Naturalistic action and praxis in callosal apraxia

We report a subject who, subsequent to closed head injury, demonstrated a severe left hand ideational apraxia and apraxic agraphia, consistent with callosal disconnection syndrome. In contrast to the left hand, performance of the right hand was unimpaired on traditional tests of gesture to command, sight of object, and with actual object use, but proved deficient on tests of spatial and constructional ability. We examined the consequences of these hand-specific deficits for the performance of naturalistic action tasks. The patient made errors with each hand; however, the right hand performed more poorly than the left hand. In addition, the types of errors made by each hand differed in a manner consistent with the results of neuropsychological testing and indicative of disconnection phenomena. We suggest that unlike gesture, naturalistic action requires the contribution of the specialized abilities of each hemisphere, integrated across callosal structures. Traditional testing of gesture may underestimate the right hemisphere's capacity for action programming.