She called it from her home phone but heard nothing. Finally, while making dinner a few hours later, she found it  in the freezer.

She was so unnerved, not just by that but also by the poor results of a subsequent mental status test, that she had an MRI done on her brain. The diagnosis: perfectly normal. Dr. Cohen said people can also overreact, attributing absent-minded actions to failing brains, when it is actually simple distractibility that is to blame.

Nancy Cutler, 51, a publication designer in Piedmont, Calif., grew worried about her brain a few years ago when she drove her car to work one day, then, forgetting she had done so, took the bus home.

“It was pretty embarrassing to have my kid call me and say, ‘what do you mean you’re on the bus?’ ”

Ms. Cutler reminded herself that she was preparing for her son’s bar mitzvah, going through a stressful period and was very distracted. But she was concerned enough to report the incident to her physician, and ask if there were certain supplements she should be taking. The doctor told her to take up activities that challenged her mind. (Ms. Cutler said she had not done anything yet, because it is “a real time commitment.”)

Dr. Cohen, who recently conducted a study of people born from 1946 to 1955, the first half of the baby boom, said he was struck by the number of respondents who believe they can do things on their own to enhance the vitality of their brains.

“There is a gradual growing awareness that challenging your brain can have positive effects," Dr. Cohen said. He said the plasticity of the brain is directly related to the production of new dendrites, the branched, tree-like neural projections that carry electrical signals through the brain “Every time you challenge your brain it will actually modify the brain,” he said. “We can indeed form new brain cells, despite a century of being told it’s impossible.”

In pursuit of his own dendritic growth, Dr. Cohen plans to take up the piano again after years of not playing. He is also sketching out a science-fiction novel he hopes to write.