Stereopsis after unilateral anterior temporal lobectomy. Dissociation between local and global measures.

In man, an impairment in stereopsis may occur following damage outside the occipital lobes. If local and global stereopsis can be dissociated, this may imply that different mechanisms mediate these functions. We tested 44 patients with unilateral anterior temporal lobe excisions and 23 normal control subjects on two tasks. In the local stereopsis task, subjects indicated which of two pins varying in disparity between 4 and 512 s of arc was closer. Results showed no threshold impairment in any group. In the global stereopsis task, random-dot stereograms varying in binocular correlation were presented in random order, and subjects indicated if the squares perceived in depth were in front of or behind the screen. At binocular correlations between 50% and 70%, left and right temporal lobectomy resulted in a deficit, which was more marked following right-sided excisions. These results are concordant with earlier work (Ptito and Zatorre, 1988), and with other evidence of a right hemisphere superiority in treating binocular depth information. Moreover, the fact that global stereopsis is impaired in the presence of intact local stereopsis suggests that closely related but not identical mechanisms are involved, and fits the notion that there is a hierarchical organization of the visual pathways originating in the striate cortex leading into temporal cortex. Global stereopsis would thus be mediated in temporal lobe areas that receive information about local disparities from more posterior regions.