It is one of those jokes neurologists regularly share when the subject turns to patients complaining of memory lapses: When you can't remember where you left your glasses, there's probably no need to worry. When you can't remember you wear glasses you're probably in trouble.
During the first several decades of life, we layer memory upon memory -- the smell of a mother's hair, the light touch of a first kiss, the multiplication tables, driving directions, telephone numbers and the skills and knowledge of an occupation. By about age 25, however, a human's memory has typically peaked and a long period of decline begins.
It proceeds at a stately pace, virtually undetectable to most, for several decades. But around about the half-century mark, cognitive performance starts to slide noticeably. Keys and glasses are forgotten. Names, places and words linger on the tip of the tongue.
When it comes to memory lapses, what's normal and what's not? It is a question preoccupying more and more Americans. In June, the Centers for Disease Control and Prevention reported that Alzheimer's disease is now the sixth leading cause of death in the United States: Almost 5.2 million in the U.S. live with the disorder, a number projected to rise. The search for effective drugs has been one of frustration -- just this year, two drugs in late-stage clinical trials failed to prove effective.