Task-dependent modulation of BOLD responses reveals distinct roles of dorsal and ventral pathways in stereoscopic depth perception

Abstract

A fundamental challenge of the primate visual system is to reconstruct 3D visual scenes from 2D retinal inputs. Binocular disparity, a slight positional displacement between the left and right retinal images, is one of the powerful cues for depth perception. Numerous physiological and neuroimaging studies have accumulated evidence of distinct roles of dorsal and ventral visual pathways for the processing of binocular disparities. These studies, however, led to inconsistent conclusions in some aspects. This may be partially because these studies used different visual stimuli triggering different activation in visual areas. For instance, some studies used dynamic random-dot stereograms (RDSs), which evoke stronger BOLD responses in dorsal pathway regions. Others used 3D object images, which evoke preferential activation in ventral pathway regions. To eliminate these stimulus effects, we asked observers to perform two distinct depth-discrimination tasks for exactly the same binocular disparity stimuli during fMRI measurements (3T, 1.5 mm isotropic voxels). Specifically, observers were asked to discriminate either position-in-depth of an entire stimulus (a position-in-depth task) or depth of the inner annulus relative to the outer (a 3D shape task), and then we examined how BOLD responses were modulated depending on the task demands in visual areas. We found that dorsal areas including hMT+, VIPS, POIPS, and DIPS showed stronger responses in the position-in-depth task than in the 3D shape task. In contrast, Brodmann's area 39 in the angular gyrus and 21 in the middle temporal gyrus were more engaged in the 3D shape task. Early retinotopic visual areas showed no difference in activation between the two tasks. We suggest that task-dependent (not stimulus-driven) multiple depth processing resides in higher-order dorsal and ventral areas. These results also shed new light on the consistency and inconsistency in the previous findings between monkeys and humans regarding the roles of dorsal and ventral pathways.

Meeting abstract presented at VSS 2017