Progress in stem cell medicine may lead to ways to restore the capacity for memory lost with aging by intervening in the activity of neural stem cells: researchers "have discovered an answer to the long-standing mystery of how brain cells can both remember new memories while also maintaining older ones. They found that specific neurons in a brain region called the dentate gyrus serve distinct roles in memory formation depending on whether the neural stem cells that produced them were of old versus young age. ... In animals, traumatic experiences and aging often lead to decline of the birth of new neurons in the dentate gyrus. In humans, recent studies found dentate gyrus dysfunction and related memory impairments during normal aging. ... In the study, the authors tested mice in two types of memory processes. Pattern separation is the process by which the brain distinguishes differences between similar events, like remembering two Madeleine cookies with different tastes. In contrast, pattern completion is used to recall detailed content of memories based on limited clues, like recalling who one was with when remembering the taste of the Madeleine cookies. ... Neuroscientists have long thought these two opposing and potentially competing processes occur in different neural circuits. The dentate gyrus, a structure with remarkable plasticity within the nervous system and its role in conditions from depression to epilepsy to traumatic brain injury - was thought to be engaged in pattern separation and the CA3 region in pattern completion. Instead, [researchers] found that dentate gyrus neurons may perform pattern separation or completion depending on the age of their cells."