[Ill.u.s.tration: Fig. 61.--Combining clutch-response with brake-response. At first, the brake-stimulus has only a weak linkage with the clutch-response, and an extra stimulus has to be found to secure the clutch-response. But whenever the clutch-response is made while the brake-stimulus is acting, the weak linkage between these two is exercised, till finally the brake-stimulus is sufficient to give the clutch-response, along with the brake-response.]

The combination of two responses is effected by linking both to the same stimulus; thus the two become united into a coordinated higher motor unit.

2. The word-habit in typewriting furnishes an example of _successive coordination_, the uniting of a sequence of movements into a higher unit. [Footnote: See p. 324.] The beginner has to spell out {412} the word he is writing, and make a separate response to each letter; but when he has well mastered the letter-habits, and, still unsatisfied, is trying for more speed, it happens that he thinks ahead while writing the first letter of a word, and _prepares_ for the second letter. In effect, he commences reacting to the second letter while still writing the first. This goes further, till he antic.i.p.ates the series of letters forming a short word while still at the beginning of the word. The letter movements are thus linked to the thought of the word as a whole, and the word becomes an effective stimulus for arousing the series of letter movements.

[Ill.u.s.tration: Fig. 62.--Learning a word-habit in typewriting. At first, besides the stimulus of the word, "_and_" it is necessary also to have the stimulus "_a_" in order to arouse the response of writing a, the stimulus "_n_" in order to arouse the writing of n, and the stimulus "_d_" in order to arouse the writing of d. Yet the stimulus "_and_" is present all this time, and its weak linkages with the writing movements are used and strengthened, so that finally it is sufficient, by itself, to arouse the whole series of writing movements.]

Many other instances of learning can be worked out in the same way, and there seems to be no difficulty in {413} interpreting any of them by the law of combination. Even "negative adaptation" can possibly be interpreted as an instance of subst.i.tute response; some slight and easy response may be subst.i.tuted for the avoiding reaction or the attentive reaction that an unimportant stimulus at first arouses, these reactions being rather a nuisance when they are unnecessary. On the whole, the law of combination seems to fill the bill very well. It explains what the law of exercise left unexplained. It always brings in the law of exercise as an ally, and, in explaining subst.i.tute response, it brings in the law of effect, which however, as we saw before, may be a sub-law under the law of exercise. These two, or three laws, taken together, give an adequate a.n.a.lysis of the whole process of learning.

The Law of Combination in Recall

Unitary response to multiple stimuli is important in recall as well as in learning. The clearest case of this is afforded by "controlled a.s.sociation". [Footnote: See p. 381.]

In an opposites test, the response to the stimulus word "long" is aroused partly by this stimulus word, and partly by the "mental set"

for opposites. There are two lines of influence, converging upon the response, "long--short" (of which only the word "short" may be spoken): one line from the stimulus word "long", and the other from the mental set for pairs of opposite words. The mental set for opposites tends to arouse any pair of opposites; the word "long" tends to arouse any previously observed group of words of which "long" is a part. The mental set, an internal stimulus, and the stimulus word coming from outside, converge or combine to arouse one particular response.

The mental set for adding has previously exercised {414} linkages with the responses composing the addition table, while the mental set for multiplication has linkages with the responses composing the multiplication table. When the set for adding is active, a pair of numbers, seen or heard, together with this internal stimulus of the mental set, arouses the response that gives the sum; but when the multiplying set is active, the same pair of numbers gives the product as the response. All thinking towards any goal is a similar instance of the law of combination.

The Laws of Learning in Terms of the Neurone

We have good evidence that the brain is concerned in learning and retention. Loss of some of the cortex through injury often brings loss of learned reactions, and the kind of reactions lost differs with the part of the cortex affected. Injury in the occipital lobe brings loss of visual knowledge, and injury in the neighborhood of the auditory sense-center brings loss of auditory knowledge.

Injury to the retina or optic nerve, occurring early in life, results in an under-development of the cortex in the occipital lobe. The nerve cells remain small and their dendrites few and meager, because they have not received their normal amount of exercise through stimulation from the eye.

Exercise, then, has the same general effect on neurones that it has on muscles; it causes them to grow and it probably also improves their internal condition so that they act more readily and more strongly.

The growth, in the cortex, of dendrites and of the end-brushes of axons that interlace with the dendrites, must improve the synapses between one neurone and another, and thus make better conduction paths between one part of the cortex and another, and also between the cortex and the lower sensory and motor centers.

The law of exercise has thus a very definite meaning when {415} translated into neural terms. It means that the synapses between stimulus and response are so improved, when traversed by nerve currents in the making of a reaction, that nerve currents can get across them more easily the next time.

[Ill.u.s.tration: Fig. 63.--The law of exercise in terms of synapse. A nerve current is supposed to pa.s.s along this pair of neurones in the direction of the arrow. Every time it pa.s.ses, it exercises the end-brush and dendrites at the synapse (for the "pa.s.sage of a nerve current" really means activity on the part of the neurones through which it pa.s.ses), and the after-effect of this exercise is growth of the exercised parts, and consequent improvement of the synapse as a linkage between one neurone and the other. Repeated exercise may probably bring a synapse from a very loose condition to a state of close interweaving and excellent power of transmitting the nerve current.]

The more a synapse is used, the better synapse it becomes, and the better linkage it provides between some stimulus and some response.

The cortex is the place where linkages are made in the process of learning, and it is there also that forgetting, or atrophy, takes place through disuse. Exercise makes a synapse closer, disuse lets it relapse into a loose and poorly conducting state.

The law of combination, also, is readily translated into {416} neural terms. The "pre-existing loose linkages" which it a.s.sumed to exist undoubtedly do exist in the form of "a.s.sociation fibers" extending in vast numbers from any one part of the cortex to many other parts.

These fibers are provided by native const.i.tution, but probably terminate rather loosely in the cortex until exercise has developed them. They may be compared to telephone wires laid down in the cables through the streets and extending into the houses, but still requiring a little fine work to attach them properly to the telephone instruments.

[Ill.u.s.tration: Fig. 64.--Diagram for the learning of the name of an object, transformed into a neural diagram. The vocal movement of saying the name is made in response to the auditory stimulus of hearing the name, but when the neurone in the "speech center" is thus made active, it takes up current also from the axon that reaches it from the visual center, even though the synapse between this axon and the speech neurone is far from close. This particular synapse between the visual and the speech centers, being thus exercised, is left in an improved condition. Each neurone in the diagram represents hundreds in the brain, for brain activities are carried on by companies and regiments of neurones. (Figure text: object seen, visual center name heard, auditory center, speech center, name spoken)]

The diagrams ill.u.s.trating different cases under the law of combination can easily be perfected into neural diagrams, though, to be sure, any diagram is ultra-simple as compared with the great number of neurones that take part in even a simple reaction.

The reader will be curious to know now much of this neural interpretation of our psychological laws is observed fact, and how much speculation. Well, we cannot as yet {417} observe the brain mechanism in actual operation--not in any detail. We have good evidence, as already outlined, for growth of the neurones and their branches through exercise.

[Ill.u.s.tration: Fig. 65.--Control, in multiplying. The visual stimulus of two numbers in a little column, has preformed linkages both with the adding response and with that of multiplying. But the mental set for adding being inactive at the moment, and that for multiplying active (because the subject means to multiply), the multiplying response is facilitated.]

We have perfectly good evidence of the law of "unitary response to multiple stimuli" from the physiological study of reflex action; and we have perfectly good anatomical evidence of the convergence and divergence of neural paths of connection, as required by the law of combination. The a.s.sociation fibers extending from one part to another of the cortex are an anatomical fact. [Footnote: See p. 56.]

Facilitation is a fact, and that means that a stimulus which could not of itself arouse a response can cooperate with another stimulus that has a direct connection with that response, and reinforce its effect.

In short, all the elements required for a neural law of combination are known facts, and the only matter of doubt is whether we have built these elements together aright in our interpretation. It is not pure speculation, by any means.

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EXERCISES

1. Outline the chapter, in the form of a list of laws and sub-laws.

2. Review the instances of learning cited in Chapters XIII-XV, and examine whether they are covered and sufficiently accounted for by the general laws given in the present chapter.

3. Draw diagrams, like those given in this chapter, for the simpler cases, at least, that you have considered in question 2.

4. Show that response by a.n.a.logy is important in the development of language. Consider metaphor, for example, and slang, and the using of an old word in a new sense (as in the case of 'rail-road').

REFERENCES

William James devoted much thought to the problem of the mechanism of learning, habit, a.s.sociation, etc., and his conclusions are set forth in several pa.s.sages in his _Principles of Psychology_, 1890, Vol. I, pp. 104-112, 554-594, and Vol. II, pp. 578-592.

Another serious consideration of the matter is given by William McDougall in his _Physiological Psychology_, 1905, Chapters VII and VIII.

See also Thorndike's _Educational Psychology, Briefer Course_, 1914, Chapter VI.

On the whole subject of a.s.sociation, see Howard C. Warren, _A History of the a.s.sociation Psychology_, 1921.

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CHAPTER XVII

PERCEPTION

MENTAL LIFE CONSISTS LARGELY IN THE DISCOVERY OF FACTS NEW TO THE INDIVIDUAL, AND IN THE RE-DISCOVERY OF FACTS PREVIOUSLY OBSERVED

You will remember the case of John Doe, who was brought before us for judgment on his behavior, as to how far it was native and how far acquired. We have since that time been occupied in hearing evidence on the case, and after mature consideration have reached a decision which we may formulate as follows: that this man's behavior is primarily instinctive or native, but that new attachments of stimulus and response, and new combinations of responses, acquired in the process of learning, have furnished him with such an a.s.sortment of habits and skilled acts of all sorts that we can scarcely identify any longer the native reactions out of which his whole behavior is built. That decision being reached, we are still not ready to turn the prisoner loose, but wish to keep him under observation for a while longer, in order to see what use he makes of this vast stock of native and acquired reactions. We wish to know how an individual, so equipped, behaves from day to day, and meets the exigencies of life. Such, in brief, is the task we have still before us.