W. S. McCulloch

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

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.

Functional Boundaries in the Sensori-Motor Cortex of the Monkey.

The method of local strychninization of the cerebral cortex of the monkey has already established the existence of functional boundaries between the major subdivisions of the sensori-motor cortex, so far as sensation is concerned. Further study of the problem by the same method, supplemented by recording the action potentials of the cortex, has confirmed the existence of functional boundaries on the “sensory” side and established the existence: of such boundaries on the “motor” side. Strychnine applied locally to any region of the cortex induces in that region typical changes of the “spontaneous” action potentials, notably the appearance of “strychnine-spikes”. Though the structural dissimilarity of the various cortical regions precludes the attribution of these spikes to any one specific structural element in the cortex, the, distribution of the spikes over the cortex differs with difference in architectonic structure of the regions strychninized. Local strychninization of any one or two square millimeters of area 4 of any subdivision of the sensori-motor cortex “fires” the whole of area 4 of this subdivision and also its postcentral portion. Outside this subdivision no spikes appear. This is evidenced by the accompanying figure obtained before and after strychninization of one square millimeter of arm area 4. Since the action potentials were taken from arm area 4 at a place as far removed as possible (12 mm.) from the locus of strychninization, the figure illustrates the occurrence of spikes throughout this area. Although the electrodes on the face area were only 2 mm. from those on the arm area, no spikes appear in the record from the face area; and although the electrodes on the leg area were only 2 mm. from the locus of strychninization in the arm area, no spikes appear in the record from the leg area.

Kritisches und Experimentelles zur Deutung der Potentialschwankungen des Elektrocorticogramms

1. Das Elektrocorticogramm (ECG) einer durch schichtweise Thermokoagulation (imchronischen Versuch) auf die zwei inneren Schichten „reduzierten“ Rinde ist augenscheinlich normal, d. h. nicht von dem ECG der normalen Rinde zu unterscheiden. 2. Eine Analyse der Strychninzacken im ECG in den verschiedenen Stadien der Vergiftung bei lokaler Strychninvergiftung der Rinde wird gegeben. 3. Die AuffassungBergers, das seine α- und β-Wellen des EEG in verschiedenen Schichten zustande kommen, wird auf Grund dieser neuen experimentellen Ergebnisse und aus theoretischen Grunden zuruckgewiesen.

Changes of Hydrogen Ion Concentration of the Cerebral Cortex

Using a glass electrode (of the Maclnnes type) with an active area of less than 0.5 mm. 2 , in conjunction with the microvoltmeter recently described by Burr, Lane and Nims (1936), having a grid-leak of 100 megohms, it is possible to measure the hydrogen ion concentration in physico-chemical systems to ±.002 pH. The same apparatus is applicable to biological systems in vivo. In the present instance it was used for a study of the pH of the cerebral cortex. One glass electrode and 2 Ag-AgCl saline-wick electrodes were placed as close together as possible (circa 2 mm.) on a selected area of the cerebral cortex of the animal. The potential difference between the wick electrodes was measured with a microvoltmeter and a Leeds and Northrup galvanometer (No. 2420). The E.m.f. between the glass electrode and either wick electrode was measured with the modified microvoltmeter and a similar galvanometer, in conjunction with a portable Leeds and Northrup potentiometer. The apparatus was so adjusted that the variations in these voltage differences could be recorded photographically with a moving-paper camera through its F/1.25 anastigmatic lens (focal distance 5 cm.). The 2 wick electrodes were placed on the cortex to determine whether or not potential gradients were so large or unstable as to invalidate a pH-measurement. The difference of potential between the glass electrode and either wick electrode can be correlated with a pH by standardization in buffers of known pH. In this preliminary note we wish to confine ourselves to a few of the results thus far obtained. 1. The D.C. potential gradients are small enough to be neglected in estimating the pH to ±.05, and stable enough throughout an experiment to permit differential measurements of pH to ±.005, a precision more than sufficient for the measurements in question. 2. In the curarized animal (monkey, cat) under constant artificial respiration the indicated pH on the cortex is constant. Increase of ventilation produces a shift towards the alkaline side (see figure), decrease of ventilation one towards the acid side. In fact, it has been possible to maintain the pH on the cortex at any specified level compatible with life by proper adjustment of the ventilation. 3. Intravenous injection of sodium bicarbonate produces a shift towards the alkaline side, of hydrochloric acid towards the acid side, of Ringer-solution no comparable effect. 4. Thermocoagulation (at 80°C. for 5 seconds) of a small area of the cortex renders this area acid (e. g., pH = 6.6) with resgect to the adjacent normal cortex (e. g., pH = 7.3). This acidity slowly increases during both the initial local vasoconstriction and the subsequent local vasodilatation and oedema of the thermocoagulated area. From these findings we feel justified in concluding that with this method one measures pH, that, though the condition of the blood circulating through the cortex affects the indicated pH, this pH is that of the transudate on the surface of the cortex immediately subjacent to the glass electrode, and finally that the pH of this transudate is largely determined by the condition of that portion of the cortex, rather than merely reflecting its vascularity. 5. Changes in pH of the cortex produce changes in its “spontaneous” electrical activity, a low pH being associated with low electrical activity, a high pH with high activity.