Lashley, Karl Spencer (1890-1958), was an American psychologist known for his research on the function of the brain in relation to behavior. Lashley developed two main principles as a result of his research. The first, the principle of mass action, states that in many types of learning the cerebral cortex acts as a whole. The second principle, the principle of equipotentiality, states that certain small areas of the brain can take on the function of larger, related areas that have been destroyed.