Gary Small, M.D., remembers the patient well. An accomplished mathematician in his early 70s, the man consulted Small after struggling with calculations, and after his wife noticed he was getting cranky. Small, director of the UCLA Center on Aging, put the mathematician through a battery of tests—and the man got top scores on all of them, including 30 out of 30 on a memory test and a whopping 140 on his IQ test. So when Small saw the patient's brain scan, he was stunned: it had all the markings of full-blown Alzheimer's disease.

"Usually, people with such profound brain changes can barely carry on a conversation," says Small. "This man was still doing high-level mathematics." Though the case is extreme, it is not unique. In fact, up to 20 percent of people autopsied who had no major memory problems are discovered to have had Alzheimer's, says Yaakov Stern, Ph.D., a neuropsychologist at Columbia University College of Physicians and Surgeons in New York City.

How does the brain continue to function—sometimes quite efficiently—despite changes that should cause severe disability? An answer, many scientists believe, is "cognitive reserve": the combination of a person's innate abilities and the additional brainpower that comes from challenging the mind. Studies show that diverse, mentally stimulating tasks result in more brain cells, more robust connections among those cells, and a greater ability to bypass age- or disease-related trouble spots in the brain. The more you work your mind, the greater your cognitive reserve. And the greater your reserve, the greater your ability to withstand the inevitable challenges of aging.

As a doctor specializing in memory problems, I have seen recent discoveries offer new hope and guidance to people who want to maintain peak brain performance. Although we don't know how to stop the devastation of dementia, we learn more each year about combating the small losses in brainpower that often come after 50—even while you enjoy the brain benefits of getting older (and yes, as you'll see below, those benefits do exist).

Your brain at 50+

Despite what our youth-oriented culture tells us, mental decline after 50 is not a given. In fact, in some ways the healthy brain gets stronger with age. Studies confirm that accumulated knowledge and expert skills (a.k.a. wisdom) increase as you get older. In addition, emotional savvy, such as not dwelling on negative thoughts, also appears to grow with age, as demonstrated in a recent Duke University study. Researchers showed a set of photos to study participants. Some of the photos were of neutral items such as household objects; others were distressing shots of violent scenes. Tested later, participants in their 70s remembered about the same number of neutral images as did those in their 20s, but the older people remembered fewer of the unpleasant ones. Cell-signaling activity in the older group suggested their brains filtered out bad memories.

Other brain functions may not improve with age, but they don't automatically wane either. One example is higher-order decision making such as choosing the best investments. Older people do as well as younger ones on tests that measure this function—as long as they aren't rushed.

And that's the catch. Some brain functions tend to decline with age, and speed is one of them. The likely reasons are loss of neural connections, blockages of blood supply, and decreases in nerve-signaling chemicals.