DEXTERITY IN ADULT MONKEYS FOLLOWING EARLY LESION OF THE MOTOR CORTICAL HAND AREA - THE ROLE OF CORTEX ADJACENT TO THE LESION

Infant monkeys were subjected to unilateral lesions of the motor corte x (mainly its hand representation). After maturation, they showed norm al use of the contralateral hand for global grip movements. However, a s compared with the ipsilateral hand, precision grip tasks requiring r elatively independent finger movements were performed with less dexter ity, particularly if adjustments of the wrist position were necessary. The purpose of this study was to investigate mechanisms which may be responsible for the rather well, although not complete, preservation o f manipulative behaviour of these adult monkeys. To this end, the hand representations were mapped bilaterally with intracortical microstimu lation in the mature monkeys, and the dexterity of both hands assessed quantitatively in a precision grip task. The behavioural effects of r eversible inactivations of the primary (Mi) and supplementary (SMA) mo tor cortical areas were then tested. The following were found. (i) The hand contralateral to the lesion exhibited subtle but significant dex terity deficits, as compared with the ipsilateral hand; the deficit wa s essentially for complex movements requiring dissociation of the thum b-index finger pinch from the other digits, involving also an arm rota tion. (ii) Reversible inactivation of the M1 hand representation in th e intact hemisphere dramatically impaired dexterity of the opposite ha nd without affecting the ipsilateral hand (contralateral to the early lesion). (iii) A relatively complete hand representation was found to occupy a new territory, medial to the old lesion. (iv) The role of thi s new displaced representation was crucial for the preserved dexterity of the opposite hand, as evidenced by its functional inactivation. In contrast, inactivation of both SMA cortices did not interfere with th e manipulative behaviour. it is thus concluded that the preserved func tional capacity of manipulations with the hand opposite the early lesi on can be essentially attributed to a cortical reorganization around t he old lesion. Under the present experimental conditions, contribution s from either the SMA or the intact M1 appear not to be crucial.