Recovery of precision grasping after motor cortex lesion does not require forced use of the impaired hand in Macaca mulatta.

We investigated recovery of precision grasping of small objects between the index finger and thumb of the impaired hand without forced use after surgically placed lesions to the hand/arm areas of M1 and M1 + lateral premotor cortex in two monkeys. The unilateral lesions were contralateral to the monkey's preferred hand, which was established in prelesion testing as the hand used most often to acquire raisins in a foraging board (FB) task in which the monkey was free to use either hand to acquire treats. The lesions initially produced a clear paresis of the contralesional hand and use of only the ipsilesional hand to acquire raisins in the FB task. However, beginning about 3 weeks after the lesion both monkeys spontaneously began using the impaired contralesional hand in the FB task and increased use of that hand over the next few tests. Moreover, the monkeys clearly used precision grasp to acquire the raisins in a similar manner to prelesion performances, although grasp durations were longer. Although the monkeys used the contralesional hand more often than the ipsilesional hand in some postlesion testing sessions, they did not recover to use the hand as often as in prelesion testing when the preferred hand was used almost exclusively. These findings suggest that recovery of fine hand/digit motor function after localized damage to the lateral frontal motor areas in rhesus monkeys does not require forced use of the impaired hand.