The science of life : fully illustrated in tone and line and including many diagrams, стр. 807

THE CORTEX AT WORK

right off to sleep during the work. This somnolence, we may note, was one of the chief obstacles to that exploration of the dog-mind that we have just discussed. At first it was simply .a nuisance that held up the experiments. But gradually it gathered an interest of its own.

Another way in which the dogs could be made sleepy was by means of unfamiliar stimuli. Some unusual noise, let us say, is sounded at fairly frequent intervals without any painful or pleasurable accompaniment such as feeding. At first, the dog’s curiosity is aroused ; it pricks up its ears and turns its head in the direction of the new sound. But as time after time the novelty is unattended by exciting consequences the animal loses its interest ; the responses get weaker and more fleeting until they vanish altogether. Now this, it is worth noting, is a process of inhibition, like the ‘“‘ extinction ” of a conditioned reflex. The pricking up of the ears and turning of the head towards a sudden sound is an inborn reflex, just as is the withdrawal of the hand when it touches a burning cigarette; the reader has the same reflex as part of his or her makeup, and tends to glance automatically in the direction of any abrupt, unfamiliar noise. A dog whose cerebral hemispheres have been removed will do it too. But the loss of interest in a persistent and meaningless stimulus is a higher phenomenon, an active inhibitory process that occurs in the cortex ; and the most striking proof of this is that the dog without cerebral hemispheres never gets used to the interruption but gives a vigorous “‘ curiosity reflex’ time after time, no matter how often the meaningless noise is repeated.

To put it in another way, the boredom that a tediously reiterated stimulus inflicts is a sign of activity in the highest part of the brain. It is a protective device. Were it not for this gift of selective indifference, the organism would be everlastingly distracted by meaningless stimuli and _everlastingly giving the “ curiosity reflex.” It is not enough to note what is important ; one must also ignore what is inessential.

Another common fact of everyday experience fits in here, for, like other forms of inhibition, this contempt bred of familiarity can be disinhibited. ‘The dog has ceased to be interested in the experimental noise ; but now if it is suddenly distracted by some further novelty—a photographer’s flash, let us say—it will respond again to the old stimulus as long as the disturbance produced by the new endures in its brain. ‘The reader

has probably noticed that when one is startled one becomes aware of all sorts of little things—the crepitation of a fire, the ticking of a clock, the sound of one’s own movements—which had previously been set down in the hemispheres as unimportant.

But we have strayed somewhat from our argument. It was with the sleepiness that appeared in inhibition experiments that we were concerned and the reason why this particular inhibition of a natural reflex interests us now is that it also has a soporific effect. The dog, like the reader, gets drowsy when it is bored. If the occasional noise is continued after the time when it ceases to call forth a “curiosity reflex,” and if the other surroundings are quiet, the animal first becomes dull and still and then falls fast asleep. Even conspicuously alert and lively animals can be thus depressed. We may note here, as another fact which presents an obvious parallel to our own species, that puppies are far more easily bored to the drowsy stage than grown dogs.

It was soon made clear that this sleepiness was not a simple exhaustion effect. It was not that the dogs were tired out by the problems presented to them. In all the cases that we have described the process of getting drowsy, strange though it may seem, was an active process just as inhibition is. That is a novel result which runs counter to our common preconceptions about sleepiness. Gradually Pavloy’s investigators became more and more convinced that the two phenomena, inhibition and sleep, were closely related to each other. Sometimes, in exceptional instances, they were even interchangeable. There was an animal which had been taught to expect food at a particular signal. ‘Then the interval between signalling and feeding was lengthened by degrees to three minutes. In most dogs, as we have already seen, the result would be as follows : the signal would be recognized, but it would initiate an inhibitory process in the brain so that there would be no visible response for, say, two minutes; then the mouthwatering and other signs of appetite would appear. But in this particular animal sleep took the place of inhibition. Whenever the signal was given the animal went promptly to sleep, its eyes shut, its muscles relaxed. It even emitted an occasional snore. Then, after a couple of minutes, it woke up suddenly and gave the usual signs of confident anticipation of the coming meal.

The Pavlov investigators have accumulated a wealth of detailed evidence to show

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