Transient responses to load perturbations of the forearm in a monkey with a chronic lesion in the internal capsule

Small electrolytic lesions were produced in the internal capsule of a monkey. The changes in muscle tone were quantified by studying the EMG responses of elbow muscles and the mechanical responses of the forearm to pseudo-random torque perturbations applied to the elbow joint. Immediately following the lesion, the EMG responses of both biceps and triceps muscles were depressed. Subsequently, biceps responses recovered and became eventually greater than in the control. Triceps responses, instead, remained low throughout the follow-up period (3 months). The mechanical behavior of the forearm was characterized in terms of the dynamic relationship between the applied torque perturbations and the resulting changes in elbow angle. After the lesion, the damping of the elbow responses decreased relative to the control. Possible mechanisms for the observed changes in the EMG and mechanical behavior are discussed.