In its casting of a cognitively sophisticated neocortex unable to fully restrain the primal emotional responses of the limbic system, MacLean’s model was a neuroanatomical cousin to Freud’s tripartite view of the mind, with its warring superego, ego and id. The theory saw its fullest expression in MacLean’s 1990 magnum opus, The Triune Brain in Evolution, which was based on wide-ranging anatomical studies of brains in animals as diverse as alligators and monkeys.
Twenty years later, MacLean rounded out his picture of the triune (three in one) brain by adding what he termed the R-complex (for “reptilian”)-structures in the brain’s core and brain stem that govern basic survival functions-to the neocortical and limbic systems he had defined previously. “Our affective behavior continues to be dominated by a relatively crude and primitive system,” he wrote. Since all mammals possess variants of these structures, MacLean concluded that they are phylogenetically ancient and that the emotional responses they produce are only weakly regulated by such newer, human structures as the neocortex. After conducting electroencephalographic recordings in patients with psychosomatic illnesses and epilepsy at Massachusetts General Hospital, MacLean had become convinced that the emotional components of these disorders were seated in deep brain structures that he called the visceral brain (and renamed the limbic system in 1952), which included the hippocampus, amygdala and cingulate gyrus. The beginnings of MacLean’s theory appeared in a 1949 paper just as he joined the faculty of the School of Medicine as an assistant professor of physiology with a joint appointment in psychiatry. MacLean once roamed through a room, Insel recalled, feeling the scalps of visiting scientists to ascertain the presence or absence of a skull protuberance he had deemed an important factor in the evolution of human intelligence. Insel remembers his colleague as irreverent and uninhibited. Insel, M.D., director of the National Institute of Mental Health, who worked alongside MacLean for 10 years at the Laboratory of Brain Evolution and Behavior in Poolesville, Md. “Paul never traveled with the herd,” said Thomas R. MacLean, who died last December at age 94, was a highly original-some say eccentric-thinker whose model of the triune brain, though now discredited, has had a lasting cultural impact. The real star of the book, however, was a theory of human neural organization that took root some 30 years earlier in writings by Paul D. Dragons won the 1978 Pulitzer Prize for general nonfiction and helped to launch Sagan’s celebrity as a spokesman for science in the 1980s. In 1977 readers were enthralled by The Dragons of Eden, a book by the astronomer Carl Sagan that explored the evolution of the human brain.
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