Praxis and the right hemisphere.

We report our observations on praxis in a strongly right-handed man following a massive stroke that resulted in virtually complete destruction of the left cerebral hemisphere. Our patient was severely impaired in pantomiming transitive gestures with the left hand and in reproducing novel non-symbolic hand and arm movement sequences. However, overlearned habitual actions like actual object use and intransitive gestures were relatively spared. Performance of axial commands was intact. Gesture recognition and discrimination were also preserved. Based on these findings, we propose that the praxis system of the right hemisphere is strongly biased toward "concrete" or context-dependent execution of familiar, well-established action routines. The right hemisphere is critically dependent on transcallosal contribution from the left hemisphere for control of the left hand in "abstract" or context-independent performance of transitive movements and in learning novel movement sequences. At least in some individuals, the right hemisphere can recognize and discriminate gestures. Possible implications of our findings for the cerebral control of praxis and for recovery from apraxia are discussed.