Dr. Mark Rosenzweig, a research psychologist whose studies in animals found that the brain reshapes itself in response to experience, in adulthood as well as in early childhood, died July 20 at his home in Berkeley, Calif. He was 86.

The cause was kidney failure, his daughter Suzanne Washburn said.

When Rosenzweig first reported in the 1950s and 60s that rodents brains developed distinct anatomical changes in response to different environments, other researchers were dismissive. Most scientists at the time considered the mature brain to be a fixed entity; individual brain cells may die, they believed, but function and structure were set in neural concrete, and learning did not alter the brains anatomy.

In a series of experiments, Rosenzweig and a team including Edward Bennett, a biochemist, and David Krech, a psychologist, compared rats raised in two environments. One was a small, barren enclosure and the other a larger space with mazes, toys like seesaws and swings, and other rats. They found that in those rats who were in the richer environment, the cerebral cortex, which is central to learning, weighed significantly more. The researchers also found different levels of cholinesterase, a brain enzyme, in the groups.

We did not invent the concept of the enriched environment, Rosenzweig later wrote, but I believe that our publications introduced the concept and the term to the neuroscience community.

Subsequent studies, with the anatomist Marian Diamond, showed that the adult brain also reshaped itself in response to new experience; the changes in anatomy were not due to development or other factors, like diet.

Rosenzweig was careful not to extrapolate his findings to people, and there is no evidence that measures like cortical weight and cholinesterase levels are markers for human intelligence. Still, the principle of an active, self-transforming brain had been established.

His work, and his colleagues, laid to rest the idea that the brain is fixed in terms of its composition; thats now regarded as nonsense, said James McGaugh, a professor of neurobiology and behavior at the University of California, Irvine. The field of neuroscience is now focused on understanding the nature and extent of the changes the brain makes in response to experience.

Mark Richard Rosenzweig was born in Rochester, N.Y., on Sept. 12, 1922, the son of a lawyer and a homemaker. He earned bachelors and masters degrees in psychology from the University of Rochester, the higher degree in 1944, then served in the Navy. After being discharged, he enrolled at Harvard, where he earned a doctorate in 1949, and then joined the psychology faculty at the University of California, Berkeley. He remained at Berkeley until his retirement in the 1990s. In 1998, he won the American Psychological Associations Distinguished Scientific Contribution Award.

Rosenzweigs wife of 60 years, Janine Chappat Rosenzweig, died in April 2008.