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Michael Pratte, Frank Tong; Assessing Tilt Illusions in Human Visual Cortex Using fMRI and Multivariate Pattern Analysis. Journal of Vision 2013;13(9):1265. doi: https://doi.org/10.1167/13.9.1265.
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© ARVO (1962-2015); The Authors (2016-present)
When a central target grating is flanked by a similarly oriented surround, the orientation of the target is repulsed away from that of the surround; when the target and surround have very different orientations (>50 degrees) the target orientation is attracted toward the surround. Although the psychophysical properties of these tilt repulsion and tilt attraction illusions are well known, there has been surprisingly little investigation of their neural basis. Here, we examined whether orientation-selective activity patterns in early visual areas might be systematically biased for a target grating when presented in the context of a surrounding grating. Participants were shown randomly oriented target gratings either in isolation or with a vertically oriented surround during fMRI scanning. We developed novel decoding analyses that provide accurate estimates of the orientation corresponding to a voxel activity pattern. These decoding techniques were used to determine how the presence of the surround orientation influenced orientation-selective responses to the target grating. In extrastriate visual areas V2 and V3, we found robust tilt repulsion and attraction effects in the decoded orientations. These biases in cortical orientation responses were consistent with typical psychophysical effects, with tilt repulsion peaking at an orientation difference of about 20 degrees between target and surround, and tilt attraction peaking at a difference around 60 degrees. In V1, however, repulsion effects were smaller and attraction effects were absent, a finding that is potentially consistent with models proposing that tilt attraction effects arise from later stages of visual processing. Our findings provide novel evidence to inform our current understanding of the neural bases of human orientation perception, by showing that contextual effects induced by a surrounding grating systematically bias orientation-selective responses in early visual areas.
Meeting abstract presented at VSS 2013
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