Bilateral and side-related reaction time impairments in patients with unilateral cerebral lesions of a medial frontal region involving the supplementary motor area.

Human subjects (nine patients with unilateral brain lesions of a medial frontal region involving the supplementary motor area, SMA, and 10 controls) performed two reaction time (RT) tasks in response to the presentation of a luminous signal: an aimed movement towards a spatially defined target involving hand lifting and pointing with the index finger, and a no-aimed movement consisting of the hand lifting phase completed by the stabilization of the limb posture without any pointing. When compared with controls, the patients exhibited a bilateral RT increase which was more pronounced in the hand contralateral to the lesion. Moreover, comparison between the two tasks showed that this contralateral RT impairment was more marked in the no-aiming than in the aiming task. These results suggest that unilateral lesions of a medial frontal region involving the SMA cause two types of RT impairment in these motor tasks. The first may concern a supramotor function which acts bilaterally and initiates the motor programme of the limb movement as the first step of preparatory processes. The second component of this RT impairment would concern a supplementary motor function which consists of the feed forward control of the coupling between the hand lifting and the appropriate posture, just before the triggering of the limb movement. This interpretation leads to the hypothesis that the SMA region, and the medial motor system in general, may have a dual motor function.