EFFECTS OF LESIONS IN THE MESIAL FRONTAL-CORTEX ON BIMANUAL COORDINATION IN MONKEYS

The hypothesis was tested that the mesial frontal cortex, including th e supplementary motor area, is engaged in bimanual co-ordination. Thre e monkeys, trained in a well-co-ordinated bimanual pull-and-grasp task , were subjected to unilateral or bilateral lesions of the mesial fron tal cortex. With unilateral lesions, the deficit consisted in a delay in movement initiation of the contralateral arm. With a bilateral lesi on, the deficit was more pronounced with marked bilateral delays in mo vement onset and slowing in reaching. However, in the three monkeys bi manual co-ordination at the moment of goal achievement remained intact with an excellent temporal co-variation of the two limbs. In the two unilateral cases, an adaptive strategy developed after a few sessions, either by catching up during reaching with the limb contralateral to the lesion (monkey M1) or by delaying movement initiation of the limb ipsilateral to the lesion (monkey M2). This outcome is discussed in te rms of Lashley's principle of motor equivalence, i.e. invariant goal a chievement with variable means. Bilateral lesions led to a transient a nd near-total impairment in movement self-initiation when all external cues were absent. It is concluded that in monkeys the mesial frontal cortex does not play a crucial role in bimanual co-ordination but rath er in movement initiation, especially when sensory cues are absent. (C ) 1998 IBRO. Published by Elsevier Science Ltd.