The science of life : fully illustrated in tone and line and including many diagrams, стр. 808

BOOK 8

that inhibition is a process which spreads. It radiates out into the adjacent cortex from its centre of activity. Ifa conditioned reflex is extinguished, or if a discrimination is applied, the inhibitory process is not confined to the particular cell-groups in the brain that are immediately concerned, but tends to invade other neighbouring cell-groups, too, and even to diffuse itself widely through the brain, Butit has to fightits way. Opposing this tendency there are antagonistic forces, so that the extent to which the spread occurs and the time for which it lasts depend on the interaction of a number of circumstances. A concrete example of the effects observed will make the idea clearer. Five points were chosen on the hind-leg of a dog and given numbers in order from the paw to the thigh—o, 1, 2, 3, and 4 (Fig. 325).

Fig. 325.

The places 1, 2, 3, and 4 were used as food-signals, one of them being touched whenever a meal was impending, and soon the animal responded by signs of appetite to such a stimulus. But the place o was differentiated from the rest, and the dog learnt not to expect anything when touched on the paw.

This differentiation, like any other discrimination, depended on inhibitory processes in the dog’s brain. Now suppose that o was touched, and immediately afterwards one of the other places, what happened ? Touching o set up an inhibition ; and it was found that the second touch, on one of the positive places, did not bring about such a marked response as usual. The inhibition had spread in the cortex from the cells connected with the paw to the neighbouring cells connected with the leg. Moreover,

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THE SCIENCE OF LIFE

Lo illustrate the experiment on inhibition described in the text.

CHAP TER 6

by comparing the results of a long series of experiments, the spreading out of this wave of inhibition through the brain could be followed in some detail. It was found that the inhibitory process weakens as it diffuses itself, being strongest at 1 and weakest at 4, and that it is opposed and finally overcome by antagonistic forces, so that in a few minutes it recedes and is swept back to its point of origin.

In other animals events were followed up in other ways. It was found that the effect might extend even to quite remote paris of the brain-cortex. After extinguishing a conditioned reflex to a flashing lamp it was possible to detect a slight and fleeting weakening of another to a musical note, so that the inhibition had spread from the sight-centre to the hearing-centre (Fig. 321).

The neighbouring areas of the brain-surface can be affected after only a single application of an inhibitory stimulus. But suppose the thing repeated, suppose that one starts a number of consecutive inhibition-waves, what will result? The spread will be stronger and wider, the inhibition of the activating forces more and more effective. Finally the whole cortex may be inhibited and even the lower reflex centres concerned with maintaining the position of the body may be invaded. The animal will then become motionless, limp, unresponding—in a word, asleep.

Herein is the answer to a question which has probably arisen already in the reader’s mind. If inhibition is such an important process in the active brain, responsible for the most delicate adjustments that it performs, how is it that in these experiments inhibition so often brings about a drowsy, sluggish state of the animal? To reply in Pavlov’s own words, ‘* To my mind all the facts which have been given in the preceding lectures dispose at once of the apparent contradiction. Internal inhibition during the alert state is nothing but a scattered sleep, sleep of separate groups of cellular structures ; and sleep itself is nothing but internal inhibition which is widely irradiated, extending over the whole mass of the hemispheres, and involving the lower centres of the brain as well.”

Inhibition is sleep controlled and localized. Sleep is inhibition at large. Under the artificial conditions of the experiments the dogs were subjected to exceptionally uniform