neuronal populations in early visual cortex.