Society for Neuroscience 2006
Oct. 14-18, 2006 in Atlanta, Georgia

Computations underlying fine and coarse depth discrimination in human stereopsis

Takahiro Doi, Seiji Tanabe, Ichiro Fujita
Graduate School of Frontier Biosciences, Osaka University, Toyonaka, Japan

Neural activity in area MT/V5 has a causal link with perceptual discrimination of coarse stereoscopic depth. Yet neurons in this area signal disparity information in binocularly anticorrelated stereograms that do not evoke sensation of three-dimensional structure. This contradiction poses a challenge on identifying the neural mechanism of stereopsis. Other areas, such as V4 and IT, support some aspects of stereopsis that MT fails to do, namely rejection of false matches and fine-disparity discrimination. We hypothesize that (1) coarse-disparity discrimination involves MT where neuronal disparity-tuning function persists in a reversed form when stereograms are anticorrelated, and that (2) fine-disparity discrimination involves V4 and IT where such reversed depth representation for anticorrelated stereograms is attenuated. To test whether distinct neural computations underlie coarse and fine disparity discrimination, we examined human stereoscopic depth discrimination for various disparity magnitudes and binocular correlations. Subjects reported whether the center region of a dynamic random-dot stereogram was perceived nearer or farther than its adjacent surround. In each block of trials, we fixed disparity magnitude and varied the proportion of dots with reversed luminance contrast between the two eyes. In blocks with a large disparity, the subjects perceived reversed depth when reversed-contrast dots dominated, and performed at chance when same-contrast and reversed-contrast dots were balanced in proportion. In blocks with a small disparity, the subjects performed at chance when reversed-contrast dots dominated, and performed correctly when the two groups of dots were balanced. The psychophysical property in coarse-disparity discrimination agrees with a binocular version of local filtering operation. On the other hand, the property in fine-disparity discrimination cannot be explained with such simple operation. We conclude that coarse-depth perception reflects the disparity energy computation, whereas fine-depth perception requires further computation to derive the depth that reflects the global-match solution. The apparent contradiction in the previous studies can be reconciled with the respective computations underlying fine and coarse stereopsis.

Supported by grants from the MEXT (17022025) and the Takeda Science Foundation