Parietal area BA7 integrates motor programs for reaching, grasping, and bimanual coordination

Abstract

Skillful interaction with the world requires that the brain uses a multitude of sensorimotor programs and subroutines, such as for reaching, grasping, and the coordination of the two body halves. However, it is unclear how these programs operate together. Networks for reaching, grasping, and bimanual coordination might converge in common brain areas. For example, Brodmann area 7 (BA7) is known to activate in disparate tasks involving the three types of movements separately. Here, we asked whether BA7 plays a key role in integrating coordinated reach-to-grasp movements for both arms together. To test this, we applied transcranial magnetic stimulation (TMS) to disrupt BA7 activity in the left and right hemispheres, while human participants performed a bimanual size-perturbation grasping task using the index and middle fingers of both hands to grasp a rectangular object whose orientation (and thus grasp-relevant width dimension) might or might not change. We found that TMS of the right BA7 during object perturbation disrupted the bimanual grasp and transport/coordination components, and TMS over the left BA7 disrupted unimanual grasps. These results show that right BA7 is causally involved in the integration of reach-to-grasp movements of the two arms.

New & noteworthy: Our manuscript describes a role of human Brodmann area 7 (BA7) in the integration of multiple visuomotor programs for reaching, grasping, and bimanual coordination. Our results are the first to suggest that right BA7 is critically involved in the coordination of reach-to-grasp movements of the two arms. The results complement previous reports of right-hemisphere lateralization for bimanual grasps.

Keywords: TMS; bimanual; coordination; grasping; reaching.

Fig. 1.

A: experiment setup. B: timeline of a single trial. C: location of stimulation sites BA7 and mTOS, as displayed in the left and right hemisphere of 1 subject. postCS, postcentral sulcus.

Fig. 2.

Trajectories for the left and right hands from movement onset to object contact (averaged across trials and then across participants). A: trajectories during the left BA7 experiment block. B: right BA7 block, with arrows highlighting effects due to TMS. C: left mTOS block. D: right mTOS block. TMS +, stimulation; TMS −, no stimulation; P +, perturbation; P −, no perturbation.

Fig. 3.

Grip aperture across normalized time, averaged trials, and then across participants. A: grip apertures measured during the left BA7 experiment block. B: right BA7 block. Shaded bars overlaying this panel indicate significant perturbation × TMS interaction effects. C: left mTOS block. D: right mTOS block. See Fig. 2 for TMS/P descriptions.

Fig. 4.

SD of grip aperture across normalized time. A: left BA7 block. B: right BA7 block. C: left mTOS block. D: right mTOS block. Same convention as Fig. 3.

Fig. 5.

SD of hand trajectories across normalized time. A: left BA7 block. B: right BA7 block. C: left mTOS block. D: right mTOS block. Same convention as previous figures.

Fig. 6.

Velocity profiles of index-finger trajectories of the left and right hands across non-normalized time, from Go signal to object contact. A: velocity for left BA7 experiment block. B: right BA7 block. C: left mTOS block. D: right mTOS block. Same conventions as previous figures.

Fig. 7.

Grip aperture for the unimanual control condition: stimulation in BA7 left hemisphere.