ELECTRICAL STIMULATION OF MONKEY BRAIN WITH VARIOUS FREQUENCIES AND PULSE DURATIONS

Abstract

Effects of altering the durations (0.01-5.0 msec.) and frequency (10-5000 cycles (sec.) of pulses upon the threshold of motor and autonomic responses evoked by electrical stimulation of different cortical and sub-cortical areas of the brain were studied in unanesthetized monkeys with permanently implanted electrodes. Pulse durations of 0.1 to 0.5 msec. were generally most effective in evoking both motor and autonomic responses. The lowest threshold for both motor and autonomic responses occurred at frequencies of 100 to 250cps. At higher frequencies a sharper increase in threshold values was seen in the case of autonomic responses. In most of the areas studied variation in frequency of stimuli resulted in change in the type of evoked motor responses. Similar changes, and at approximately the same frequencies, were obtained from widely separated areas of the gray and the white matter of both cortical and subcortical areas of the brain. Motor responses evoked from the tectum opticum never followed the rhythm of stimulus. Even at frequencies below 10 cps perfectly summated response occurred. Since the stimulation of efferent fibers with frequencies above 30 cps may result also in smooth action of effectors, it was suggested that differentiation as to whether efferent fibers or cells or both are being stimulated could be achieved by using low frequency stimulation. Negative had lower thresholds than positive pulses. Bidirectional waves were generally as effective, and in some cases had lower thresholds than the negative pulses. Bidirectional waves of 0.1-0.2 msec. duration, and frequencies of about 100 cps seem to be the most appropriate stimulus to evoke smooth motor responses. For detailed analysis of motor phenomena low frequencies (up to 10 cps) should also be used.