WEDNESDAY, Aug. 27, 2014 (HealthDay News) -- Someone who has been mugged in a dark alley will likely never want to return there, having associated that location with a fear of being attacked.

But neuroscientists working with mice say they've discovered the brain circuit that controls how memories are linked with positive and negative emotions. And in rodent tests, they've manipulated brain cells to reverse the emotions attached to a memory.

In essence, they made once-reluctant mice want to return to that dark alley, by replacing their negative emotions with positive ones through stimulation of a key memory region in the brain.

The findings explain what happens inside the brain when psychotherapy helps lift a person out of depression, said senior author Susumu Tonegawa, director of the RIKEN-MIT Center for Neural Circuit Genetics at the Massachusetts Institute of Technology in Cambridge, Mass.

"The psychiatrist will talk with a patient suffering depression and try to make them recall positive memories they have had in the past," Tonegawa said. "Apparently, this will reduce the effect of the bad memories they have had or the very strong stress they have had. But unless you look into the inside of the brain, you can't tell what's going on underneath the behavior."

When people create memories, they store a great deal of context along with the memory itself, he explained. "The memory information stored is not only about what happened, but also about the context in which the event occurred," Tonegawa said. Part of that context includes how a person felt about the event.

Showing that the context of negative memories can be changed is promising, said Dr. Paul Sanberg, professor of neuroscience at the University of South Florida in Tampa. "If we can harness that information and carry it further, we may be able to come up with new clinical insights," he said.

The brain region that creates these emotional links could prove a useful target for new medications and therapies intended to treat disorders such as depression or anxiety, Sanberg said.