Left inferior parietal representations for skilled hand-object interactions: evidence from stroke and corticobasal degeneration.

Patients with ideomotor apraxia (IM) are frequently more impaired in the production and imitation of object-related (transitive) than non-object-related, symbolic (intransitive) gestures, but reasons for this dissociation, and its anatomical underpinnings, remain unclear. Our theoretical model of praxis (Buxbaum, 2001) postulates that left inferior parietal lobe (IPL) gesture representations store information about postures and movements of the body and hand for skillful manipulation of familiar objects; in contrast, bilateral fronto-parietal dynamic calculations provide constantly-updated information about the current position and movement of the body and hand for both familiar and novel, transitive and intransitive movements. This account predicts distinct patterns of IM in patients with left IPL damage versus bilateral fronto-parietal involvement. Consistent with predictions, 16 stroke patients with left IPL damage were more impaired with transitive than intransitive gestures, whereas 4 patients with bilateral fronto-parietal damage due to corticobasal degeneration (CBD) were not [F (1, 18) = 8.5 p < .01]. Additionally, the hand posture component of transitive gestures was the most impaired aspect of gesture in CVA, but tended to be the least impaired aspect of gesture in CBD [F (3, 54) = 5.1, p < .005]. Finally, CVA patients were more impaired with transitive hand postures than meaningless or intransitive hand postures, whereas CBD patients showed the opposite pattern. These data indicate that the left IPL mediates representations of skilled hand-object interactions, as distinct from dynamic coding of the body in space, and suggest that the IPL maps between representations of object identity in the ventral stream and spatial body representations mediated by the dorsal system.