The Excitable Cortex ol i\w ( 'hiiiijian/.t-e, ( )iaii<(-rian. and ( lorilla i:i9 



And it was clear, in our experieiiee. that the motor cortex of an 

 individual heinibphere and of both hemispheres in one individual does not. 

 as its surface is j^one over point for point in a systematic localisation 

 experiment, yield th(> whole series of movements that can he yielded hy 

 similar examination of a series, even a small series, of hennspheres. 

 Movements will api>ear in one hemisphere which do not appear in another, 

 or. puttintj it in another way. will appear in one experiment which do 

 not appear in another experiment. We think that this is probably 

 lari;ely owing to "facilitation." When the motor cortex in any individual 

 hemisphere is systematically explovi-d point to point by the electrode, 

 particular motor respon.ses when once evolved tend to reappeai- from 

 adjacent cortical points. These cortical points form i^roups, each <^roup 

 occupyinij a small cortical area whence the same motor resj)onse is elicited. 

 •Such an area is probably partly the result of the facilitation exercised in 

 regard to the response characteristic of it by the influence of one point 

 upon another in it. This facilitation of one response would act in the 

 direction of I'estricting the appearance of other responses which neverthe- 

 less might be latent in the cortex : it would tend to deviate the response 

 (v. infra). 



Functional Instability of Cortical Motor Points. 



This raises the (|uestion of the functional instability of a motor cortical 

 point (5). In addition to the influence of depth of narcosis, freedom of 

 blood supply, local temperature, and such effects of experimental exposure 

 of the cortex as " drying "' or inspissation of applied Locke's solution, the 

 motor responses of a cortical point may be easily and greatly modified by 

 precurrent, especially closely precurrent. stimulation either of itself or 

 of neighbouring, especially closely adjacent, cortical points. The motor 

 response from a given point, though it may, as the maps of cortical localisa- 

 tion usually depict, remain approximately the same throughout a lengthy 

 experiment, even from hour to hour, when similar stimuli are repeated at 

 intervals not too brief, may yet vary considerably in result of precurrent 

 stimulations not too distant in time and place. Experiments in which a 

 large field of cortex is examined systematically point for point by electrical 

 .stimulation to determine the functional localisation are likely to display 

 the influence of previous stimulation of one point upon another. 



Three phenomena of this kind, presumably all closely akin, make them- 

 selves evident in an examination of the motor cortex, namely, facilita- 

 tion of response, reversal of response, and deviation of response. 

 Of these the first, noted by various observers (e.g. 15), and particularly 

 fully studied recently by T. Graham Brown (3, 4) in the chimpanzee as 

 well as in macacus and other monkeys, is characterised by a change of the 

 cortical point's response in the direction of increase, with or without other 

 modification. It may be induced by stimulation of the point itself or by 

 stimulation of other points. Reversal of response (5) is a change super- 



