My husband, at 74, is the baby of his bridge group, which includes a woman of 85 and a man of 89. This challenging game demands an excellent memory (for bids, cards played, rules and so on) and an ability to think strategically and read subtle psychological cues. Never having had a head for cards, I continue to be amazed by the mental agility of these septua- and octogenarians.

The brain, like every other part of the body, changes with age, and those changes can impede clear thinking and memory. Yet many older people seem to remain sharp as a tack well into their 80s and beyond. Although their pace may have slowed, they continue to work, travel, attend plays and concerts, play cards and board games, study foreign languages, design buildings, work with computers, write books, do puzzles, knit or perform other mentally challenging tasks that can befuddle people much younger.

But when these sharp old folks die, autopsy studies often reveal extensive brain abnormalities like those in patients with Alzheimer’s. Dr. Nikolaos Scarmeas and Yaakov Stern at Columbia University Medical Center recall that in 1988, a study of “cognitively normal elderly women” showed that they had “advanced Alzheimer’s disease pathology in their brains at death.” Later studies indicated that up to two-thirds of people with autopsy findings of Alzheimer’s disease were cognitively intact when they died.

“Something must account for the disjunction between the degree of brain damage and its outcome,” the Columbia scientists deduced. And that something, they and others suggest, is “cognitive reserve.”