Different mechanisms of fear extinction dependent on length of time since fear acquisition.

@article{Myers2006DifferentMO,
  title={Different mechanisms of fear extinction dependent on length of time since fear acquisition.},
  author={Karyn M. Myers and Kerry James Ressler and Michael Davis},
  journal={Learning \& memory},
  year={2006},
  volume={13 2},
  pages={
          216-23
        }
}
Fear extinction is defined as a decline in conditioned fear responses (CRs) following nonreinforced exposure to a feared conditioned stimulus (CS). Behavioral evidence indicates that extinction is a form of inhibitory learning: Extinguished fear responses reappear with the passage of time (spontaneous recovery), a shift of context (renewal), and unsignaled presentations of the unconditioned stimulus (reinstatement). However, there also is evidence to suggest that extinction is an "unlearning… 

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...

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