From the APR/MAY issue of Seed:
Our ability to form new memories diminishes as we grow older. Even the best musicians find new notes much harder to memorize at 50, and largely maintain their careers through polished performances of earlier learned classics. In my case, the experiments behind important new biological advances no longer are easily encoded within my brain on first exposure. I must repeatedly recall experimental details before they become installed in my temporal lobes. Most days I keep my diminished brain powers out of my immediate consciousness, just as I try to avoid my face in the mirror. No one wants to anticipate a future in which our bodies sag and wrinkle and our brains slow down. Other days I wonder why our ability to remember inevitably declines.
New facts may be hard to get into mature brains because there is no free space to store them. When we are young, the brain is steadily increasing in size, constantly generating new neurons. However, once adolescence has passed, the size of the brain is fixed and the generation of neurons—a process called neurogenesis—becomes largely restricted to the hippocampus, the cortical region where new memories are formed. Research has shown that as mice age, they form fewer and fewer new hippocampal neurons, and their capacity to learn correspondingly declines. Equally important, blocking neurogenesis by interfering with DNA synthesis quickly shuts down a mouse's ability to learn. Storage of new information in the adult brain may very well require the generation of new nerve cells, as well as newly freed space created by the death of neurons bearing memories that we no longer recall.
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Unfortunately, there is no direct way to count the number of newly forming nerve cells in humans. So we have no way, as yet, to confirm the obvious hunch that age-related diminishment in human memory, like that of the mouse, is due to a slowing down of hippocampal neurogenesis. We do know, however, that when chemotherapy temporarily stops hippocampal neurogenesis, cancer patients find themselves much less able to recall the details of their daily lives.
This decrease in neurogenesis may be a reflection of the evolutionary imperative for long-living animals to have long-lasting memories. In prehistory—before books or maps or Google—recollections of how, say, to cross a mountain range or find water-filled oases were necessary assets to hunter-gatherers. Until the development of written languages, all of human experience and culture had to be carried in our brains. Older people with vast memories of the past were necessarily more respected than their younger, less-experienced counterparts. Today, however, much of our culture is stored in books, in musical scores, in enduring works of art, and now in the hard drives of computers. These days, fast-learning, web-savvy 25-year-olds may have more to give their communities than their experienced 50-year-old equivalents. The latters' accumulated wisdom may not be as applicable to our ever faster-moving future.
In fact, the most important long-term medical challenge now facing advanced human civilizations may not be to stop cancer or Alzheimer's disease. Rather, it may be to slow down the rate at which we lose the ability to generate new adult nerve cells. Here experiments with mice may lead the way. For instance, when mice use treadmills to run long distances each day, they make new nerve cells at double the rate of their sedentary peers. I hope science will show that the same holds true for humans. I may still be on full salary because I still regularly play tennis with young pros who jump and lunge—and force me to do the same.
—Nobel laureate James D. Watson is the co-discoverer of the double helix.