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Andrew Welchman, Tim Preston, Sheng Li; Functional specialisation for the perception of disparity-defined depth in the human visual cortex. Journal of Vision 2008;8(6):541. doi: 10.1167/8.6.541.
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© ARVO (1962-2015); The Authors (2016-present)
Electrophysiological and fMRI studies have revealed selectivity for binocular disparity across primate visual cortex. However, the functional role of this activity in supporting depth perception remains largely unknown. To investigate this relationship, we recorded fMRI responses in the human visual cortex whilst observers viewed random dot stereograms depicting planes with either crossed or uncrossed horizontal disparity. We used multivariate analysis methods (linear support vector machines) to determine cortical regions whose fMRI responses support robust classification of planes presented at different depth positions, thereby indicating selectivity for disparity-defined depth. We contrasted classification for correlated planes against classification for anti-correlated stimuli (i.e. negative correlation between the luminance contrast of corresponding dots in the two eyes) that do not support depth perception. For correlated stimuli, classification of crossed- vs. uncrossed disparity was above chance across the cortical hierarchy from V1 to higher dorsal (hMT+/V5) and ventral regions (LO), with especially high accuracies in intermediate dorsal regions (V3, V3A). Importantly, pattern classification for anti-correlated stimuli highlighted differences between dorsal and ventral areas. Whilst fMRI responses in extrastriate ventral areas (VP/V3, V4) supported the classification of anti-correlated stimuli, such classifications were abolished in dorsal areas (V3B/KO and V7) and higher temporal regions (LO). This suggests a functional distinction between regions, with perceived depth related to disparity processing in dorsal extrastriate regions and higher temporal areas. A further functional distinction was suggested by classifying the fMRI data according to the specific disparity content of the stimuli. In particular, classification performance indicated representations related to the specific depth position in dorsal areas, whilst representations related to the sign (near vs. far) of depth in LO. Our findings provide novel evidence that disparity processing in the dorsal stream is selective for the specific depth position, whilst higher temporal regions encode perceived depth in a categorical manner.
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