"The Forgetting" by David Shenk

The normal memory is in a state of constant, imperceptible revision: Impressions are perpetually rewritten, exhaustively reworked. Old memories are durable because we've revisited and remade them every time we've experienced a related perception. They can be untrustworthy in a court of law because we've sifted and summarized them so extensively, perhaps integrating a chance resemblance or fortuitous suggestion. We even reshape them by selective forgetting, cutting back a tree here or there in order to keep the forest in view.

Sensations don't go directly or easily into the brain's long-term memory matrix. Short-term memory enables us to learn -- to negotiate the new and unfamiliar, juggling them until we can fit them into our permanent data store. As we get older we sometimes have to make a conscious effort to remember something new, perhaps effectively "underlining" it in our short-term memory buffer by writing it down. But for an early Alzheimer's sufferer, such efforts often fail. Malformations ("plaques and tangles") have grown within the brain's cortex, impeding the neurons' initial firing, interfering with short-term memory creation.

Still in the early stage, my mother-in-law is pretty content right now: Friendly home health aides help her with the laundry and shopping, get the mail and tell her when to take her pills. A geriatric care manager makes sure the aides arrive on schedule and that prescriptions are refilled. My husband oversees her bill paying; my adult son balances her checkbook; I replace her clothes as they wear out. It takes a village to do the work of one normal brain, but for now she likes living in the jerry-rigged village we've built at the upper tip of Manhattan. She's always wanted us to call every day and now we do. I'm not sure what my husband discusses with her, but she and I often happily revisit my home-decorating misadventures. She's lost her impatience with my blunders, and I've begun to rather cherish them.

The future will doubtless be a lot tougher. As Alzheimer's progresses -- slowly, over years -- the plaques and tangles spread and memory attenuates until, late in the second stage, identity itself is obliterated. In the final stage the brain "forgets" how to regulate basic body functions like swallowing.

"And I'm at risk for it, you know," my husband commented. "But by then," I replied -- much too quickly -- "there'll be a cure. Or ... or therapies. Something."

Researchers do talk about the possibility of a cure -- it'll be here in 10 years, one of them tells Shenk exultantly. Responding to the prospect of millions of octogenarian boomers wandering around on a last long strange trip, Alzheimer's research is well funded and promises rich rewards. Scientists are making progress: There are now a few drugs on the market that stimulate memory for a limited period, even as the disease continues to ravage the brain.

Shenk describes the research -- theory, technique and furious rivalries between contending schools of opinion -- in vivid, laypersons' language. He goes further as well, to speculate on the limits of the human lifespan itself.

Little-understood environmental factors aside, the reason Alzheimer's is so common nowadays is that we're beginning to live on what biologists call "manufactured time." Inherited traits that would have prevented us from reaching the age when we're most likely to reproduce have been winnowed out by natural selection. What happens to us in our later years, however, doesn't affect our chances of reproducing. And so diseases rarely seen before the 20th century have become more common as science has created new ways to keep us alive into our 80s and beyond.

Next page: From "King Lear" to "Memento"