Karl Lashley

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Karl Spencer Lashley (June 7, 1890 – August 7, 1958) was an American psychologist and behaviorist, well remembered for his influential contributions to the study of learning and memory. Based on years of research in which he failed to find the biological locus of memory (or "engram"), he was one of the first to claim that memories were not localized in one part of the brain, but were widely distributed throughout the cortex. Lashley's work has had wide-ranging implications for the study of learning, memory, and brain functions. However, his understanding of the human mind was limited to the structure of the physical brain. The complexity of human thought and behavior is based on internal aspects, namely the mind and spirit. Understanding the impact behavior in the physical world has on human beings only begins with understanding the way the brain stores memories. Its effects on human minds and spirits may prove to be much more far-reaching and even more indelible.


Karl Spencer Lashley was born on June 7, 1890 in Davis, West Virginia. As a child he showed great interest in animals, and his mother, Maggie Lashley, encouraged his intellectual pursuits.

He entered the University of West Virginia at the age of fifteen, and signed up for the zoology class under the famous neurologist, John Black Johnston. At that time, Lashley decided to pursue a career in zoology.

Lashley graduated in 1910, and obtained a teaching fellowship at the University of Pittsburgh, where he received his master’s degree in bacteriology. He then went on to study genetics at Johns Hopkins University, under zoologist Herbert S. Jennings. At the same time, Lashley became associated with the psychiatrist Adolf Meyer, and psychologist John B. Watson, the association that significantly influenced Lashley’s life and made him turn toward studies of animal behavior. During three years of postdoctoral work on vertebrate behavior (1914–1917), 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 in Minneapolis, where his prolific research on brain function gained him a professorship in 1924. Over the next several years Lashley collaborated with Shepherd Franz, at St. Elizabeth's Hospital in Washington, DC, examining effects of lesions of the frontal cortex on learning abilities in rats. This work was the foundation for Lashley’s major work, Brain Mechanisms and Intelligence (1929).

Lashley was appointed professor at the University of Chicago in 1929, where he taught until 1935. In 1929, he served as president of the American Psychological Association. He moved to Harvard University in 1935, and stayed there until his retirement in 1955. He also served as director of the Yerkes Laboratories of Primate Biology in Orange Park, Florida, from 1942 to 1955.

Lashley married Edith Ann Baker in 1918. After Edith died in 1948, Lashley remarried in 1957 to Claire Imredy Schiller, widow of the Hungarian psychologist Paul Harkai Schiller. Lashley died suddenly, on August 7, 1958, while vacationing in Poitiers, France.


Lashley was both an experimental researcher and a theoretician. His early works included a study of sexual behavior of the hydra and detailed study of the behavior of seabirds. In both of those areas he followed the behaviorist theory of John B. Watson, to whose approach he remained loyal throughout his career.

Lashley’s later 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 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—memory. This work led Lashley to another area of study, the one that he focused on during the rest of his career, and the area he became the most famous for: cerebral localization.

The study of cerebral localization was an area in psychology that generated numerous controversies in the first half of the twentieth century. Many scientists including Franz Joseph Gall, Paul Broca, and others believed that particular local areas in the brain were responsible for particular brain functions, therefore advocating for exact cerebral localization. On the other side, researchers like Marie-Jean-Pierre Flourens held that brain functions are dispersed around the brain.

Combining the insights from behaviorism and his research with trained rats, Lashley hypothesized the existence of a local area in the brain where memories of learned responses are stored. Lashley called this area the "engram" (memory trace). The idea that learned responses are localized in the brain was in line with behaviorism, and Lashley devoted his career to finding such areas. He used mazes of various difficulty and size, where he trained rats to find the exit. After the rats memorized where the exit was, Lashley would perform different lesions on their brains, under the assumption that he would be able to remove the memory traces that their brains created. However, after years of research, Lashley failed to find the location of any such “engrams.”

In 1929, Lashley published his masterpiece Brain Mechanisms and Intelligence: A Quantitative Study of Injuries to the Brain, where he summarized years of his research on cerebral localization. In it he renounced the existence of a precise local area in the brain that is responsible for holding memories. The study had a profound impact on the young field of physiological psychology, and on future research on the brain.

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. Different areas of the brain participate in learning, and the memory traces are dispersed over the entire cortex.
  • 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. Locus is thus not important.

As Lashley (1950) put it himself:

This series of experiments has yielded a good bit of information about what and where the memory trace is not. It has discovered nothing directly of the real nature of the memory trace. I sometimes feel, in reviewing the evidence of the localization of the memory trace, that the necessary conclusion is that learning is just not possible. It is difficult to conceive of a mechanism that can satisfy the conditions set for it. Nevertheless, in spite of such evidence against it, learning sometimes does occur.


Based on his work with animals, and the application of that knowledge to universal principles of behavior, Lashley is regarded as a pioneer in ethology and comparative psychology.

One of the fields in psychology that owes the most to Karl Lashley is the field of physiological psychology. Lashley’s work on localization of learning and memory shaped future research in brain studies, showing that the brain was more complex than was thought. Research on “engrams” continued after Lashley’s death, with the work of Donald O. Hebb, a student of Lashley.

However, other lines of research also developed, based on Lashley's failure to find localized memory traces, implying instead that learning affects a wide area of the cortex. Lashley’s insights were in line with modern theories used by connectionists: theories of artificial intelligence, cognitive science, neuroscience, psychology, and philosophy of mind.


  • Lashley, Karl S. 1923. The behavioristic interpretation of consciousness. Psychological Bulletin 30: 237–272, 329–353.
  • Lashley, Karl S. 1929. Brain mechanisms and intelligence. University of Chicago Press.
  • Lashley, Karl S. 1930. Basic neural mechanisms in behavior. Psychological Review 37: 1–24.
  • Lashley, Karl S., ed. 1932. Studies in the dynamics of behavior. University of Chicago Press.
  • Lashley, Karl S. 1935. The mechanism of vision, Part 12: Nervous structures concerned in the acquisition and retention of habits based on reactions to light. Comparative Psychology Monographs 11: 43–79.
  • Lashley, Karl S. 1950. In search of the engram. Society of Experimental Biology Symposium 4: 454–482.

ISBN links support NWE through referral fees

  • Orbach, Jack, ed. 1981. Neuropsychology after Lashley: Fifty Years since the Publication of Brain Mechanisms and Intelligence. Lawrence Erlbaum Associates. ISBN 0898590884
  • Sheehy, Noel, ed. 2002. Biographical Dictionary of Psychology. London: Routledge. ISBN 0415285615

External links

All links retrieved October 5, 2022.


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