Human cortical activity correlates with stereoscopic depth perception

Stereoscopic depth perception is based on binocular disparities. Although n eurons in primary visual cortex (V1) are selective for binocular disparity, their responses do not explicitly code perceived depth. The stereoscopic p athway must therefore include additional processing beyond V1. We used func tional magnetic resonance imaging (fMRI) to examine stereo processing in V1 and other areas of visual cortex. We created stereoscopic stimuli that por trayed 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 deter mine the stereoacuity threshold (the smallest detectable disparity) and the upper depth limit (largest detectable disparity). fMRI was then used to qu antify cortical activity across the entire range of detectable interplane d isparities. Measured cortical activity covaried with psychophysical measure s of stereoscopic depth perception. Activity increased as the interplane di sparity increased above the stereoacuity threshold and dropped as interplan e disparity approached the upper depth limit. From the fMRI data and an ass umption that V1 encodes absolute retinal disparity, we predicted that the m ean response of V1 neurons should be a bimodal function of disparity. A pos t hoe analysis of electrophysiological recordings of single neurons in maca ques revealed that, although the average firing rate was a bimodal function of disparity (as predicted), the precise shape of the function cannot full y explain the fMRI data. Although there was widespread activity within the extrastriate cortex (consistent with electrophysiological recordings of sin gle neurons), area V3A showed remarkable sensitivity to stereoscopic stimul i, suggesting that neurons in V3A may play a special role in the stereo pat hway.