Karl S. Lashley was a major figure of twentieth-century neuropsychology, whose work on the relation between brain function and behavior, especially learning and memory, has proved seminal.
Karl Spencer Lashley, one of the titans of the twentieth century in neuropsychology, was born in 1890 in Davis, West Virginia and died in 1958 in Poitier, France. A small-town Appalachian boy with the heart of a naturalist, he grew up to become a world-class animal psychologist. His research and theorizing concerning the relation between brain and behavior has had a lasting influence on psychology and neurology. That influence was further enhanced by the contributions of some of his students, particularly Frank A. Beach, Donald O. Hebb, and Roger W. Sperry, who was awarded the Nobel Prize in medicine in 1981.
The major question Lashley set for himself was: how does the brain work in the process of remembering things? His research convinced him that contemporary theories of learning and memory were erroneous – that learning could never be explained in terms of the formation of simple connections at synapses of particular neurons. His contributions to neuropsychology are collected in a volume of his selected papers, and his research and theories are reviewed in two volumes by Jack Orbach (see the Further Reading section).
From childhood, Lashley’s mother encouraged him in intellectual pursuits. He learned to read at the age of four and made avid use of the household library. But reading was not the only source of his attraction to science. He had the instincts of an observer and displayed a deep interest in animal and plant life. A favorite boyhood pastime was to wander in the woods, observing and collecting various species, including butterflies, snakes, frogs, snails, mice, and raccoons. As this interest in animals persisted throughout his life, he was never without a pet. At one time he owned a cat and a parrot, but the combination created unexpected problems when the voluble bird displayed a tendency to adopt the cat’s kittens. Other pets included cockateels, monkeys, and dogs. One dog, ‘Till Eulenspiegel’, developed a fondness for daiquiris and ‘pink ladies’.
Lashley had a mechanical aptitude that appeared early. He was fascinated by his mother’s sewing machine and learned to use it efficiently, later, as an adult, making sails for his boats and drapes for his home. Meanwhile, as a substitute for sewing, his father bought him a jigsaw to use in woodworking. This started Lashley on a hobby that became a lifelong pleasure. He produced a steady flow of elegantly designed and finely executed articles, even living room furniture. After retirement, he continued his cabinet-making and repair work, including the remodeling of his house. As a laboratory researcher, Lashley found that this aptitude served him well, and he demanded similar skills from his students, requiring them to construct their own devices for experiments.
When Lashley was seven, his father fell prey to gold fever and decided to take his family northwest to prospect. Years later he recalled the excitement of the gold rush and meeting with such swashbucklers as Swiftwater Bill and Klondike Pete, who sported nugget jewelry and recounted tall tales about fabulous strikes and the dangers of encountering bird-sized Alaskan mosquitoes!
At college, Lashley’s plan was to major in Latin. In order to fill a vacant hour in his schedule, he enrolled in a course in zoology. The teacher of this class had a profound influence upon the 16-year-old boy: ‘Within a few weeks in his class I knew that I had found my life’s work.’ After earning his degree at the University of West Virginia, Lashley was awarded a teaching fellowship in biology at the University of Pittsburgh and enrolled as a Master’s candidate. His thesis was on the permeability of the eggshell. But his most important contact that year was with another teaching fellow, Karl Dallenbach, who later said of Lashley: ‘Though he had never taken a course in psychology, he was permitted to take my laboratory course. In this small class, we worked intimately together on various experiments. Lashley was intensely interested and was an outstanding student. In this course, he showed the promise that he later fulfilled.’ After receiving his Master’s degree, Lashley accepted a fellowship at Johns Hopkins University to work with the eminent biologist H. S. Jennings on the invertebrate paramecium. His doctoral research was on asexual reproduction of hydra. He received his PhD in genetics in 1914. Though interested in learning as a topic, he remained a nativist all his life.
At age 11, Lashley had a few piano lessons but he found practicing scales impossibly boring. Then, at 18, he picked up the violin and learned to play without formal instruction. He claimed that he first heard classical music at 20 and was immediately fascinated by it. His first wife, a pianist, introduced him to the literature of chamber music. It didn’t take long before he taught himself to play the cello (Figure 1). He collected an extensive library of instrumental music, joined the Jacksonville Orchestra and organized a small group of Florida musicians to meet regularly at his home (called ‘Fiddler’s Cove’) for the playing of chamber music. He was a trustee and benefactor of the Jacksonville College of Music. Always on the alert for behavior that might shed light on how the nervous system worked, he once calculated the speed of finger movements involved in playing a fast cadenza on the piano, and compared this with the known speed of neural transmission. The comparison revealed that the intervals between successive finger movements were too short to support the theory that each movement is aroused by motor impulses set off by sensory impulses from the preceding finger movement. There is just not enough time for a sensory message from the finger to go to the brain and pass to the motor area and then for a motor impulse to trigger the next finger movement. Lashley cited this example to support the notion of ‘central patterning’ of complex motor sequences in the brain (something that today we call ‘motor programs’).
Lashley’s interest in psychology, first aroused in Pittsburgh, continued at Hopkins. While majoring in zoology, he took two minors: one with Adolph Meyer, professor of psychiatry; the other with John B. Watson, who became the father of behaviorism. Watson’s impact on him was so great that, 44 years later, Lashley asserted: ‘Anyone who knows American psychology today knows that its value derives from biology and from Watson.’ In 1914, he joined Watson to carry out field experiments on homing, nesting, and reproductive behavior of sooty and noddy terns on the Dry Tortugas (west of Key West, Florida).
During the First World War, Lashley was assigned to educate the public and the military on the dangers of venereal diseases. Working together, he and Watson showed and discussed movies designed to further the campaign against these afflictions. The movies illustrated what damage could be wrought on the genitals. In later years, Lashley enjoyed telling of the time when they went to a small town and distributed advertisements announcing a free movie. The advertisements included no mention of the subject matter and, according to Lashley’s account, he and Watson were fortunate to escape in one piece from the sheriff and enraged citizenry.
While holding a postdoctoral scholarship, Lashley continued to work with Watson, studying the effects of strychnine and other drugs on maze learning in rats. At the same time he journeyed frequently to Washington, DC to study the brain-lesioned monkeys of the psychologist Shepherd Ivory Franz. Eventually, he acquired the surgical and histological skills to embark on an ablation program of his own on the neural basis of learning and memory. This program brought him worldwide recognition, and his research career was solidly launched.
After stints in academic posts at Minnesota and Chicago, Lashley was chosen in 1935 for a chair at Harvard. The invitation came from a search committee charged by the President of the University to find ‘the best psychologist in the world’. Not bad for someone who never took a didactic course in psychology! Finally, in 1942, he was appointed Director of the Yerkes Laboratories of Primate Biology in Orange Park, Florida, where he wrote some of his most memorable papers, including In Search of the Engram (1950), in which he concluded that he could not find the memory trace in any one place in the brain.
In his 1929 monograph (see Further Reading), Lashley enunciated his controversial concepts of mass action and equipotentiality. Empirically, the term mass action summarizes the results of many brain ablation experiments – that the loss of the maze habit in rats is determined by the size of the lesion and not by its locus in the brain. Theoretically, mass action refers to a theory of how the cerebral cortex works, that it is the pattern of activity, independent of its locus, that is relevant, and that the memory trace is reduplicated and distributed in the brain. The concept equipotentiality refers to the fact that intact neurons can take over the function of destroyed cells. In his later years, Lashley discarded this concept and preferred to cite the facts of sensory and motor equivalence. Examples include the recognition of unfamiliar visual stimuli by the eye not used during monocular learning to recognize those stimuli (interocular transfer), and the performance of skilled movements by the hand not practiced during learning (intermanual transfer). Lashley loved to point out impishly that right-handers can write with their left hands, with their feet (on the beach), and even with their noses. In short, neurons that are not used during the course of learning can still show the effects of learning – that is, they can mediate memories (referred to as Lashley’s lesson by Orbach, 1998). The recovery of function in patients after suffering from brain lesions also points to the same fact, that neurons inactive during learning can still show the effects of learning.
By the early 1920s, Lashley’s research results on Pavlovian conditioning led him to break away from Watson’s theorizing on learning. He found Watson’s Stimulus–Response formula troubling because it failed to include the brain in the causal sequence. Does a stimulus cause a response? No, answered Lashley, a stimulus excites the brain, and it is the resultant activity in the brain that is responsible for the response. Thus, he revised Watson’s formula to read Stimulus–Brain–Response. In this way, he provided for psychological functions that need to be sustained in the brain, such as selective attention and the memory trace. Unfortunately, the model of the day assumed that brain processes were linear – that is, the neural activity was thought to flow directly from input to output, as in a telephone line. It wasn’t until 1938 that Lashley came up with a mechanism to explain how neural activity is sustained after the stimulus has ceased. This mechanism, called the reverberatory circuit, was borrowed from the neuroanatomical descriptions of Lorente de Nó. But it was Lashley’s student Hebb who illustrated the wide application of reverberatory circuits for neuropsychological theory in a landmark book published in 1949. Hebb’s theory of cell assemblies in the brain took the neuropsychological community by storm.
In 1952, Lashley wrote: ‘I have never been able by any operation on the brain to destroy a specific memory. From such experiments, I have been forced to conclude that the memory trace is diffuse, that all memories are somehow represented in all or almost all parts of the cerebral cortex. Whatever the nature of the trace, it must be reduplicated throughout wide areas.’
Finally, in 1957, he wrote: ‘The neuron is a living organism, subject to continual variation in its functional capacity according to its metabolic state. It shows an all-or-none response in that it does or does not fire. But its ‘all’ may vary greatly from time to time. Comparison of the nervous system with a digital computer (and with soldered wire circuits) implies a uniformity in the action of neurons which is contrary to fact.’ Thus, Lashley dismissed the rigid circuitry of early theories of artificial intelligence.