The science of life : fully illustrated in tone and line and including many diagrams, стр. 804

BOOK 8

the equivalent human process, whenever we learn that although X has a certain meaning, something else rather like it has not, we undergo what is at bottom an inhibitory process, like these processes that we are studying in dogs.

As another variant of this discriminating education, the animal can be taught to distinguish between combinations of stimuli. For example, it can learn that a whistle by itself means food, but that the same whistle accompanied by the simultaneous flash of a light means nothing. This evidently corresponds to our learning that what does in a particular set of circumstances will not do if certain added conditions are present. It may be tempting to skate on a frozen pond, but not if a dangerboard is added to the scene. The same fundamental process of selective blottingout is at work.

An inhibitory process, fundamentally identical with the others but having rather a different effect, appeared in the so-called “delayed conditioned reflexes.” A dog has learnt to expect food at some signal —say, at the flash of a bright light. Then the interval between the flash and the feeding is slowly lengthened, from day to day, until it reaches two or three minutes. At first the dog begins to salivate and lick its lips as soon as the flash appears, but gradually it learns to bide its time. When the lesson is completely learnt it shows no sign of response at all for about a minute and a half or two minutes after the flash—that is, until shortly before the food appearsand then it begins to show the usual signs of expectation. The pause between the flash and the signs of expectation here is really a period of suspense, of internal conflict ; the flash has produced a tendency to respond, but a simultaneous inhibitory process has been evoked which holds the nerve-centres concerned in check for most of the regular interval between signal and meal.

The most obvious human parallel is the learning of movements which have to be properly timed—such as changing the gears of an automobile, or judging the bounce of a tennis-ball and striking it at the correct moment.

In the experiments that we have just discussed the inhibition concerns conditioned reflexes. But even inborn reflexes can be inhibited as a result of training. Boxers learn to keep their eyes open, even when an opponent’s glove jerks towards them, by inhibiting the natural protective blink. A

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THE SCIENCE OF LIFE

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striking demonstration of this power of education was given in Pavlov’s laboratory when a dog was taught to regard an electric shock as a dinner-gong. At first the shock was so weak as to be a barely perceptible tickle, but as the training progressed it was stiffened up stage by stage, until an astonishing state of affairs was reached. The application of a surprisingly powerful shock produced no sign of pain or displeasure, not even the quickening of pulse-rate and breathing that usually accompanies an unpleasant surprise, but was followed by mouth-watering, tail-wagging and the other physical accompaniments of expectant appetite. In other dogs a prick of the skin, deep enough to draw blood, was robbed of its terrors and became an apparently welcome signal of approaching food. We see the same kind of thing in the keen cricketer who does not notice the sting of the ball he catches, or the medical student who overcomes his or her natural aversion to dissection.

These experiments of Pavlov’s show very clearly how the reactions of the brain to the stimulations that are constantly streaming in from the sense-organs are controlled by a set of delicately adjusted inhibitory processes. One of the most striking peculiarities of these inhibitions is their extreme sensitiveness. They are anything but blunt and brutal restraints. They are fragile. If, for instance, there is any extraneous disturbance during one of these experiments (such as the noise of people talking in the corridor outside the room, or a faint whiff of some strange odour in the air) the inhibitions are the first things to suffer.

If the animal so disturbed happens to be undergoing an extinction experiment, and has already got some reflex partly or wholly extinct, this disturbance will upset all the newly-acquired restraint. The animal may revert to the old responsiveness even when its reactions have almost fallen to zero. Orifa discrimination experiment is in progress the dog will go back upon the discrimination it has learnt and respond to the wrong notes or combinations of stimuli as well as the right ones. Or again, this effect of disturbance in diminishing inhibition is very clearly shown in the case of a “delayed * conditioned reflex, in which two or three minutes intervene between signal and meal. Suppose that during this period, in which the animal waits and withholds its response, there is some slight distraction, its effect is to produce immediate salivation and appetite signs. The pause,