Hemispheric asymmetries in motor function: II. Ordering does not contribute to left-hemisphere specialization.

Patients with unilateral brain lesions of vascular origin were administered tests designed to determine if the left hemisphere is specialized for the ordering of motor acts. Patients with left- and right-sided lesions responded similarly to manipulations of ordering demands in two motor tasks, one a test of manual sequence learning and the other a test of speeded performance. A detailed analysis of performance on an Ideational Apraxia task indicated that the deficit of patients with left-sided lesions was best characterized as a deficit in generating appropriate acts rather than ordering them. It was concluded that there is no evidence for left-hemisphere specialization for the ordering of motor acts. It was hypothesized that there are two broad stages of motor function: generation of motor "target" acts (which is lateralized to the left hemisphere) and the ordering and execution of these acts (which are not lateralized).