Dr. Huttenlocher’s research, rooted in work he began in the 1970s on samples of cerebral cortex taken from human cadavers, helped establish the brain as an adaptive organism — neural plasticity, as it is called — but one in which the ability to adapt declines with age.

“I stumbled on the whole thing,” Dr. Huttenlocher told The Chicago Tribune in 1993. “It was something that nobody expected. It took quite a long time until people began to accept that this really happens.”

Dr. Huttenlocher’s work is “like the Bible in this area of science,” said Dr. Eric Kandel, a professor of brain science at Columbia University who won the Nobel Prize in Physiology or Medicine in 2000. “He saw implications of it that many people who only had a basic science background might have overlooked.”

Dr. Huttenlocher was the chief of pediatric neurology at the University of Chicago in the mid-1970s when he began using an electron microscope to photograph billions of synapses. Then he began counting them. He counted at the lab. He counted at home.

“He had all of these pictures of synapses in our house,” his daughter said.

He had set out to study differences in the brains of people who were intellectually disabled, but he decided there might be more to learn by studying the control group, the so-called normal brains.