Backus, B., Fleet, D.J., Parker, A.J. and Heeger, D.J.
Human cortical activity correlates with stereoscopic depth perception.
Journal of Neurophysiology 86(10):2054-2068, 2001.

Stereoscopic depth perception is based on binocular disparities. Although neurons in primary visual cortex (V1) are selective for binocular disparity, their responses do not explicitly code perceived depth (Cumming and Parker 2000). The stereoscopic pathway must therefore include additional processing beyond V1. We used functional magnetic imaging (fMRI) to examine stereo processing in V1 and other areas of visual cortex. We created stereoscopic stimuli that portrayed two planes of dots in depth, placed symmetrically about the plane of fixation, or else asymmetrically with both planes either nearer or farther than fixation. The interplane disparity was varied parametrically to determine the stereoacuity threshold (the smallest detectable disparity) and the upper depth limit (largest detectable disparity). fMRI was then used to quantify cortical activity across the entire range of detectable interplane disparities. Measured cortical activity covaried with psychophysical measures of stereoscopic depth perception. Activity increased as the interplane disparity increased above the stereoacuity threshold, and dropped as interplane disparity approached the upper depth limit. The V1 measurements were consistent with a post-hoc analysis of electrophysiological recordings of single neurons in macaques. Although there was widespread activity within the extrastriate cortex (consistent with electrophysiological recordings of single neurons), area V3A showed remarkable sensitivity to stereoscopic stimuli, suggesting that neurons in V3A may play a special role in the stereo pathway.
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