Study demonstrates "fear extinction" during sleep A new study suggests people's fears can be 'extinguished' while they sleep. US NEUROLOGISTS have redefined what's meant by "a good night's sleep," suggesting that anxiety disorders can be tackled by therapies administered while people are in the land of nod. Researchers from Northwestern University in Chicago found that people could be made to overcome fears while they were asleep, without exposing them to the "stimulus" that scared them in the first place. While previous research has found that new memories can be formed during sleep, the latest study shows that pre-existing memories can also be altered during sleep. The findings, reported today in the journal Nature Neuroscience, raise the prospect of "fear extinction" therapy for dozing patients. In the study, 15 participants were trained to fear certain chemical smells. The researchers administered "uncomfortable but tolerable" electric shocks while they were exposed to an odour. Sweat tests revealed that they experienced a "fear response" when they smelt the same odour later in the day. The researchers then exposed them to the smell, without the associated electric shock, while they slept. After they woke they exhibited less fear response to the odour, even though they had no memory of smelling it while they slept. MRI tests of the hippocampus, the part of the brain involved in memory formation, found that the odour prompted less activity after patients had been exposed to it in their sleep. "Our findings provide robust evidence that re-exposure to an unreinforced fear context during sleep enhances fear extinction in a stimulus-selective manner," the paper says. The researchers say it is difficult to tell definitively whether the effect is a result of "unlearn¬ing" the association between the smell and the electric shock, or "new learning" that the smell is not a threat. But they tend toward the latter explanation, saying re-exposure to the odour appears to have "induced the formation of a qualitatively unique memory trace ... rather than simply weakening or eliminating the original trace. "Sleep represents a critical period for memory consolidation in which new memories are (easily altered) and vulnerable to modulation," they say.