The executive and ideational components of apraxia.

Fifty six left brain-damaged (LBD) patients and 38 right brain-damaged (RBD) patients were requested to perform two measures of praxis using the hand ipsilateral to the side of lesion. One task required the accurate imitation of actions made by the examiner and taxed executive abilities; the other task required the patient to pantomime the use of objects that were shown, but not handed, and it also involved an ideational component, i.e., the evocation of gestures. The cut-off point discriminating a normal from a pathological performance was set at the level of the poorest score found in 60 control patients. On both tests apraxia was found to be associated primarily with left brain-damage. However, while on the use of objects test practically only LBD patients failed, on the imitation test there was also a sizeable proportion of RBD patients who showed a mild apraxia. When the performance on the two tests was contrasted, there were as many LBD patients as RBD patients who scored remarkably lower on movement imitation than on pantomiming the use of objects, whereas the opposite dissociation was found almost exclusively in LBD patients. We infer from these data that the left hemisphere dominance for praxis is more marked at the ideational stage and that there is also a minor right hemisphere participation in the control of the executive stage of gesture.