A hurricane, a car accident, a roadside bomb, a rape — extreme stress is more common than you might think, with an estimated 50 to 60 percent of Americans experiencing it at some point in their lives. About 8 percent of that group will be diagnosed with post-traumatic stress disorder, or PTSD. They will have flashbacks and nightmares. They will feel amped up, with nerves on a permanent state of high alert. They won’t be able to forget.

One of the only effective treatments for PTSD is ‘exposure therapy,’ in which people are repeatedly exposed to their fear — such as a painful memory — in a safe context. This treatment works partly because of how our brain encodes memories.