Those who have endured the rigors of cancer therapy talk about “chemo brain,” the memory and concentration problems that accompany radiation and chemotherapy. Now researchers led by neurologist Michelle L. Monje of Harvard University have found the root of these cognitive difficulties: damaged stem cells.

In the hippocampus, a brain region vital for laying down new memories, “stem cells continue to add new circuit elements,” says Stanford University neuroscientist Theo D. Palmer, who helped Monje find out why brain fogginess can persist for years after cancer treatment has ended. They discovered that the chemicals and radiation used to kill tumor cells damage the stem cell reservoir in the hippocampus and nearly halt the formation of new neurons in both children and adults.

Radiation treatment also triggers a response from microglial cells, the immune cells of the central nervous system. Because the inflammatory cells stifle neuronal growth, some experts think that the microglia may be the real culprit behind radiation-induced brain defects. The researchers’ previous work in rats showed that anti-inflammatory drugs helped to restore some neurogenesis.

Without such intervention, stem cells damaged by radiation do not seem to recover, according to Monje. But there is hope: exercise has been shown to stimulate neurogenesis in healthy animals and in people, so Monje thinks there is a good chance that being active would help improve cognition in cancer survivors, too.