The organic changes involved in learning

The organic changes involved in learning probably constitute an extension of those factors responsible for the development of the nervous system in embryo. In learning, as distinguished from fatigue, constant stimulation modifies behavior in the direction of increased rather than decreased response. Organization, not the connection between previously isolated nerve paths, is the distinguishing feature of learned response. Experiments by Lashley on removal of the cortex in rats following the learning of maze habits indicate that the engram is not sharply localized. It is difficult to estimate the extent to which such results apply to man, but we should be cautious in accepting even tentatively any simplified account of learning in terms of isolated neural elements. The laws of contiguity, use, and effect are open to criticism as explanatory principles; and the three major theories of learning (trial and error, conditioned response, and configural) mainly describe special types of learning problems.

Practically nothing is known of the structuro-functional changes in neural tissue which accompany learning. We can distinguish roughly four types of speculation--theories based upon evidence for (1) the growth of new connections between neurones, (2) changes in the conductivity of the synaptic membrane, (3) alterations in the size and structure of the neurone, and (4) establishment of potential gradients. The first two types of theory seem the most promising, but they are not so stated as to make a critical analysis possible. In spite of the vast literature on the subject, we must conclude that the problem of learning is still largely unsolved. The wide variety of description and explanation now offered under this topic raises the question whether all learning has a common organic basis. Its continuity with growth suggests an affirmative answer; but we cannot be sure of even this fundamental matter until the physiological mechanisms of a wide variety of habits, skills and memories have been ascertained.