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Stephanie McMains, Sabine Kastner; Collinear alignment modulates competitive interactions in human extrastriate cortex. Journal of Vision 2008;8(6):777. doi: 10.1167/8.6.777.
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© ARVO (1962-2015); The Authors (2016-present)
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 mechanisms that subserve the grouping of collinear elements can reduce 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, sixteen oriented colored gabors were displayed within the upper right visual quadrant (entire stimulus 4°×4°). The stimuli were either presented simultaneously (SIM) or sequentially (SEQ: each quarter of the stimulus (4 gabors) presented alone). In addition, stimuli either formed a line segment (4 gabors aligned) or did not. Greater activity was observed in extrastriate cortex for the SEQ condition compared to the SIM condition, regardless of stimulus alignment. 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 a line segment compared to when the stimuli did not form a perceptual group, suggesting that grouping by collinearity overcame competition. These results, combined with previous findings employing illusory contour figures, suggest that perceptual grouping can influence neural competition in a bottom-up fashion outside the focus of attention, likely representing a general principle of scene segmentation whereby processing is biased in favor of potential foreground elements.
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