Threshold movements produced by excitation of cerebral cortex and efferent fibers with some parametric regions of rectangular current pulses (cats and monkeys)

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Abstract
The threshold currents for the production of 1st perceptible movements by excitation of some zones of the motor cortex of a series of cats and monkeys were measured. The waveform used was a unidirectional rectangular pulse (with the anode on the pia). Train repetition frequency was constant at 1/min. Ranges of pulse duration, pulse repetition frequency, and train duration used were 0.1-500 m. sec., 1/60 to 7500 pps., 0.1-5 sec., respectively. The current of every stimulating train was observed and measured by means of a series resistor, a direct coupled amplifier, and a cathode ray oscilloscope. The data from 2 unanesthetized monkeys (1 with a reversible implanted electrode) are presented. In general the thresholds for pulses longer than about 5-10 m. sec. (and for shorter pulses at frequencies giving a high duty cycle) are relatively insensitive to frequency changes. No pulses at frequencies less than 5 pps. showed any frequency sensitivity. For short pulses (less than about 5 m. sec.) the frequency sensitive region is from 10-2000 pps. In a parametric region showing a high frequency sensitivity (fast change of threshold current with change of frequency) one observed (with increasing current) summation of subthreshold stimuli and smooth threshold movements, easy spread to adjacent parts, after-discharges, traveling seizures, and generalized convulsions. This is best seen in the conscious animal. With short pulses, single contractions with each pulse are seen at very low frequencies, without frequency sensitivity, without summation, and without spread. For the short (0.1 m. sec.) pulses the coulomb threshold is less than that for the longer (1 m. sec.) pulses, despite a higher current threshold for the shorter pulses. For 5-sec. trains, short pulses (0.1-1.0 m. sec.) at low repetition frequencies (less than 5 to 10 pps.) apparently excite efferent fibers directly without exciting cortex; whereas at higher frequencies (greater than 5-20 pps.) such pulses excite mainly cortical strata. Damage to cortex probably results from stimulation with this waveform (with something less than 800 trains at or near threshold at 1-min. intervals).

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