The neural bases of the reach-grasp movement in humans: Quantitative evidence from brain lesions

: Visually guided grasping is a fundamental building block of animal behavior, the specific neural mechanisms of which remain poorly documented in the human brain. We have mapped the causal contribution of different brain parts to grasping behavior by studying the kinematic parameters of 33 patients with brain tumors, engaged in actions directed toward objects of different sizes. Using motion capture techniques, we analyzed the dynamics of grip aperture and wrist transport. Voxel-based lesion-symptom mapping analysis was applied to correlate lesion volumes with specific behavioral deficits. Results showed that lesions in the anterior and lateral bank of the intraparietal sulcus produced impaired finger scaling related to object size. Conversely, impaired velocity of finger aperture was associated with lesions in the dorsal premotor cortex (PMd). Grip aperture deficits following dominant hemisphere lesions were bilateral and were unilateral when following nondominant hemisphere lesions. Impaired wrist transport during reaching was associated with lesions in the first segment of the superior longitudinal fasciculus. Our work highlights an architecture of the grasping network in humans, with unique species-specific features. We hypothesize a model of human neural architecture in which object geometry for hand preshaping is first coded in the left anterior intraparietal cortex and then shared with the right hemisphere. Execution of the motor program of hand preshaping is then performed by the PMd on the corresponding side.