A kinesthetic deficit after partial transection of a pyramidal tract in monkeys

Abstract

Summary Monkeys were trained to perform a thermal and a kinesthetic discrimination in immediate succession; the response was a sequence of movements involving the turning of a handle with the right hand. In the left medullary pyramid 25–45% of the fibers were transected. After motor function was fully recovered, as evaluated by general observation, the monkeys were retrained to stable performance. Comparison of 1000 trials before with 1000 trials after the lesions showed a deficit in kinesthetic discrimination; temperature discrimination was unaffected. Simple response latency, choice latency and the time required to reach for the manipulandum were increased even after the smallest pyramidal lesion. The time required to perform the turning response was also increased, but this was, in part, due to retraining. The effect of continued training was determined in 2 monkeys by comparing 2 postoperative observation periods 6 months apart. Both sensory and motor performances were unaffected in a monkey with a control lesion in the right dorsal column. In interspersed trials a visual cue present during the inter-trial period facilitated temperature discrimination. This did not improve the subsequent kinesthetic discrimination, and thus no evidence was obtained for involvement of the pyramidal tract in a mechanism for rapid modality switching. The pyramidal tract must have a sensory function because many of its fibers terminate in the dorsal horn; that it may be related to kinesthesis is consistent with the recent finding that afferent transmission is modulated only during movement.