New nerve cells, produced naturally by adult neural stem cells present in the brain, appear to be essential for learning and memory. The old idea that brain cells are not renewed (that "you start with as many brain cells as you'll ever have, and it's downhill from there") was tossed out the window by scientists years ago. Researchers showed in the early 1990's that the adult brain continues to make new neurons, a process termed "neurogenesis", throughout life (Gage has a good review of the early years.) Now researchers at Kyoto University in Japan have shown in mice that new brain cells are necessary for learning and for memory. The new research, published in Nature Neuroscience, indicates that neural stem cells in the adult brain continue to produce new brain cells that are important for memory and learning. The new study supports work published earlier this year showing that new brain cells can affect learning and memory. Last year research showed that transplanting adult neural stem cells into brain-injured mice could restore some memory, and in March 2008 another study showed that injecting human umbilical cord blood stem cells into the brains of aging animals boosted neurogenesis. A recent study also suggested that stimulating specific molecules in the brain could reactivate adult neural stem cells. Exercise has also been shown to stimulate neurogenesis in the brain. So various methods seem to have the potential to help the mouse remember where he put his cheese, or where you left your keys, by stimulating your own brain adult stem cells.