Monday, September 15, 2008

Hippocampal involvement in contextual modulaton of fear extinction

Ji, J. & Maren, S. (2007). Hippocampal involvement in contextual modulaton of fear extinction. Hippocampus, 17, 749-758.

Responding to an extinguished CS is susceptible to many recovery effects. The first is renewal, in which changing the context favors recall of extinguished fear memory. Examination of its several forms (ABA, AAB, ABC) led researchers to postulate that following extinction the meaning of the CS becomes ambiguous and requires context to disambiguate; inhibitory association is "gated" so that its activation requires the simultaneous presence of the CS and the extinction context. The second is spontaneous recovery, or the return of conditional responding with the passage of time. Studies suggest that renewal and spontaneous recovery appear to result from a similar control mechanism, rather than simply erasure of the original fear memory. Therefore, some see SR as another renewal effect that occurs outside of the "temporal extinction context". Third is reinstatement, in which the extinguished response returns after extinction if the animal is merely exposed to the US alone in a distinct context. This, likewise, appears to be a context-dependent process.

These all suggest that extinction involves new learning, and that this learning is especially sensitive to context. The hippocampus, mPFC, and amygdala have been implicated in this learning. One model holds that when the animal is tested within the extinction context, hippocampus drives mPFC inhibition of LA. When animals are presented with an extinguished CS outside of the extinction context, the hippocampus may inhibit mPFC activation and thus promote excitation in the LA to renew extinguished fear under these conditions. Another model posits direct projection from hippocampus to LA subserving contextual modulation of extinction.

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