Stimulus-induced tremor in chronic monkeys

Bipolar electrodes were chronically implanted in many subcortical structures and over the sensorimotor cortex in monkeys. Phasic motion was recorded with piezoelectric cartridges, strapped to the animals' limbs, at numerous recording sessions for many months. The most common response observed was tremor which followed the frequency of stimulation to 20–30/sec, then remained in that frequency range as the stimulus frequency was increased to 100/sec. However, in all animals non-following tremor was also observed, and occasionally the response remained below 10/sec with stimulus frequencies to 100/sec. The amplitude of both following and non-following phasic motion was influenced by stimulus intensity, simultaneous stimulation of two electrodes, and barbiturate anesthesia. In contrast, the frequency of the non-following phasic motion was independent of all variables investigated. At high stimulus frequencies, high amplitude, non-following phasic motion was often super-imposed on a low amplitude response at the frequency of stimulation. In addition, the amplitude of the non-following tremor was modulated at frequencies of 1–2/sec, and occasionally the modulation was related to the respiratory cycle. Evidence is presented that the frequency of the non-following phasic motion is established at the level of the spinal cord.