Two papers published online in Nature offer an insight into the specific mechanisms in the brain that control fear.
Andreas Lüthi and colleagues suggest that feelings of fear for stimuli previously linked to unpleasant consequences are triggered by rapid switching in the balance of activity between two circuits in the brain. The team discovered that two distinct populations of neurons in a part of the brain called the amygdala are involved in different aspects of fear memory - one in the elimination of established fear responses, and the other in the ability to refresh those memories. They found that selectively activating one of the two populations triggered large changes in behavioural state. The authors suggest that in normal situations, selective activation of these two types of cells is sparked by sensory and contextual information that we receive from our surroundings.
In a related study, Denis Paré and colleagues uncovered another cellular mechanism underlying our ability to unlearn established fear memories. They propose that a different population of neurons within another part of the amygdala, called intercalated amygdala neurons, also helps us to become 'not scared' of previously scary stimuli. By destroying these neurons in rats they observed a decrease in the extinction of learnt fear memories, meaning the rats remained afraid. Both studies shed light on the various cells and circuits in the brain that maintain learned fear, and could provide therapeutic clues for fear and anxiety disorders, such as post-traumatic stress disorder.
Andreas Lüthi (Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland) Author paper 
Denis Paré (Rutgers State University, Newark, NJ, USA) Author paper 
Abstracts available online:
(C) Nature press release.
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