APA Dictionary of Psychology

Karl S. Lashley’s generalization that the size of a cortical lesion, rather than its specific location, determines the extent of any resulting performance decrement. Proposed in 1929 following experimental observations of the effects of different brain lesions on rats’ ability to learn a complex maze, the concept reflects Lashley’s belief that large areas of the cortex function together in learning and other complex processes. This concept is no longer considered completely valid. Also called law of mass action; principle of mass action. See also equipotentiality.