THE LEFT-HEMISPHERE AND THE SELECTION OF LEARNED ACTIONS

The left hemisphere's dominance for movement is well known. The basis of its dominance is less clear. We have tested 16 left hemisphere (LH) patients, 17 right hemisphere (RH) patients and 12 neurologically nor mal controls on a battery of five tasks. The tasks were based on anima l lesion and recording studies, and human imaging and magnetic stimula tion studies that identified two distributed systems that are importan t for the selection of motor responses and object-oriented responses. The LH patients were impaired on three response selection tasks: learn ing to select between joystick movement responses instructed by visual cues; learning to select between analogous object-oriented responses instructed by visual cues; learning to select movements in a sequence. Although we replicated the finding that LH damage impairs sequencing, some of the impaired tasks had no sequencing element. We therefore ar gue that the LH deficits are best explained as an impairment of respon se selection. This was confirmed by showing that LH subjects were unim paired on a more demanding task-object discrimination learning-which i mposed a greater memory load but had no response selection element. Mo reover, the LH deficits could not be attributed to disorganization of movement kinematics. The lesions of the impaired LH group were widespr ead but always included the distributed systems known to be important for response selection-the dorsolateral frontal and parietal cortices, striatum, thalamus and white matter fascicles. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.