The Fundamental Laws of Human Behavior
SEVENTH LECTURE
Max Meyer
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Learning. The susceptibility of nervous conductors. Variations of the nervous path. (1) Two kinds of variation of response. (2) Sensory condensation. (3) Motor condensation. Graceful motion. Inhibition. How a child comes to fear a fire.
THUS far we have spoken of animal activities only under two heads, as reflexes, and as selected groups of reflexes, that is, instincts. Both these kinds of activities may be given a, common name. We may call them hereditary activities, because the characteristic response following each particular stimulation is in these cases completely determined by the strictly biological inheritance of the individual from his ancestors,— is in no way shaped by the geographical and social inheritance resulting from the fact that the individual has been given by his ancestors life and the training of his youth in a particular geologic-physical and social environment. This shaping during life of the individual's manner of acting may be referred to by the word "learning," or by the word "habit." We are, perhaps, inclined to use the word "learning" chiefly in connection with social institutions, "schools" of all kinds, "habit" chiefly in connection with altogether fortuitous circumstances. For us, however, in [.his treatise, these words are synonymous. Regarded as phases in the behavior of the human or animal individual,
(86) acquiring a habit and learning are identical. In each animal we find — the higher the species to which the individual belongs, the more numerous—learned or habitual, in addition to hereditary, activities. Our next task is the description of the changes which hereditary nervous activities undergo in order to become habit,—the description of the process of learning, or rather, of the various kinds of processes of learning.
It is plain that any shaping during life of an individual's sensori-motor activity presupposes: first, that there is a variation in the succession of sensory excitation and motor response, and secondly, that this variation is fixed and thus caused to reappear. The variation is possible only if the main flux of the excitation once takes a path different from the one which is to be expected according to the shortest hereditary connection of sensory and motor points. The fixation of this path is possible only if (we might also say: fixation means that) nervous conductors are in some way susceptible to excitations traveling through them, so that one of the reasons why an excitation may be thought to choose a particular path, is simply this, that it has once before taken this path. Of course this statement may, and must, be made more definite with respect to our assumption that the main path of the nervous flux is the path of least resistance. We must say, then, that the susceptibility of nervous conductors consists in their resistance being reduced by a flux occurring in them,—this resistance to stay reduced for a considerable time after the flux has terminated. We may add, at once, remembering that the counterpart of learning is forgetting and that forgetting is a gradual process: that we have the right to assume that the reduced resistance of any nervous conductor slowly rises again to its original measure.
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The question then is: do the pictures of streaming which we have used in order to have before our mind a clear idea of nervous activity and its peculiarities, aid us also in imagining the susceptibility of a neuron as one of its natural properties? It seems that this question can be answered unhesitatingly in the affirmative. We are hardly more familiar with any other natural event than that of a stream broadening, washing out its channel; and a dry river bed being gradually obliterated by being filled with debris and dust is not a very unusual phenomenon either. Our picture of nervous activity, with regard to the susceptibility of conductors, is therefore quite consistent . The question is then left, if it is also consistent with regard to the possibility of a variation of the main I r 1h along which the excitation is conducted. Now, this question has been answered already, in our discussion of instinct. We said that an instinct is not only a collecting, but also a selecting agency, and we explained the selection of a path for an excitation, by referring to the familiar physical principle underlying the action of a jet-pump. No further assumption is necessary, only a reference to an assumption already made. A variation of the main path taken by an excitation can, accordingly, be the result of a second nervous process deflecting the first by dint of its own greater intensity. That a variation of the nervous path can be brought about also by other factors, without any deflection, will appear later.
We may distinguish several classes of variation of the nervous path. If the stimulation as well as the motor response is so simple that the whole nervous activity can be called a reflex, the variation of the nervous path can consist either in the motor response occurring at a motor point which is not the point corresponding to the sensory point stimulated or in the very same motor response following
( 88) the application of a stimulation to a point which is not the corresponding sensory point. Let us use variation of response as a technical term for this kind of variation of the nervous path. Naturally, we can apply the same term also to cases in which the nervous path is neither at its sensory nor at its motor end quite so simple. Indeed, looking for a striking concrete example, we can hardly help giving one in which the nervous path is complex. A small child, taking beer into his mouth, spits it out, reflexly, or rather instinctively, for the motor activity may be quite complex. Any bitter substance stimulates definite sensory organs of the mouth, and the motor response of the facial and other muscles is that which we call spitting. The bitter substance is thus removed. The large number of people who habitually drink beer can have acquired this habit only by a variation of response taking place and becoming fixed. Once, instead of spitting, the very different motor response of swallowing occurred, as a variation, and this variation, becoming fixed, became the habit of drinking beer. In this example a different response came to follow the same stimulation. As an example of the same response following a different stimulation we may mention this. A baby, during the first three or six weeks of his life, responds to any sudden noise by quickly closing his eyes. It is easy to observe this. One need only clap his hands or whistle in order to see the baby wink. On the other hand, one may closely approach the baby's open eye with a finger or a stick without causing the slightest winking, unless the eye is actually touched. When the baby is a few months old, all this has changed. Noises rarely call forth the response of winking, but when any object is brought near the eye, the latter is closed. This reaction then remains our habit all through our adult life. Once, of course, the variation must have
( 89) occurred for the first time.—the variation of the nervous path which consists in a visual stimulation taking the place of an auditory stimulation, while the motor response remains the same.
If the stimulation is complex and the motor response correspondingly complex, the variation of the nervous path can consist in the complex response being called out by a greatly simplified stimulation, possibly a stimulation of a single sensory point. The habitual nervous activity then becomes similar to an instinct, for a complex reaction in response to a simple stimulation is, as we have seen, characteristic of an instinct. In order to have a brief term for this kind of a variation of the nervous path, let us call it sensory condensation, thus referring to the fact that at the sensory end of the system of nervous conductors the flux (when represented reversely in a diagram) no longer spreads out, but is condensed into a narrow channel.
An example, rather complex in all its aspects, but very familiar and therefore well illustrating our case, is the following. In playing a certain piece of music on the piano, at a particular place in the music each one of several fingers has to perform a definite movement,—what movement, is indicated by as many notes as there are fingers to move. The beginner, in order to strike the correct chord, looks at every note. But after some time of practice, we observe that he plays exactly the same complicated chord even when some of the notes, without his knowledge, have been erased or changed by us. Obviously these notes are no longer needed for the response, and a simpler stimulation now brings about the same motor response. Typewriting, reading, proofreading, weaving, attending to any machine, —any kind of skillful activity can illustrate this same kind of variation of the nervous path. The complex activity is ultimately called forth by a part of the original stimu-
( 90) -lation; sometimes to the detriment of the subject, as when a proofreader overlooks a typographical error, reading the whole word although not all. of the word is there to act on his eyes.
If the motor response is complex (because it is an instinctive response or the stimulation is complex), the variation of the nervous path can consist in the response being greatly simplified, possibly reduced to a response at a single motor point. Thus it would become similar to a reflex response, were it not, perhaps, for the complexity of the stimulation. In order to have a brief term for this kind of variation of the nervous path, let us call it motor condensation, thus referring to the fact that at the motor end of the system of nervous conductors the flux no longer spreads out, but is condensed into a narrow channel. Watch a child receiving his first instruction in writing and you will frequently observe, not merely a moderate activity in the shoulder, wrist, and finger joints, as when an adult is writing, but in addition to an excessive activity in these joints a tense bending of those fingers of the writing hand which do not hold the pen, and also of the fingers of the other hand, even a twisting of the head and the feet as if no writing were possible without them. We say in such a case that the person acts awkwardly. The disappearance of awkwardness is generally equivalent to the dropping of all movements unnecessary for the end in question. A graceful motion is simply a motion no element of which appears to be superfluous. The acquisition of graceful motion means motor condensation.
Of course, our distinction of three classes of variation of the nervous path, variation of response, sensory condensation, and motor condensation, does not imply that each of these variations must occur in separation from the others. On the contrary, we must expect to find
( 91) in actual life usually mixtures of them. As example of such a mixture may serve the behavior of a person falling into a river. If this is his first experience of the kind, the motor response is exceedingly complex. The hands wove about in all directions, but chiefly they are thrown up in wild attempts to catch anything which might be in reach a straw, as the proverb says while the feet make movements which, when slowly executed, might be useful in climbing a tree, but which can serve no very useful purpose in the surrounding fluid. The skillful swimmer, on the other hand, merely makes a few moderate downward strokes with his outstretched arms and hands, and thus reaches the surface and stays there, possibly not moving his feet at all. The variation of the nervous path in this case obviously consists in a variation of response as well as a motor condensation. As a matter of fact, a sensory condensation is also involved, for during the process of learning to swim many stimulations of various sensory points (perhaps a teacher's words and example) are effective, which later become superfluous.
The process of learning, the acquisition of a habit, has a negative aspect of great importance, so far as the motor end of the nervous activity is concerned. What a person does not do, whether he reacts towards his physical or social environment in a definite way or remains inactive in this way, is frequently of the greatest consequence to others. When I am walking along a highway and meet strangers, my chief interest is in not being attacked and robbed. What they positively do, concerns me but little, possibly not at all. How important the negative aspect of human activity is appears from the fact that the great moral law of the decalogue contains almost exclusively negative rules. Our scientific interest, therefore, can not be restricted to the positive aspect of an individual's
( 92) motor activity, It is clear, even from the mere outline of nervous activity as given thus far, that the negative aspect of the motor result of a nervous process can never be a reduction of a nervous excitation to nothing, but only a deviation to a motor point other than the one at which we had a certain right, according to our knowledge of the nervous connections, to expect its motor response. When we are interested in a person's or animal's not doing a certain thing, we use as a technical term the word "inhibition. " From the point of view of this treatise, inhibition means that the individual does something else instead of what we thought or feared he might do, while we do not care what it is that he actually does. From a purely neurological point of view the term inhibition in this sense is superfluous. Nevertheless we shall have to use it because of its great significance from the social point of view, for many social institutions of great. importance, for example all those connected with crime and criminal law, cannot do without this concept of inhibition. The law which inhibits murder by stating "Thou shalt not kill" does not state what we shall do instead of killing. The law giver is not concerned with our action provided it does not consist in killing. But it is of great value to us in any sociological application to be aware of the fact that, at the bottom, the prevention of crime is not a problem of preventing action, but altogether a problem of substituting a socially valuable reaction to a certain stimulation for a socially harmful reaction.
Thus far, we have hardly done more than define the terms, variation of response, with its negative aspect inhibition, sensory condensation, and motor condensation. We must now make clear in detail, in specific instances illustrating these cases, what goes on in the nervous system when the nervous path is modified and this modifica-
( 93) -tion becomes fixed,—to make sure that the fundamental assumptions hitherto put down are sufficient for an under-standing of the behavior of any animal, even the highest, or that they must be supplemented in a particular way. Let us study in detail, first, the fundamental nervous processes underlying the "education" of the proverbial child who learns to fear the fire. This is a variation of response. The instinctive activity called out by the flame impressing the sensory points of the eyes consists in a drawing nearer of the body or its limbs to the flame. The habit which exists in later life is a withdrawing of the body or its limbs from the flame. The variation
of the nervous path, therefore, must consist in this, that the excitation coming from the eyes, which at first passes into certain muscles, for example, those stretching out the arm, later passes into the very antagonistic set of muscles, drawing in the arm. In Figure 28 we represent this schematically as if the eyes stimulated were only a single sensory point (Sa) and each of the muscle sets only a single motor point (Ma approaching, Mb, receding). The only difference from our previous method of drawing the figures is that the two reflex arches open in opposite directions, right and left, instead of both
( 94) downwards. Sa and Ma, are corresponding points. It is natural, therefore, that the child's finger is stretched out toward the light. The problem is how we can explain that later the path Sa S1a S2ab M2ab M1b Mb, marked in the figure by a zigzag line, has a lesser resistance than the path Sa S1a M1a Ma, so that then the finger is withdrawn as soon as the fame becomes simply visible. We have stated that such a change must mean the fixation of a variation, and this variation may be the result of a second nervous process deflecting the first one from its original course, in the direction of Ma, to another course in the direction of Mb. The second nervous process, therefore, must have Mb as its motor end. What kind of a sense organ is then represented by the sensory point Sb,, corresponding to this motor point Mb? We know that. the drawing of limbs toward the body, while resulting from many different stimulations, is especially the result of pain stimulation. An animal suffering from strong pain of any kind does not stretch out its limbs, but draws them in, curves them, and the whole body too, if possible. It writhes. We know further, from direct and indirect experience, that pain stimulation is quite generally the essential factor in any kind of variation of response. Sb represents, therefore, the pain nerve ends of the finger stretched out. In what way, then, in our diagram of Figure 28, do the two nervous processes, from the eyes and from the finger tip, influence each other?
In order that one of two nervous processes deflect the other from its course, they must obviously exist in the nervous system (Figure 28) simultaneously. This does not imply, however, that one of them may not begin before the other. In our case, Sa is stimulated first, and only because of the excitation traveling from Sa to Ma . and causing the finger to approach the flame, occurs
( 95) stimulation of Sb by the flame. From this moment on. Sa and Sb are stimulated simultaneously. Whether now the finger is moved into the flame or away from it, depends, so far as it is a strictly mechanical event, simply on the relative force with which the one (at Ma) or the other (at Mb) of the antagonistic sets of muscles contracts. This muscular force here depends on the relative intensity of the nervous flux towards Ma and towards Mb. It seems rather obvious that the excitation at Sb, caused by the burning of the finger in or near the flame, must be exceedingly strong, so that, as a purely mechanical effect, the reaction must be that of a withdrawal. But even if the two reactions were not as mechanical phenomena mutually exclusive,—even if they were not antagonistic, but anatomically independent movements,—the laws of nervous function alone would practically suppress the one in favor of the other, because the stronger nervous process would sufficiently deflect the flux of the weaker nervous process. It is this suppression of one nervous process by another nervous process (and not at all the mechanical annihilation of one muscular pull by another) which we arc here concerned with. Let us see how this is brought about.
The flux originating from SQ divides at the point S1a. The larger part of it takes the path S1a M1a a smaller part the path S1a S2ab M2ab M1a, because the resistance of the latter is, according, to the diagram, three times that of the former. It is evident that at the point M2ab, too, a division of the flux must take place. Although the resistance of the path M2ab M1b Mb is no greater than that of the path M2ab M1b Ma most of the flux must go on from M2ab in the direction of M1a, because the flux over S1a M1a Ma according to our assumptions, acts as suction upon the contents of the conductor M2ab M1a , whereas
(96) no similar force acts in the direction of M1b . Practically all of the flux which originates from Sa finds its outlet therefore in Ma , only an insignificant fraction going to Mb. But immediately the excitation of the point Ma brings about the strong stimulation of Sb the outstretched finger being burnt. A strong flux now goes on from Sb to Mb, mostly over S1b M1b , partly over S1b S2ab M2ab M1b ; but even the part taking the latter path is strong compared with the flux originating from Sa. At once all the conditions are changed which determine the flux originating from Sa. The strong current over S1b S2ab M2ab draws the contents of the conductors S1a S2ab and Sa S1a . Consequently most of the flux from S1a takes the direction of S1b M1b Mb ; little takes the direction of M1a . From M2ab , again, most of the flux is drawn, by the strong current over S1b M1b Mb , in the direction of M1b ; little in the direction of M1a since the weak current over S1a M1a Ma draws only weakly in this direction. All this will appear even more natural to us when we recall that in our physical analogy, the jet-pump, the suction effect is not simply proportional to the velocity of the fluid, but to the square of the velocity. We have the right to assume, therefore, that the flux of a weak nervous process is very readily consumed by the deflecting effect of one which is only considerably stronger. Consequently, in spite of the continuance of the stimulation at Sa (the eyes), there is now practically no motor excitation at Ma (no tendency to stretch out).
How is the fixation of this variation brought about, to the effect that later, when Sa is stimulated, the response occurs at Mb, without any stimulation at all occurring at Sb ? We have assumed that each neuron is susceptible to any flux occurring within it, so that its resistance is lessened in proportion to the flux and the time during which it continues. Under the assumption
( 97) of such a susceptibility the fixation of the variation of response is a plain enough matter. The direct path from Sa to Ma is made up (in the diagram of Figure 28) of three units of length, the path from Sa to Mb of five units. The reduction of the resistance of the latter path need not be by any means enormous, in order to make the flux in the direction of Ma so weak as to be practically insignificant.
