THE FUNCTION OF THE BRAIN IN RELATION TO INTELLIGENCE
It has been said that the problem of learning is the most important single problem in psychology. In a certain sense, Aristotle prophetically recognized this fact when he formulated his "laws of learning." For many centuries thereafter, the problem remained in the hands of philosophers and received no more than speculative attention until Ebbinghaus began his original and epochal experiments, shortly after 1880. Since that time, an important experimental literature has been developed from the investigation of conditions which are essential for efficient learning and retention. Later, the rapid development of animal psychology and child psychology as experimental subjects greatly broadened the study, particularly with respect to the theoretical understanding of learning.
That learning in a higher animal depends upon the setting up of changes in nervous tissue, and in particular upon changes in the brain, was widely assumed long before the experimental investigation of learning was undertaken. As a result, beginning even before 1840, an extensive knowledge of the structural and physiological characteristics of the human brain has been developed in neuroanatomy and related sciences. Although it was actually a general realization of the importance of the brain in behavior which led to these investigations, knowledge derived from them has not proved of much direct assistance in throwing light on the manner in which the nervous system functions in learning and in retention. There have been no direct experimental studies of the neural changes which are brought about when learning takes place, although studies of reflex action and of the nerve impulse have excited great interest in the question. At present, psychologists have virtually abandoned "synaptic resistance," "drainage," and related hypotheses of a vague nature