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Purpose: Visual processing is dissociated between a dorsal (occipitoparietal) stream for action and a ventral (occipitotemporal) stream for perceptual recognition. Visually guided grasping requires processing of object shape, but for the purposes of action rather than perceptual recognition. By comparison, visually-guided reaching requires transporting the hand to the target location but not shape processing. We used functional magnetic resonance imaging (fMRI; 4 Tesla) to determine whether grasping (compared to reaching) produced activation in dorsal areas, ventral areas, or both. Methods: Rectangular objects of varying length and orientation were mounted on a rotating drum that subjects viewed directly without mirrors. On each trial, one of the objects was illuminated and the subject grasped the rectangle along the long axis using a precision grip (with the finger and thumb). In a control condition, subjects reached and touched, but did not grasp, the target object. Event-related single trials took advantage of the hemodynamic delay to dissociate true grasping-related activation from potential motion artifacts. Results: In each of six subjects, grasping produced greater activation than reaching in the anterior intraparietal (AIP) cortex. Negligible grasp-specific activation was observed in ventral stream object areas. Conclusions: These results suggest that the processing of shape required to form a grasp involves dorsal but not ventral stream regions. The dorsal stream area that was activated is a likely human homologue of monkey AIP, an area containing neurons that code object shape and fire during grasping.
