The project involves studies of motor control in subhuman primates, in healthy human subjects, and in patients with parkinsonism and Huntington's disease. Work in primates has shown that rapid ballistic movements depend on central programs operating relatively "open loop," whereas small precisely controlled movements utilize the continuous feedback provided by the "servo" properties of transcortical reflexes. Reflexes of motor cortex neurons are heightened during the servo-mode occurring with small precise movements, while these reflexes are attentuated during the open-loop mode underlying ballistic movement. Studies in normal human subjects have shown that the response to a kinesthetic stimulus involving muscle stretch consists of: 1) a spinal reflex; 2) a longer latency reflex traversing motor cortex and the pyramidal tract; 3) an even longer latency intended response which involves cerebellum, thalamus, motor cortex, and pyramidal tract. The latency of this third component is in fact kinesthetic reaction time (KRT). KRT may have a minimum value of 70 msec even for a kinesthetic stimulus which reflexly inhibits the muscle which is to respond. KRTs are significantly faster than auditor or visual reaction times. A variety of abnormalities of these responses have been detected in patients with neurological disorders. BIBLIOGRAPHIC REFERENCES: Evarts, E.V.: Neuronal Representation of Acquired Movement Patterns in Primates. In Fentress, J.C. (Ed.): Simpler Networks and Behavior. Sunderland, Mass., Sinaur Associates, 1976, pp. 266-273. Evarts, E.V.: The Interaction of Central Commands and Peripheral Feedback in Pyramidal Tract Neurons (PTNs) of the Monkey. In Herman, R.M., Grillner, S., Stein, P.S.G., and Stuart, D.G. (Eds.): Neural Control of Locomotion. New York, Plenum, 1976, pp. 808-817.