Dr. Paul D. MacLean, the neuroscientist who in the 1960s developed the then-revolutionary concept of the three-part brain -- one part controlling involuntary actions, one controlling emotions and the third involved in thought processes -- died of a heart attack Dec. 26 at his home in Potomac, Md. He was 94.
His conception of the “triune brain” has since been dismissed by most neuroscientists as simplistic, but his ideas laid the foundation for much of the brain research that was to follow and played a major role in popularizing the subject for a lay audience.
He thought that the complex emotions and violent behaviors of humans were produced by imbalances in the control of the brain by one or more of the three areas. Violence, for example, would result when the part of the brain controlling the most primitive instincts predominated.
He speculated that each of these areas developed independently as evolution drove animals into more complex behaviors.
The oldest and simplest part of the brain, according to the triune model, is the “R-complex” or reptilian brain, which primarily comprises the brain stem and the cerebellum. Its primary function is controlling heartbeat, breathing muscles and balance, but it is also responsible for aggression, territorial instincts and similar behaviors -- all of them factors crucial for survival in even the most primitive animals.
Next to develop was what MacLean termed the limbic system, including the amygdala and hippocampus. These parts of the brain house the primary centers of emotion and are crucial in converting information into long-term memory. The limbic system is involved in primary activities related to food and sex, as well as the mediation of emotions and feelings.
Last to develop was the neocortex, which constitutes about five-sixths of the brain’s mass. It makes language possible and is necessary for logical thinking and planning for the future.
The idea worked its way into many publications in the 1960s and ‘70s, including the works of Arthur Koestler and Carl Sagan’s Pulitzer Prize-winning “The Dragons of Eden: Speculations on the Evolution of Human Intelligence.”
MacLean himself elucidated his ideas in his 1990 book, “The Triune Brain in Evolution: Role in Paleocerebral Functions.”
Researchers now know that the evolution of the brain was much more complex than the simple pattern suggested by MacLean.
Furthermore, more sophisticated techniques now available show that the functions supposedly residing in each of the areas are spread more widely throughout the brain.
Nonetheless, MacLean’s experiments stimulated other scientists to begin exploring what many have termed the “big questions” of life, such as consciousness and philosophy, rather than restricting themselves to more mechanical problems such as vision, hearing and movement.
Paul Donald MacLean was born May 1, 1913, in Phelps, N.Y. He graduated from Yale University in 1935, then received his medical degree there in 1940. During World War II, he was a military medical officer stationed in New Zealand.
After brief stints in private practice in Seattle and as a public health officer at Massachusetts General Hospital, in 1949 he joined the faculty of Yale, where he performed most of his research on the triune brain.
In 1957, he joined the National Institute of Mental Health in Bethesda, Md., where he worked until his retirement in 1985.
His wife of 64 years, Alison Stokes MacLean, died in 2006.
MacLean is survived by four sons, Paul Jr. of New York City, David of Middletown, R.I., Alexander of Lincoln, Mass., and James of Rockville, Md.; a daughter, Alison Cassidy of Potomac; a brother; 13 grandchildren; and two great-grandchildren.