The science of life : fully illustrated in tone and line and including many diagrams, стр. 806

BOOK 8

interacts with and opposes a tendency of excitation and inhibition to spread about in the brain, from part to part, that we shall presently discuss.) In this, the experiments point towards an explanation of a number of states in which one part of the brain is excited and holds others in check—such as concentration, attention, and obsession.

$5 The World of a Dog

Besides the light thrown upon our own mental processes, this work on conditioned reflexes enables us to probe into many niatters that were once mysteries of the dogmind. It is possible to learn how much this animal hears, smells, and feels, and to realize how far its world is like our own. Does it see things as we see them and hear what we hear ?

The method of study, which developed out of the discrimination experiments just described, is not difficult to understand. In one case a dog was taught to expect food whenever a black screen was held before it, but not when a white screen was similarly exposed. After the lesson had been learnt and the distinction mastered, a pale grey screen was presented instead of the white one. ‘This was found to make little difference to the animal ; black was still a food-signal and grey had no effect. Day by day, the grey screen was made darker and darker, in order to find out at what stage the animal would begin to confuse the two and mistake the gradually darkened grey for a food-signal. The result was surprising. It was found that the dog had far better powers of distinguishing shades than the experimenters could claim. Ultimately, there were two screens, both of which looked black to the human eye, but which were obviously perfectly distinct to the dog; one of them produced copious salivation whenever it appeared, while the other had little or no effect. But in other respects the canine eye was found inferior to the human. It cannot distinguish shapes as clearly as the human eye can, and (when they had excluded differences in sheer brightness) the investigators were unable to get any unequivocal evidence of colour vision in their animals. The dog, it would seem, looks out on a black-and-white world rather like the world that the cinematograph presents; the projector is rather out of focus so that the shapes are blurry, but there is a much more delicate appreciation of luminous and shadowy patches than we can ever experience.

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CHAPTER 6

When the - experimenters turned their attention to the dog’s hearing they found a marked superiority to that of humanity. The animals could distinguish variations in the loudness and softness of a note far more delicately than any man, and they had a truly wonderful power of judging rhythms ; one of the dogs discriminated without difficulty between a metronome ticking at one hundred beats to the minute and another at ninety-six. Dogs can also hear notes which are so high in pitch as to be beyond the range of the human ear, as Galton showed many years ago. Equipped with a Galton whistle, you can go for a walk, whistle for your dog, and he will come running, though neither you nor any other human being can hear a _ sound. Pavlov has built up conditioned reflexes on the basis of these apparently inaudible notes.

It would be fascinating to use these methods for exploring the canine nosefor our powers of smell are as nothing compared with a dog’s—but, unhappily, little has been done in this direction. The difficulty in this case is a technical one ; the problem of how to generate smells of known and constant intensity has yet to be solved.

But these suggestive investigations into the world as it appears to a dog are only a side-branch from the main trend of Pavlov’s work. Let us return to the study of inhibition, for it is there that the mostilluminating hints are found to help in our study of the human mind.

§ 6 Boredom, Alertness, and Sleep

While the experiments on inhibition were being conducted, it was noticed that the animals often became curiously dull and drowsy. Perhaps in the early stages of an investigation all had gone briskly and well, the dog being alert and mastering its lessons without difficulty ; but when experiments were begun which involved considerable inhibition—such as the learning of a subile distinction between similar stimuli—the drowsiness made its appearance. (Can we not remember as a human parallel how Master Everyboy is bright and happy at his play, how quickly he gets drowsy at his lessons, how quickly he wakes up again when he is freed, and how suddenly he is sobered when his conditioned or natural reflexes are corrected by parental ‘ don’ts * ?)

Some dogs showed this effect more than others ; one or two bad learners even went