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Karl S. Lashley (1890–1958), born in Davis, West Virginia, was an American psychologist and behaviorist well-remembered for his influential contributions to the study of learning and memory. His failure to find a single biological locus of memory (or "engram", as he called it) suggested to him that memories were not localized to one part of the brain, but were widely distributed throughout the cortex.

While working toward his Ph.D. in genetics at Johns Hopkins University, Karl Lashley became associated with the influential psychologist John B. Watson. During three years of postdoctoral work on vertebrate behavior (1914-17), he began formulating the research program that was to occupy the remainder of his life.

In 1920 he became an assistant professor of psychology at the University of Minnesota, Minneapolis, where his prolific research on brain function gained him a professorship in 1924. He was later a professor at the University of Chicago (1929-35) and Harvard University (1935-55) and also served as director of the Yerkes Laboratories of Primate Biology,Orange Park, Florida from 1942 to 1955.

His work included research on brain mechanisms related to sense receptors and on the cortical basis of motor activities. He studied many animals, including primates, but his major work was done on the measurement of behavior before and after specific, carefully quantified, induced brain damage in rats. He trained rats to perform specific tasks, then lesioned specific areas of the rat cortex, either before or after the animals received the training. The cortical lesions had specific effects on acquisition and retention of knowledge.

By 1950, Lashley had distilled his research into two theories. The principle of "mass action" stated that the cerebral cortex acts as one—as a whole—in many types of learning. The principle of "equipotentiality" stated that if certain parts of the brain are damaged, other parts of the brain may take on the role of the damaged portion.

• 1923 "The behavioristic interpretation of consciousness." Psychological Bulletin
• 1929 "Brain mechanisms and intelligence."
• 1930 "Basic neural mechanisms in behavior." Psychological Review

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