When I was in college in the 1970s, we were taught that no new neurons ever formed in the mammalian brain. We were born, it was argued, with all the neurons we would ever get. Yet, a decade earlier, in 1962, a young scientist named Joseph Altman had reported in the prestigious journal Science that new neurons could form. Altman went on to publish other studies, but his research was ignored and rejected. Then, in the 1970s, another young scientist, Michael Kaplan, discovered neurogenesis (the birth of new neurons) in the brains of rats. Kaplan published 19 papers on the subject, but his work met with enormous resistance, and he eventually left research science and went on to a career as a rehabilitation doctor. It wasn't until the 1990s that scientists accepted the idea of neurogenesis in adult mammalian brains and discovered newborn neurons not just in the brains of rats but in the brains of people too.
Neurogenesis does not occur everywhere in the brain but is evident in the hippocampus and olfactory bulb and perhaps in the cerebral cortex. New neurons are born not from mature nerve cells but rather develop from neural stem cells that remain in our brains throughout life. Indeed, in some brain areas, there is continual turnover of neurons—old ones die and new ones are born—and these new neurons can participate in circuits that underlie learning.
If new neurons are being formed every day in our brain, how can we hold onto these cells and not let them simply die away? An answer has been lurking around in the scientific literature since the 1970s, when Kaplan reported that an enriched environment enhances the number of new neurons. In animals, placing them in cages full of interesting toys or giving them learning tasks promotes the survival of these newborn cells. New neurons born in the hippocampus may participate in the formation of long-term memories as well as in spatial perception. London taxi drivers who have extensive knowledge of the London city streets have larger-than-normal hippocampi. Perhaps, much neurogenesis has occurred in their hippocampi.
Physical exercise also increases the number of newborn neurons. But some conditions, such as excessive stress and depression, hamper the growth of newborn nerve cells.
A person in their eighties still grows new neurons, but to harness the potential of these newborn cells, we may need to keep challenging ourselves, explore new things, exercise, and see the proverbial cup not as half empty, but as half full.