An “Extinction” Phenomenon on Stimulation of the Cerebral Cortex. ∗

Abstract

In the course of stimulation experiments on the motor cortex of the monkey an interesting phenomenon was met, which we wish to report.

The monkeys (macacus) were anesthetized with “Dial” and ether; in two experiments ether was used exclusively. The motor cortex was exposed for stimulation. The electrical stimulations, which were given for a few seconds, consisted of several patterns of various pulses, obtained from a thyratron stimulator after Schmitt, 1 60 cycle alternating current or an ordinary inductorium. Both biphasic and half-rectified pulses were used. The durations of the various stimulatory periods and the intervals between them, though variable at will, were rigidly controlled by a mechanical device driven by a synchronous A.C. motor. Unipolar and bipolar stimulation, polarizable and non-polarizable electrodes were used. The responses of the contralateral musculature were recorded with the isotonic method on a smoked paper kymograph. The anesthesia, the temperature of the animal and the external conditions of the cortex were kept as constant as possible over long periods of time. The wave form of the various stimulations was checked with the cathode ray oscillograph.

Under all of these experimental conditions the stimulations of a single focus of the motor cortex result in very constant responses, provided the intervals between the successive stimulatory periods are long enough. Then the only variation of the responses is that described in a previous paper 2 as “waves” or intrinsic fluctuations in cortical excitability. Usually, if very strong stimuli are avoided, intervals of half a minute to one minute are sufficient to obviate any disturbing influence resulting in variation of response, apart from the “waves” just mentioned. When the intervals between the stimulatory periods are reduced to 6 seconds or less the well-known phenomenon of primary facilitation, a marked increase in succeeding responses, promptly appears.