Illusory contour formation modulates competitive interactions in human extrastriate cortex

When multiple stimuli appear simultaneously in the visual field, they are not processed independently, but rather interact in a mutually suppressive way suggesting that they compete for neural representation. The biased competition model of selective attention suggests that both top-down and bottom-up processes can help resolve this competition by reducing suppressive interactions between competing stimuli. Both top-down attention and bottom-up visual salience (via pop-out visual stimuli) have been found to reduce competition in extrastriate cortex. Here we investigated whether the low- to intermediate-level visual cortical mechanisms that subserve illusory contour formation can reduce the competitive interactions between competing stimuli in a bottom-up fashion. FMRI activity in visual cortex was investigated while subjects performed a target detection task at fixation. While subjects performed this central task, four illusory contour inducers were displayed at four nearby locations within the upper right visual quadrant. The four stimuli were either presented simultaneously (SIM) or sequentially (SEQ: each stimulus presented alone in one of the four locations). In addition, stimuli either formed an illusory contour (inducers rotated inward) or did not (inducers rotated outward). Greater activity was observed in extrastriate cortex for the SEQ condition compared to the SIM condition, regardless of if the stimuli formed an illusory contour. This supports the biased competition model that predicts greater competition, and thus reduced activation, when the stimuli are presented simultaneously. Greater activity was also observed during the SIM condition in V3/V4 when the stimuli could be grouped together to form an illusory contour compared to when the stimuli did not form a perceptual group. This suggests that in the illusory contour condition the four inducers were grouped together into a single perceptual group, reducing competition among the simultaneously presented inducers. These results suggest that early contour formation can influence neural competition in a bottom-up fashion.