It is hard to teach an old dog new tricks, because the old tricks employ familiar, much-used neural paths, while new tricks require the connecting up of groups of neurones not in the habit of working together; and the flow of nerve energy is more easily accomplished in the neurones accustomed to working together. One who learns to speak a foreign language late in life never attains the facility and ease that might have been reached at an earlier age. This is because the neural paths for speech are already set for his mother-tongue, and, with the lessened plasticity of age, the new paths are hard to establish.

The connections between the various brain areas, or groups of neurones, are, as we have seen in an earlier chapter, accomplished by means of association fibers. This function requires millions of neurones, which unite every part of the cortex with every other part, thus making it possible for a neural activity going on in any particular center to extend to any other center whatsoever. In the relatively unripe brain of the child, the association fibers have not yet set up most of their connections. The age at which memory begins is determined chiefly by the development of a sufficient number of association fibers to bring about recall. The more complex reasoning, which requires many different associative connections, is impossible prior to the existence of adequate neural development. It is this fact that makes it futile to attempt to teach young children the more complicated processes of arithmetic, grammar, or other subjects. They are not yet equipped with the requisite brain machinery to grasp the necessary associations.

[Fig. 18.]—Diagrammatic scheme of association, in which V stands for the visual, A for the auditory, G for the gustatory, M for the motor, and T for the thought and feeling centers of the cortex.

Association the Basis of Memory.—Without the machinery and processes of association we could have no memory. Let us see in a simple illustration how association works in recall. Suppose you are passing an orchard and see a tree loaded with tempting apples. You hesitate, then climb the fence, pick an apple and eat it, hearing the owner's dog bark as you leave the place. The accompanying diagram will illustrate roughly the centers of the cortex which were involved in the act, and the association fibers which connect them. (See Fig. 18.) Now let us see how you may afterward remember the circumstance through association. Let us suppose that a week later you are seated at your dining table, and that you begin to eat an apple whose flavor reminds you of the one which you plucked from the tree. From this start how may the entire circumstance be recalled? Remember that the cortical centers connected with the sight of the apple tree, with our thoughts about it, with our movements in getting the apple, and with hearing the dog bark, were all active together with the taste center, and hence tend to be thrown into activity again from its activity. It is easy to see that we may (1) get a visual image of the apple tree and its fruit from a current over the gustatory-visual association fibers; (2) the thoughts, emotions, or deliberations which we had on the former occasion may again recur to us from a current over the gustatory-thought neurones; (3) we may get an image of our movements in climbing the fence and picking the apple from a current over the gustatory-motor fibers; or (4) we may get an auditory image of the barking of the dog from a current over the gustatory-auditory fibers. Indeed, we are sure to get some one or more of these unless the paths are blocked in some way, or our attention leads off in some other direction.

Factors Determining Direction of Recall.—Which of these we get first, which of the images the taste percept calls to take its place as it drops out of consciousness, will depend, other things being equal, on which center was most keenly active in the original situation, and is at the moment most permeable. If, at the time we were eating the stolen fruit, our thoughts were keenly self-accusing for taking the apples without permission, then the current will probably discharge through the path gustatory-thought, and we shall recall these thoughts and their accompanying feelings. But if it chances that the barking of the dog frightened us badly, then more likely the discharge from the taste center will be along the path gustatory-auditory, and we shall get the auditory image of the dog's barking, which in turn may call up a visual image of his savage appearance over the auditory-visual fibers. It is clear, however, that, given any one of the elements of the entire situation back, the rest are potentially possible to us, and any one may serve as a "cue" to call up all the rest. Whether, given the starting point, we get them all, depends solely on whether the paths are sufficiently open between them for the current to discharge between them, granting that the first experience made sufficient impression to be retained.

Since this simple illustration may be made infinitely complex by means of the millions of fibers which connect every center in the cortex with every other center, and since, in passing from one experience to another in the round of our daily activities, these various areas are all involved in an endless chain of activities so intimately related that each one can finally lead to all the others, we have here the machinery both of retention and of recall—the mechanism by which our past may be made to serve the present through being reproduced in the form of memory images or ideas. Through this machinery we are unable to escape our past, whether it be good or bad; for both the good and the bad alike are brought back to us through its operations.

When the repetition of a series of acts has rendered habit secure, the association is relatively certain. If I recite to you A-B-C-D, your thought at once runs on to E, F, G. If I repeat, "Tell me not in mournful numbers," association leads you to follow with "Life is but an empty dream." Your neurone groups are accustomed to act in this way, so the sequence follows. Memorizing anything from the multiplication table to the most beautiful gems of poetic fervor consists, therefore, in the setting up of the right associative connections in the brain.

Association in Thinking.—All thinking proceeds by the discovery or recognition of relations between the terms or objects of our thought. The science of mathematics rests on the relations found to exist between numbers and quantities. The principles and laws of natural science are based on the relations established among the different forms of matter and the energy that operates in this field. So also in the realm of history, art, ethics, or any other field of human experience. Each fact or event must be linked to other facts or events before it possesses significance. Association therefore lies at the foundation of all thinking, whether that of the original thinker who is creating our sciences, planning and executing the events of history, evolving a system of ethics, or whether one is only learning these fields as they already exist by means of study. Other things being equal, he is the best thinker who has his knowledge related part to part so that the whole forms a unified and usable system.

Association and Action.—Association plays an equally important part in all our motor responses, the acts by which we carry on our daily lives, do our work and our play, or whatever else may be necessary in meeting and adapting ourselves to our environment. Some sensations are often repeated, and demand practically the same response each time. In such cases the associations soon become fixed, and the response certain and automatic. For example, we sit at the table, and the response of eating follows, with all its complex acts, as a matter of course. We lie down in bed, and the response of sleep comes. We take our place at the piano, and our fingers produce the accustomed music.