This outstanding physiological psychologist was born in West Virginia and received his Ph.D. in genetics at Johns Hopkins in 1915. He remained there for two additional years, working with the behaviorist John B. Watson and the biologist H. S. Jennings, then accepted a teaching position at the University of Minnesota (1917-26). Afterward he spent three years as research psychologist at the Behavior Research Foundation in Chicago, six years in teaching at the University of Chicago, and twenty years as professor of neuropsychology at Harvard (1935-55). Between 1942 and 1955 he also served as director of the Yerkes Laboratories in Orange Park, Florida. He was elected president of the American Psychological Association in 1929 and president of the Society of American Naturalists in 1947.Lashley was basically a researcher who applied the behaviorist approach while avoiding the controversies over consciousness in which that school became involved: “To me the essence of behaviorism is the belief that the study of man will reveal nothing except what is adequately describable in the concepts of mechanics and chemistry, and this far outweighs the question of the method by which the study is conducted” (1923). In carrying out his research, however, he departed from the rigid behaviorist orientation by focusing attention on the functioning of the total organism rather than on the measurement of stimuli and responses. In so doing he adopted an approach that was close to the holistic viewpoint of the Gestalt school.Although Lashley conducted important investigations in the fields of color vision, instinct, sex, heredity, conditioning, and other aspects of animal behavior, he contributed most heavily to the problem of localization of functions in the brain. Prior to the 1900s most of the research had dealt with specific localizations; for example, it had been shown that stimulation of one part of the motor cortex leads to arm movements and stimulation of another part produces leg movements. In 1907, however, S. I. Franz reported new findings that called this theory of specific localization into question. While still at Johns Hopkins, Lashley collaborated with Franz on experiments in which they had white rats go through learning experiences, and then destroyed portions of their cerebral tissue and recorded the effects both on the previously formed habits and on future learning. By this method they discovered that the same function may be mediated by two parts of the brain on different occasions. In 1917 they published a joint paper in which this revolutionary discovery was reported. See FRANZ.During the 1920s Lashley carried this research further by destroying different amounts of the brain tissue and noting the effect on sensory discrimination and intelligence. In one series of experiments he compared a rat’s ability to learn before and after extirpation by measuring the time and errors required to learn a maze. For another series he invented a “jumping stand,” which consisted of a platform from which the rat jumped at doors on which certain signs, such as a triangle or circle, had been drawn. If the animal jumped at the correct stimulus, the door swung open and it was rewarded with food; if it made a mistake, the door failed to open and it fell into a net. See CONCEPT FORMATION.The results of these experiments were described in numerous monographs and brought together in a book, Brain Mechanisms and Intelligence, published in 1929. In it Lashley proposed the principle of equipotentiality, an elaboration of the theory of vicarious functioning which had been anticipated by Flourens about a century before. This view holds that all parts of the nervous system are so closely interrelated that if one part is destroyed, an “equipo- tential” area can take over its functions. Lashley believed this to be a basic biological principle for activities such as intelligence and motor learning. In one of his experiments he destroyed part of the motor cortex of monkeys and produced a temporary paralysis of the part of the body mediated by that area. According to the earlier theory of specific localization, no recovery of function would be possible, yet Lashley showed that the lost ability actually reappeared in time, although in somewhat less efficient form.Lashley found that the principle of equipotentiality does not apply to every function. If the visual areas of a rat’s brain are destroyed, it loses its pattern vision but not its ability to discriminate brightness, since that function is mediated on a lower level. Human beings, on the other hand, need the visual cortex for both these functions. Lashley further discovered that in lower animals learning and retention depend more on the amount of intact cortex than on its particular location. The experiments that led to this discovery were also done on rats. Lashley first gave them an opportunity to learn how to escape from a problem box. Then he destroyed various amounts of cortex in different subjects. Up to about 15 per cent of cortical destruction resulted inno impairment of any motor or sensory task; beyond that point, the more he destroyed, the harder it was for the rats to escape from the box and to relearn the procedure.Lashley termed the brain activity “mass action,” a concept that is closely related to equipotentiality. In essence it means that large amounts of equipo- tential brain tissue work together in the learning and retention process, so that no matter where the tissue is destroyed, the loss in ability is in proportion to the extent of the damage. The mass action principle, however, applies far more fully to lower animals than to human beings, since there is greater localization of function in the human brain. See CEREBRAL CORTEX.