The study adds to our understanding of how memories are made and altered in the brain, and could help to relieve sufferers of post-traumatic stress disorder (PTSD) of the fearful memories that disrupt their lives. The results are published in Nature Neuroscience.

The brain secures memories by transferring them from short-term to long-term storage, through a process called reconsolidation. It has been shown before that this process can be interrupted with drugs. But Joseph LeDoux of the Center for Neural Science at New York University and his colleagues wanted to know how specific this interference was: could the transfer of one specific memory be meddled with without affecting others?

"Our concern was: would you do something really massive to their memory network?" says LeDoux.

Scary music

To find out, they trained rats to fear two different musical tones, by playing them at the same time as giving the rats an electric shock. Then, they gave half the rats a drug known to cause limited amnesia (U0126, which is not approved for use in people), and reminded all the animals, half of which were still under the influence of the drug, of one of their fearful memories by replaying just one of the tones.

When they tested the rats with both tones a day later, untreated animals were still fearful of both sounds, as if they expected a shock. But those treated with the drug were no longer afraid of the tone they had been reminded of under treatment. The process of re-arousing the rats' memory of being shocked with the one tone while they were drugged had wiped out that memory completely, while leaving their memory of the second tone intact.

LeDoux's team also confirms the idea that a part of the brain called the amygdala is central to this process - communication between neurons in this part of the brain usually increases when a fearful memory forms, but it decreases in the treated rats. This shows that the fearful memory is actually deleted, rather than simply breaking the link between the memory and a fearful response.

Greg Quirk, a neurophysiologist from the Ponce School of Medicine in Puerto Rico, thinks that psychiatrists working to treat patients with conditions such as PTSD will be encouraged by the step forward. "These drugs would be adjuncts to therapy," he says. "This is the future of psychiatry - neuroscience will provide tools to help it become more effective."