Scientific Understanding of Consciousness
Amygdala Mediates Defensive Behavior
Nature 454, 600-606 (31 July 2008)
Switching on and off fear by distinct neuronal circuits
Cyril Herry, Stephane Ciocchi, Verena Senn, Lynda Demmou, Christian Müller, Andreas Lüthi
Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, CH-4058 Basel, Switzerland
Switching between exploratory and defensive behaviour is fundamental to survival of many animals. The amygdala is a key brain structure mediating defensive behaviour in states of fear and anxiety. Such states can be induced by classical auditory fear conditioning, in which an initially neutral auditory stimulus (the conditioned stimulus, CS) comes to elicit a fear response after pairing with an aversive foot shock (the unconditioned stimulus, US).
Subsequent repetitive presentations of the CS alone induce a progressive decrease in the fear response, a phenomenon called extinction.
Whereas firing of amygdala neurons is critical for the retrieval of conditioned fear memories, their firing after the extinction of conditioned fear is thought to be constrained by local inhibitory circuits activated by the medial prefrontal cortex (mPFC),
Converging evidence from animal studies indicates, however, that the basolateral complex of the amygdala (BLA), comprising the lateral (LA) and the basal (BA) nuclei, actively participates in fear extinction. Although fear extinction is an active learning process eventually leading to the formation of a consolidated extinction memory, it is a fragile behavioural state that is readily influenced by context.
Changing context results in the immediate recovery of the previously conditioned fear response, a process known as fear renewal.
In vivo pharmacological studies indicate that the hippocampus, which is reciprocally connected to the BLA, processes contextual information during fear conditioning, extinction and renewal.
Thus, bi-directional changes in fear behaviour during extinction and context-dependent renewal are likely to be encoded within a distributed network containing the BLA, the mPFC and the hippocampus.
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