When I was in college in the 1970's, 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 1970's, 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 1990's that scientists accepted the idea of neurogenesis in 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.