Within grasp but out of reach: evidence for a double dissociation between imagined hand and arm movements in the left cerebral hemisphere

What roles are played by the cerebral hemispheres in planning object-orient ed reaching and grasping movements? In an attempt to address this question. we compared the abilities of the left and right hemispheres of commissurot omy patient J.W. to imagine hand manipulation (i.e.. grasp) or arm transpor tation (i.e.. reach) movements. A graphically rendered manipulandum (dowel) was briefly presented to the left (LVF) or right (RVF) visual fields in a variety of different orientations. In the grasp selection task (experiment 1). J.W. was required to determine which side of a dowel his thumb would be on if he were to engage the stimulus in a power grip using either his domi nant (right) or non-dominant hand. In the reach selection task (experiment 3). J.W. judged which end his elbow would be on if he treated the dowel as an armrest for his dominant or non-dominant forearm. No actual movements we re allowed in either task. Movements selected in the imagery tasks were com pared with those chosen during actual motor control under comparable circum stances. These comparisons revealed a left hemisphere advantage for represe nting grasping movements involving the right hand, and reaching movements i nvolving the left arm. The right hemisphere. by contrast, displayed moderat e accuracy when representing grasping movements with the left hand, but app eared incapable of imagining reaching movements with either arm. The double dissociation between imagery for hand and arm movements in the left cerebr al hemispere is consistent with the hypothesis that grasping and reaching c omponents of prehension involve dissociable planning mechanisms. (C) 3000 E lsevier Science Ltd. All rights reserved.