The science of life : fully illustrated in tone and line and including many diagrams, стр. 805

THE CORTEX AT WORK

as we have seen, is a period of internal struggle between excitation and inhibition, and the distraction weakens the inhibition so that the response breaks through. A very strong extraneous disturbance, however, has the opposite effect to a moderate one. It does not simply extinguish the inhibition. It goes deeper. It interrupts the whole conditioned reflex, and so abolishes the positive response.

Besides such adventitious distractions as noises and stray whiffs, there are physiological ones, such as a sore on the skin or a full bladder (for Pavlov’s dogs had been trained to propriety on this matter, as men are). These also upset the inhibitory side of the balance before they disturb the positive responses. The name given by Pavlov to such an interruption of inhibition, whether it springs from external or internal causes, is “ disinhibition.”

These experimental results are evidently of the first importance. They are providing us with a new language, in which we can express all sorts of phenomena of everyday mental life in terms of analysable and measurable processes. The distracting effect of a barrel-organ in the street, the faulty timing of a tennis player who is “ put off his game” by the barracking of the crowd, the general irritability and weakening of control that a toothache produces—a hundred kinds of restlessness and worry can be interpreted in terms of disinhibition.

Perhaps this difference in sensitiveness between excitation and inhibition, so that inhibition is the first to go, explains the action of small doses of alcohol, which elates by weakening the restraining processes. It has already been shown experimentally that caffeine, the essential drug of coffee and tea, sways the balance in favour of excitation. It thus, but by a different method, releases impulses from inhibition and extinguishes discriminations. In a dog taught that a touch on the paw meant food while a touch on the back meant nothing, injection of caffeine weakened the differential inhibition so that the animal responded by salivation to a touch in either place.

A complicated thing, this thinking brain of ours! We can get a rough, very metaphorical picture of its activity by returning to the architectural details which were set forth in the last chapter. The fibres which stream up to the hemispheres spread out, we saw, in such a way that various parts of the body are connected to and represented by various parts of the cortex. ‘There is a sight-centre, a hearing-centre, a movement-

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centre, and so on, and these main areas are more minutely subdivided. We all come into the world with a plan of our bodies mapped out on our brains in this fashion. Over the surface of that plan, while we are alive, there is a continual play of excitation and inhibition, of activity and restraint, like the shimmer of light and shade on the underside of a bridge. Some minds, we shall note, are naturally brighter than others; they have a preponderance of excitatory over inhibitory processes. Others are darker, being more reserved and controlled. But we live and learn, and as we do so the first crude plan with which we started gets marked and written over. As our conditioned reflexes develop, an intricate network of connecting paths between point and point spreads over it, and as we inhibit and discriminate a further pattern condenses on this web, some groups of cells becoming permanently excitatoryin tendency and some inhibitory, according to the meaning of the impulses which come along their fibrous connections. Pavlov likens the cerebral cortex in this stage to a patchwork, and speaks of it as a “‘ mosaic of functions.” This mosaic is the record of our individual lives, the form in which our experience and our little share of wisdom are engraved. But it is a living mosaic. It can be revised and altered, its detail can be made finer and more precise, as it both guides and is moulded by the rippling excitations which run over its surface and well up from below.

Pavlov’s investigations into these two interacting processes of our brains go much deeper than does this descriptive introduction. The fundamental empirical laws governing their interactions are being worked out, and already generalizations are emerging which explain many of the known facts. Thus it seems (and the cases of inhibition already described illustrate the point) that whenever an experimental stimulus falls alone on the cortex, unsupported by any natural reflex such as feeding, it will sooner or later become inhibitory. That is an example of the simple laws into which our mental processes are being translated. But the details of the analyses are too complicated to be set forth heres

We may, however, mention very briefly the following fact. If any point on the cortex is in a state of excitation, its activity tends to inhibit the activity of other parts of the brain-surface ; and, conversely, an area of inhibition tends to create excitation and more active response elsewhere. (This phenomenon, which is called “ induction,”

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