Marian Diamond, taking a close look at the brain of a rat, demonstrated the impact of the environment on brain development. (Eric Luse/San Francisco Chronicle)

Marian Diamond, a pathbreaking neuroscientist whose research — including a study of Albert Einstein’s preserved brain — showed that the body’s three-pound seat of consciousness was a dynamic structure of beautiful complexity, capable of development even in old age, died July 25 at an assisted-living community in Oakland, Calif. She was 90.

A daughter, Ann Diamond, confirmed her death but did not know the cause.

Dr. Diamond, a professor emerita of integrative biology at the University of California at Berkeley, was for decades known on campus as the woman with the hat box. Inside the container, decorated on the outside with a floral print and carried by a bright blue string, was a preserved human brain.

It was the crucial prop for a lesson she spent a half century teaching: that the brain was, as she once wrote, “the most complex mass of protoplasm on this earth and, perhaps, in our galaxy.”

Dr. Diamond was considered a foundational figure in modern neuroscience. Crucially, she provided the first hard evidence demonstrating the brain’s plasticity — its ability to develop, to grow, even in adulthood. “In doing so,” her colleague George Brooks said in a statement, “she shattered the old paradigm of understanding the brain as a static and unchangeable entity that simply degenerated as we age.”

Dr. Diamond in 2010, holds a preserved brain and her signature flowered hat bag. (Elena Zhukova)

Her breakthrough occurred in the early 1960s, when — building on the work of psychologist Donald O. Hebb — she began studying the brains of lab rats. Rats that were raised alone, in small and desolate cages, had more trouble navigating a maze than did rats that were raised in “enriched” cages, with toys and rat playmates.

Studying their brains under a microscope, Dr. Diamond found that the cerebral cortices of the rats in enriched cages were about 6 percent thicker than the rats in the “impoverished” cages.

Her findings, published in a 1964 paper with three colleagues, were a pivotal contribution to the long-running debate between nature and nurture, which seeks to determine the extent to which a person is shaped by their genes or by their life experiences.

At the time, genes were believed to play the all-important role. “The idea that the brain could change based on environmental input and stimulation was felt to be silly,” said Robert Knight, a UC-Berkeley professor of psychology and neuroscience. “And that’s the boat she completely sunk.”

Dr. Diamond went on to develop a rich theory of brain plasticity, one that she sometimes summarized with a phrase more commonly heard at a gym than a neuroscience classroom: “Use it or lose it.” In papers and lectures often directed at a wide audience, she outlined five factors crucial to brain development at any age: diet, exercise, challenge, newness and — perhaps surprising for a laboratory researcher — love.

She said she stumbled upon the fifth factor while performing her experiments with lab rats, which weren’t living long enough for her to study their brains in old age. Although the cages were well cleaned, many of the rats were dying after about 600 days, or roughly 60 years in a human time span. But some were living much longer.

The difference, she found, was touch. By holding the lab rats against her lab coat and petting them each day, she found that she could increase their life span — and found that these rats generally had thicker cerebral cortices.

She was born Marian Cleeves in Glendale, Calif., on Nov. 11, 1926. Her father was a physician from northern England, and her mother, a classics scholar, dropped out of her doctoral program to raise Dr. Diamond and her five older siblings.

Dr. Diamond attended Glendale Community College — she said she was afraid of leaving her parents at home alone, as empty nesters — before transferring to UC-Berkeley, where she lettered in tennis and graduated with a bachelor’s degree in 1948. She soon became the anatomy department’s first female graduate student, receiving her master’s degree in 1949 and her doctorate in 1953, the same month her first child was born.

She was named Cornell University’s first female science instructor in 1955 and returned to UC-Berkeley five years later. She remained at the school until her retirement in 2014, and in recent years was one of the most popular professors on YouTube, where videos of her anatomy course were viewed more than 1 million times.

Dr. Diamond faced gender discrimination early on, according to Gary Weimberg, who co-directed a 2016 documentary of Dr. Diamond, “My Love Affair with the Brain.” In one of her first major articles, she told him, a male colleague failed to properly credit her contributions; although she performed the bulk of the research and initiated the project, her name was the last of four and — as though it were incidental — was listed in parentheses.

Dr. Diamond said she confronted the colleague, who told her that he had never written an article with a female co-writer. “Treat it like another name,” she said. Ultimately, her name appeared first.

Dr. Diamond made headlines in the 1980s for performing research on Einstein’s brain. After the physicist’s death in 1955, his brain was removed and preserved — without permission from Einstein’s family — by pathologist Thomas Harvey.

Decades later, Dr. Diamond asked to study several sections of Einstein’s brain. In 1984, three years after making her request, the brain finally arrived in the mail: four thin slices, preserved in formaldehyde and stored inside a mayonnaise jar.

Placing the tissue under a microscope, Dr. Diamond found an unusually high amount of glial cells, which were thought to be a relatively unimportant part of the tissue that held the brain together. Her discovery launched renewed interest in the role of glial cells, which are now believed to play a crucial role in cognitive processes.

Dr. Diamond’s marriage to Richard M. Diamond, a nuclear chemist, ended in divorce. Her husband of 35 years, Arnold Scheibel , a neuroscientist at the University of California at Los Angeles, died in April.

Survivors include four children from her first marriage, Catherine Diamond of Taipei, Taiwan, Ann Diamond of Mazama, Wash., and Richard C. Diamond and Jeff Diamond, both of Berkeley.

Her children often witnessed Dr. Diamond’s appreciation for the brain firsthand.

Once, Ann Diamond recalled, her mother flew across the country to visit her at a summer camp. Dr. Diamond carried two hat boxes onto the plane: one bearing peach pie, the other a human brain. Ann Diamond ate the pie.

As for the brain, Dr. Diamond hoisted the formaldehyde-soaked organ out of the box with her bare hands, and encouraged campers to don gloves and touch it themselves.