Visuospatial contextual processing in the parietal cortex: an fMRI investigation of the induced Roelofs effect.

Neighboring contextual elements can dramatically affect the manner in which the brain processes the perceptual characteristics of an object. Indeed, many well-known visual illusions rely on misleading contextual cues to create misperceptions of size, length or orientation (e.g., in the Ebbinghaus, Muller-Lyer or rod-and-frame illusions, respectively). However, little is known about the brain regions underlying these integrative computations. The current study used fMRI to delineate the brain areas responsible for processing visuospatial contextual information. Participants were asked to determine whether a small target was positioned left or right of midline in the presence of an offset rectangle designed to induce a shift in the participant's perception of straight-ahead (the induced Roelofs effect). We found localized, bilateral regions in superior parietal cortex and precuneus that were specifically active when participants judged the target location in the presence of this shifted context; significantly less activation was present when a color judgment was made with identical stimuli, or when the location judgment was made without a Roelofs-inducing frame. We propose that this portion of parietal cortex is selectively involved in processing visuospatial contextual information. Additional findings support the notion that perceptual judgments of target location based on an egocentric frame of reference fall within the purview of the dorsal stream of visual processing, rather than the ventral stream.