The processing of three-dimensional shape from disparity in the human brain.

Three-dimensional (3D) shape is important for the visual control of grasping and manipulation and for object recognition. Although there has been some progress in our understanding of how 3D shape is extracted from motion and other monocular cues, little is known of how the human brain extracts 3D shape from disparity, commonly regarded as the strongest depth cue. Previous fMRI studies in the awake monkey have established that the interaction between stereo (present or absent) and the order of disparity (zero or second order) constitutes the MR signature of regions housing second-order disparity-selective neurons (Janssen et al., 2000; Srivastava et al., 2006; Durand et al., 2007; Joly et al., 2007). Testing the interaction between stereo and order of disparity in a large cohort of human subjects, revealed the involvement of five IPS regions (VIPS/V7*, POIPS, DIPSM, DIPSA, and phAIP), as well as V3 and the V3A complex in occipital cortex, the posterior inferior temporal gyrus (ITG), and ventral premotor cortex (vPrCS) in the extraction and processing of 3D shape from stereo. Control experiments ruled out attention and convergence eye movements as confounding factors. Many of these regions, DIPSM, DIPSA, phAIP, and probably posterior ITG and ventral premotor cortex, correspond to monkey regions with similar functionality, whereas the evolutionarily new or modified regions are located in occipital (the V3A complex) and occipitoparietal cortex (VIPS/V7* and POIPS). Interestingly, activity in these occipital regions correlates with the depth amplitude perceived by the subjects in the 3D surfaces used as stimuli in these fMRI experiments.