The neurological basis of learning.

Abstract

THE NEUROLOGICAL BASIS OF LEARNING SIR JOHN GADDUM* I.Introduction Anyone who is interested in the activities of individual neurons in the cerebrum would like to form some picture of their relation to the activities ofthe whole animal. Much work has been done on this topic and everyone must make his own summary and consider the meaning ofthe facts. The speculations given below are an attempt to do this. They are mostly either unoriginal or improbable, but it is possible that one or more may provide the stimulus for further experiment. II.Brain Models The biological success ofanimals depends partly on their power to make movements and to adapt these movements in the light ofsignals coming from the outside world. When the conditions are standardized it is possible to get a constant response to a given signal. This fact led in the nineteenth century to the belief that a reflex was a mechanical response which did not depend on the previous history ofthe animal. There is, of course, some truth in this belief, but it is surprising what simple nerve nets can learn from experience. Bees can learn that a blue background is associated with food [i]. They can also learn the way to food and to communicate this information to other bees using the sun as a guide—and the weight oftheir brain is only about 2.5 mg. Octopus vulgaris can learn that some white shapes are signals of danger and will then take appropriate avoiding action [2]. The flatworm (planarian) Dugesia dorotocephala was shown [3] to be able to learn * Institute ofAnimal Physiology, Babraham, Cambridge, England. Present address: io Fendon Close, Cambridge, England. 436 SirJohn Gaddttm · Neurological Basis ofLearning Perspectives in Biology and Medicine · Summer 1963 that a bright light was a signal for an electric shock, which made it contract its whole body. Later work showed that these animals could learn to be guided by light through a simple Y-maze, and led to other surprising discoveries, which are not directly relevant to the present discussion . These animals are less than 2 cm. long, and their nervous system consists of two small ganglia, one of which is directly concerned with learning. These facts show that learning may occur in a small organ. Similar learning also occurs in a wide variety ofother simple animals [4]. It is tempting to suppose that the mechanism which controls behavior and modifies it in the light ofexperience has something in common in all animals. The word "plastic" is applied to nerve nets whose properties are altered by experience, and it is thought that the study ofplastic nerve nets in simple animals may throw some light on the mode of action of the brains ofcomplex animals. The main seat ofthe intellect must be a plastic nerve net of some kind, though it is possible to have plasticity without intellect. In the higher mammals, intelligence is associated with the cerebral cortex since: (1) the species ofanimal with the most complicated cortex are in general the most intelligent; (2) when the cortex is absent through disease, intelligence is absent too; and (3) extirpation ofthe whole cortex destroys intelligence. It does not follow, however, that the cerebral cortex is the only part of the brain where plastic changes occur. Most nerve nets are to some extent plastic and the difference between them is a matter ofdegree. According to Gastaut and others [5], the thalamus and the reticular formation play a major part in the formation ofconditioned reflexes. There has been much speculation about how plastic nerve nets work, and various theories have been suggested in the form ofmodels which are thought to resemble real brains. In the early part ofthis century the brain was thought to be like a telephone exchange, each sensation being switched through to oneparticular center and each motoractcaused byanother particular center. The function of the brain was to connect these two centers with one another in some unknown way. This theory was taken to imply that each sensation, andeach motor act, depended on a localized area ofthe brain and would disappear ifthat area was destroyed. This is true in the sense that visual sensations are analyzed in the occipital cortex and...