New research showing how memories take shape may lead to better treatments for unwanted memories as well. Building on research dating to the 1990s and farther back, studies today are showing specific ways to reduce the fear-laden portion of traumatic memories, such as those that occur in post-traumatic stress disorder.

Some approaches center on stress hormones, which help to entrench memories of emotionally significant events. For example, epinephrine and cortisol, both released by the adrenal glands, act on receptors in the brain to sear the memory in place, quickly and often indelibly.

“Stress hormones make stronger memories,” says Jim McGaugh of the University of California, Irvine. McGaugh’s early work showed that epinephrine helps rats form a quick association between a place and a mild shock.

Usually rats given this conditioning will refuse to re-enter the place, expecting the shock to follow. But when given the drug propranolol, which blocks epinephrine receptors, before training, the animals show no alarm.

In a classic experiment published in 1994 in Nature, McGaugh teamed up with Larry Cahill, also at Irvine, to study the effects of propranolol on emotional memories in humans. Volunteers listened to a story with a tragic ending or one with a similar, but emotionally neutral, plot. Propranolol impaired memories for the tear-jerker but not for the neutral story, indicating that the epinephrine released during the emotional scenes otherwise would have intensified the memory.

McGaugh’s work led other researchers to pick up on the implications for PTSD. Cahill, Roger Pittman of Harvard University and their colleagues published a study in 2002 in which victims of traumas, including car accidents, rape, and medical emergencies, received propranolol or a placebo within six hours of arriving at the hospital and continuing for 10 days.