© 1992 SCIENTIFIC AMERICAN, INC The Biological Basis of Learnin and Individuality Recent discoveries suggestg that learning engages a simple set of rules that modify the strength of connections between neurons in the brain. These changes play an important role in making each individual unique by Eric R. Kandel and Robert D. Hawkins ver the past several decades, there surgeon at the Montreal Neurological Institute. has been a gradual merger of two In the 1940s Penfield began to use electri­ originally separate fields of science: cal stimulation to map motor, sensory and neurobiology,O the science of the brain, and language functions in the cortex of patients cognitive psychology, the science of the mind. undergoing neurosurgery for the relief of Recently the pace of unification has quick­ epilepsy. Because the brain itself does not ened, with the result that a new intellectual have pain receptors, brain surgery can be car­ framework has emerged for examining percep­ ried out under local anesthesia in fully con­ tion, language, memory and conscious aware­ scious patients, who can describe what they ness. This new framework is based on the abil­ experience in response to electric stimuli ap­ ity to study the biological substrates of these plied to different cortical areas. Penfield ex­ mental functions. A particularly faSCinating plored the cortical surface in more than 1,000 example can be seen in the study of learning. Elementary as­ patients. Occasionally he found that electrical stimulation pects of the neuronal mechanisms important for several dif­ produced an experiential response, or flashback, in which ferent types of learning can now be studied on the cellular the patients described a coherent recollection of an earlier and even on the molecular level. The analysis of learning may . experience. These memorylike responses were invariably therefore provide the first insights into the molecular mecha­ elicited from the temporal lobes. nisms underlying a mental process and so begin to build a Additional evidence for the role of the temporal lobe in bridge between cognitive psychology and molecular biology. memory came in the 1950s from the study of a few patients Learning is the process by which we acquire new knowl­ who underwent bilateral removal of the hippocampus and edge, and memory is the process by which we retain that neighboring regions in the temporal lobe as treatment for knowledge over time. Most of what we know about the world epilepsy. In the first and best-studied case, Brenda Milner of and its civilizations we have learned. Thus, learning and mem­ the Montreal Neurological Institute described a 27-year-old ory are central to our sense of individuality. Indeed, learning assembly-line worker, H.M., who had suffered from untreat­ goes beyond the individual to the transmission of culture able and debilitating temporal lobe seizures for more than from generation to generation. Learning is a major vehicle for 10 years. The surgeon William B. Scoville removed the medi­ behavioral adaptation and a powerful force for social prog­ al portion of the temporal lobes on both sides of H.M.'s ress. Conversely, loss of memory leads to loss of contact with brain. The seizure disturbance was much improved. But im­ one's immediate self, with one's life history and with other hu­ mediately after the operation, H.M. experienced a devastat­ man beings. ing memory deficit: he had lost the capacity to form new Until the middle of the 20th century, most students of be­ long-term memories. havior did not believe that memory was a distinct mental Despite his difficulty with the formation of new memories, function independent of movement, perception, attention H.M. still retained his previously acquired long-term memory and language. Long after those functions had been localized store. He remembered his name, retained a perfectly good use to different regions of the brain, researchers still doubt­ of language and kept his normal vocabulary; his IQ remained ed that memory could ever be assigned to a specific region. in the range of bright·normal. He remembered well the The first person to do so was Wilder G. Penfield, a neuro- ERIC R. KANDEL and ROBERT D. HAWKINS have collaborated on studies of the neurobiology of learning. Kandel is University MIRROR DRAWING EXPERIMENT in patients with temporal Professor at the College of Physicians and Surgeons of Columbia lobe lesions gave the first hint, in 1960, that there are two University and senior investigator at the Howard Hughes Medical distinct types of learning systems. One form, which is spared Institute. He received anA.B. from Harvard College, anM.D . from by the lesions, involves tasks that have an automatic quality the New York University School of Medicine and psychiatric train· such as the skilled movements illustrated in this experiment. ing at Harvard Medical School. Hawkins received a B.A. from The subject, who can see his hand only in the mirror, tries to Stanford University and a Ph.D. in experimental psychology from trace the shape of a star. The second type of learning depends the University of California, San Diego. He is associate professor on conscious awareness and cognitive processes and is abol­ in the Center for Neurobiology and Behavior at Columbia. ished by the lesions. SCIENTIFIC AMERICAN September 1992 79 © 1992 SCIENTIFIC AMERICAN, INC events that preceded the surgery, such psychological distinction-a division in task better after five days of practice as the job he had held, and he remem­ the way all of us acquire knowledge. than on the first day, he may respond, bered vividly the events of his child­ Although it is still not clear how many "What are you talking about? I've never hood. Moreover, H.M. still had a com­ distinct memory systems there are, re­ done this task before." pletely intact short-term memory. What searchers agree that lesions of the tem­ Whereas explicit memory requires H.M. lacked, and lacked profoundly, poral lobes severely impair forms of structures in the temporal lobe of ver­ was the ability to translate what he learning and memory that require a con­ . tebrates, impliCit memory is thought to learned from short-term to long-term scious record. In accordance with the be expressed through activation of the memory. For example, he could con­ suggestion of Neal j. Cohen of the Uni­ particular sensory and motor systems verse normally with the hospital staff, versity of Illinois and Larry R. Squire of engaged by the learning task; it is ac­ but he did not remember them even the University of California at San Diego quired and retained by the plasticity in­ though he saw them every day. and of Daniel L. Schacter of the Univer­ herent in these neuronal systems. As a The memory deficit following bilater­ sity of Toronto, these types of learning result, impliCit memory can be studied al temporal lobe lesions was original­ are commonly called declarative or ex­ in various reflex systems in either ver­ ly thought to apply equally to all forms plicit. Those forms of learning that do tebrates or invertebrates. Indeed, even of new learning. But Milner soon dis­ not utilize conscious participation re­ simple invertebrate animals show ex­ covered that this is not the case. Even main surprisingly intact in patients with cellent reflexive learning. though patients with such lesions have temporal lobe lesions; they are referred profound deficits, they can accomplish to as nondeclarative or implicit. he existence of two distinct forms certain types of learning tasks as well Explicit learning is fast and may take of learning has caused the reduc­ as normal subjects can and retain the place after only one training trial. It of­ tionists among neurobiologists to memory of these tasks for long periods. ten involves association of simultane­ askT whether there is a representation on Milner first demonstrated this residual ous stimuli and permits storage of in­ the cellular level for each of these two memory capability in H.M. with the dis­ formation about a single event that types of learning process. Both the neu­ covery that he could learn new motor happens in a particular time and place; ral systems that mediate explicit memo­ skills normally [see illustration on page it therefore affords a sense of familiar­ ry and those that mediate impliCit mem­ 181. She, and subsequently Elizabeth K. ity about previous events. In contrast, ory can store information about the as­ Warrington of the National Hospital for implicit learning is slow and accumu­ sociation of stimuli. But does the same Nervous Diseases in London and Law­ lates through repetition over many tri­ set of cellular learning rules guide the rence Weiskrantz of the University of als. It often involves association of se­ two memory systems as they store as­ Oxford, found that patients such as quential stimuli and permits storage of sociations, or do separate sets of rules H.M. can also acquire and retain memo­ information about predictive relations govern each system? ry for elementary kinds of learning that between events. Implicit learning is ex­ An assumption underlying early stud­ involve changing the strength of reflex pressed primarily by improved perfor­ ies of the neural basis of memory sys­ responses, such as habituation, sensiti­ mance on certain tasks without the sub­ tems was that the storage of associa­ zation and classical conditioning. ject being able to describe just what tive memory, both implicit and explicit, It immediately became apparent to has been learned, and it involves mem­ required a fairly complex neural circuit. students of behavior that the difference ory systems that do not draw on the One of the first to challenge this view between types of learning that emerged contents of the general knowledge of was the Canadian psychologist Don­ from studies of patients with temporal the individual.