Characterising human disparity tuning properties using population receptive field mapping.

Our visual percept of small differences in depth is largely informed by binocular stereopsis, the ability to decode depth from the horizontal offset between the retinal images in each eye. While multiple cortical areas are associated with stereoscopic processing, it is unclear how tuning to specific binocular disparities is organised across human visual cortex. We used 3T functional magnetic resonance imaging to generate population receptive fields in response to modulation of binocular disparity to characterise the neural tuning to disparity. We also used psychophysics to measure stereoacuity thresholds compared to backgrounds at different depths (pedestal disparity). Ten human participants (7 female) observed correlated or anticorrelated random-dot stereograms with disparity ranging from -0.3° to 0.3°, and responses were modelled as 1-dimensional tuning curves along the depth dimension. First, we demonstrate that lateral and dorsal visual areas show the greatest proportion of vertices selective for binocular disparity. Second, with binocularly correlated stimuli, we show a polynomial relationship between preferred disparity and tuning curve width, with sharply tuned disparity responses at near-zero disparities, and broader disparity tuning profiles at near or far disparities. This relationship held across visual areas and was not present for anticorrelated stimuli. Finally, the individual thresholds for psychophysical stereoacuity at the 3 different pedestal disparities were broadly related to population receptive field tuning width in area V1, suggesting a possible limit for fine stereopsis at the earliest level of cortical processing. Together, these findings point to heterogeneity of disparity processing across human visual areas, comparable to non-human primates.Significance Statement Binocular disparity arises from the horizonal separation of the two eyes and provides information for determining depth and 3D structure. We used functional magnetic resonance imaging and population receptive field mapping to measure tuning of multiple visual areas to binocular disparity in the human visual cortex. We additionally measured psychophysical thresholds for detecting binocular disparity and correlated these with the neural measures. The width of the disparity tuning was related to the preferred disparity across all visual areas. Disparity tuning widths in V1 were also related to psychophysical thresholds. These findings in the human are broadly comparable to non-human primates.