The Characteristics of Learned Response

In the adult organism there are few, if any, reactions which have not been modified by environmental agencies. Work decrement, hysteresis, adaptation, movement sequences, habit, associative memory, and conditioning are names which differentiate these modifications in terms of the stimulus-response relations which are involved. Upon such a basis we can make out two types of phenomena: (1) In work decrement, hysteresis, and adaptation, constant stimulation modifies behavior in the direction of decreased response. (2) In memory, conditioning, and habit, constant stimulation modifies behavior in the direction of increased response; that is, a reaction is called out by a stimulus which, before training, was unable to elicit it. Most psychologists regard the second type of phenomena as learning and assign phenomena of the first type to another topic, such as fatigue. Accepting this distinction as useful, we may proceed at once to analysis of the learned response.

Despite the recognized importance of learning, progress toward a psychological understanding of the process has been very slow and halting. To be sure, many studies have been made of the conditions under which learning occurs; and much time and energy have been spent in plotting the curve of acquisition and the curve of retention for various responses. We know that organisms differ in their rate of learning; that plateaus, amnesia, and transfer of training are often involved; that retention is tested by recall and recognition; that interpolated activity may facilitate or inhibit retention; and so on. These facts are valuable in themselves, but they do not suggest an integrating principle by which all may be welded into a unit. They are still capable of varied interpretation, and therein lies the dilemma of the physiological psychologist. He is asked to explain the mechanics of the learning process; but he finds such a divergence of description and opinion upon the psychological topic that he may well question whether it can be studied as a unitary problem.

Psychologists offer little help by saying that learning is due to association. At first ideas were conceived to become associated together; later it was stimuli and responses. As common-sense descriptions of learning, such formulations are, perhaps, permissible; but when presented as a concise statement of the learning process, they lack point. What is a stimulus in terms of a pattern of receptor excitation? What is a response in terms of adjustor and effector integration? If we are to understand learning, we shall have to deal with it in these terms. Descriptions of learning as association reveal our almost total ignorance of its physiological basis. An early notion assumed that specific paths were worn down between the neural elements (brain centers or receptors and effectors) which were active during training. The influence of repetition in forming this path was considered analogous to the erosion of a river bed.

A path once traversed by a nerve-current might be expected to follow the law of most of the paths we know, and to be scooped out and made more permeable than before; and this ought to be repeated with each new passage of the current. Whatever obstructions may have kept it at first from being a path should then, little by little, and more and more, be swept out of the way, until at last it might become a natural drainagechannel.

This method of validating connections or associations is still with us. In fact, most psychologists today phrase their analysis of the learning process in such a way that it can be appliqued on an atomistic neurology. Learning (or fixation) is now referred to the synapse or some even less understood concept rather than to nerve paths. Conditioning has been substituted for association, reactions for ideas, new terms for old--and "verbal magic," one and all.

It is very significant that so many positive statements about the nature and characteristics of learning turn out to be untrue. Perhaps we need first to develop a purely psychological statement of learning, before we become involved in hypothetical, unsubstantiated, and hampering physiology. We might reverse this argument, however, and state that an actual study of the physiological mechanism involved in a variety of learned responses is the only means to an understanding of the psychological principles involved.

Point-for-point correspondence between receptor and effector elements is rarely demonstrated in any learning experiment. On the contrary, effector response is frequently independent of the receptor cells stimulated in successive repetitions of the training situation. A transfer of the habit of brightness discrimination also took place between the eye first stimulated and the other, which had not been used in training. Of course, it can be maintained that corresponding points in the two retinas have a common cortical neurone; but this has not been proved. The stimulus to any reaction above the level of a spinal reflex involves not the excitation of certain definite sensory cells but the excitation of cells of a system of certain ratios, and response may be given to that ratio even though the particular cells involved have not previously been excited in the same way during the formation of the habitual reaction. . . . The common elements are not common neurones, but common ratios of excitation in different neuronic systems.

While we need not consider any of this as final, it does show that the characteristics of learned response demand more careful consideration than is usually given them in stimulus-response formulations. Organization is the distinguishing feature of learned response; yet it has been neglected both psychologically and physiologically.