PET scans of the brains of a person with normal memory ability and someone diagnosed with Alzheimer's Image courtesy of the National Institute on Aging/National Institutes of Health hide caption

itoggle caption Image courtesy of the National Institute on Aging/National Institutes of Health

Tennessee women's basketball coach Pat Summitt sits with her dog at her home in Knoxville, Tenn. Summitt plans to coach "as long as the good Lord is willing" despite recently being diagnosed with early onset dementia. Debby Jennings/AP/University of Tennessee hide caption

itoggle caption Debby Jennings/AP/University of Tennessee

A woman with Alzheimer's disease walks in a corridor at a retirement house in Angervilliers, France, last month. Sebastien Bozon/AFP/Getty Images hide caption

itoggle caption Sebastien Bozon/AFP/Getty Images