Abstract
Posterior parietal cortex in primates contains several functional areas associated with the visual control of body effectors such as the eye, arm, hand, and head. Thus, one might expect that different parietal regions would be maximally activated by visual stimuli within the depth range associated with the corresponding effector. For example, the ventral intraparietal area (VIP) in the monkey responds preferentially to moving stimuli near the head (Colby et al, 1993, J. Neurophys.). We used functional magnetic resonance imaging (fMRI) to examine how visually-driven activation in posterior parietal cortex varies as a function of stimulus distance from the head. Solid 3-D graspable objects were placed in near space (∼ 17cm), grasping space (∼ 43cm), or beyond normal grasping range (∼ 95cm). At each of the three distances, the objects were either stationary or alternately loomed and receded. Visual angle and velocity were equilibrated across distances. Although the pattern of activation in putative area VIP showed inconsistent depth-specificity across subjects, an area in the superior parieto-occipital (SPO) sulcus did show depth-specificity. SPO activation was highest for objects near the head, moderate for objects within arm's reach, and considerably lower for objects beyond grasping range. This preference for near and grasping space was observed for both moving and stationary objects. SPO may correspond to the human parietal reach region, perhaps including the homologue of monkey area V6A, an area which also responds best to objects presented in near space (Claudio Galletti, personal communication).