Cognitive representations of hand posture in ideomotor apraxia

Ideomotor apraxia (IM) is a disorder of skilled action characterized by spatiotemporal errors in pantomiming object use and in using objects. Recent evidence suggests that at least some patients with IM may exhibit particular deficits in forming hand configurations appropriate for object use. Sirigu et al. [Cortex 31 (1995) 41] reported an apraxic who positioned her hand inappropriately when attempting to use objects in accordance with stored knowledge of object-specific manipulation, but in reaching tasks could grasp the same objects appropriately in response to their structure. To this point, however, apraxics' ability to respond to functional and structural attributes of objects has not been empirically assessed. We investigated the hypothesis that patients with IM (n=9) due to left inferior parietal damage would be impaired in producing and recognizing hand postures associated with familiar objects, indicating deficient memorial representations for object-specific hand postures. In contrast, we predicted relatively unimpaired ability to produce and recognize appropriate hand postures with novel objects, indicating integrity of "on line" spatiomotor procedures coding hand position in response to object structure. Apraxics' performance was contrasted with 10 healthy controls and 8 brain-lesioned non-apraxics. Consistent with our predictions, the apraxics responded abnormally with familiar objects but normally in recognizing hand postures appropriate for novel objects. In addition, performance with objects calling for a prehensile response (pinch or clench) was superior to that with objects evoking a non-prehensile response (palm or poke). These data suggest degradation of inferior parietally-mediated representations of the precise hand postures associated with familiar objects. Furthermore, they are consistent with possible over-reliance upon dorsal system procedures for calculating precision and power grip in response to object structure.