Karl Lashley

Karl Lashley

Karl Lashley

Noted for:  The search for the “engram”; representation and processing in the mammalian cerebral cortex; localization and mass action in the brain; the problem of serial order in behavior.

Karl Lashley joined the Harvard faculty in 1935, and in the ensuing twenty years he expanded his research on the representation and localization of sensory and motor activity in mammalian brains.  Concurrent with the latter half of his tenure at Harvard, Lashley also served as the director of the Yerkes Laboratories of Primate Biology in Orange Park, Florida from 1942 to 1955.

Trained by the founder of the behaviorist movement in American psychology, John B. Watson, Lashley eventually parted ways with the evolving theoretical perspective of his mentor by emphasizing the neurobiological basis for sensorimotor and memory representations in the brain.

Lashley pioneered experimental work conducted on rats with surgically induced brain lesions, by damaging or removing specific areas of a rat’s cortex, either before or after the animals were trained in mazes and visual discrimination.   Lashley made several fundamental discoveries about how the brain stores and processes information. By implanting insulating chips of mica in rats’ cortexes and showing that they had few effects on learning and behavior, he established that (contra to Gestalt theories of the era) the cortex processed information in the pattern of activity and connectivity among neurons, not in global field and wave effects propagating through a medium. By showing that lesions that undercut slabs of cortex had far more severe consequences lesions that were perpendicular to the cortex, he helped show that the principal circuits of the cortex ran up and down into the white matter rather than side-to-side across the cortical surface. His famously unsuccessful search for the “engram” – the localized trace of the memory for a maze in a trained rat’s brain – led him to propose the principle of "mass action," in which learning is distributed across all parts of the brain rather than stored in a single regions, with the degree of impairment proportional to the amount of brain that was damaged. His complementary principle of "equipotentiality" stated that in the event of damage to one area of the brain, other parts of the brain can sometimes assume the role of the damaged region. Though his views are now considered too extreme, the principle that memories are not localized to a single spot in the brain is now well accepted.

In 1951 Lashley published a famous paper called “The Problem of Serial Order in Behavior,” in which he pointed out that complex sequential behavior (such as playing a piece on the piano) could not be executed by one response sending a proprioceptive signal back to the brain which would then trigger the next response in the sequence – there simply wasn’t enough time for the neural signals to travel up to the brain and back down. Instead, behavior had to be controlled by a central, hierarchically organized program. This insight has guided the study of motor behavior ever since, and influenced Noam Chomsky’s critique of Skinner’s theory of language and the development of Chomsky’s theory of generative grammar.

Lashley was a pioneer of neuroscience before the term existed, and seeking to understand the connection between the physical structures of the brain and psychological processes of learning, memory, and planning. He eschewed the theoretical perspectives of his time in an attempt to avoid being hampered by a priori assumptions.  He is listed as number 61 on the American Psychological Associations list of the 100 most eminent psychologists of the 20th century. 


Eminent psychologists of the 20th century.  (July/August, 2002). Monitor on Psychology, 33(7), p.29.

Wideman, N. (1999).  Constructing scientific psychology:  Karl Lashley’s Mind-Brain Debates.  Cambridge University Press.