The fast and reliable nature of fear conditioning allowed an extensive analysis of related neural components in mammalian brain ( Blanchard and Blanchard, 1972 Davis, 1992 LeDoux, 2000). As a result, the CS comes to elicit the conditioned response (CR), an orchestration of autonomic, behavioral, and endocrine responses that prepares the organism for the upcoming threat. In a typical fear conditioning, an initially neutral conditioned stimulus (CS) is contingently paired with an aversive unconditioned stimulus (US).

Pavlovian fear conditioning is among a handful of learning paradigms in which well defined neural circuits have been delineated. A prerequisite for testing the SPM hypothesis, therefore, is to identify the neural circuit for the target behavior and to focus on the key sites for learning-induced synaptic plasticity. In mammalian brain, complexity of most learning tasks unavoidably demands a network of neurons, making it impossible to monitor all potentially relevant synapses. Despite the near-unanimous consensus that learning induces detectable synaptic modifications in the brain, as originally proposed by Hebb (1949) and formularized in the “synaptic plasticity and memory (SPM)” hypothesis ( Martin et al., 2000), direct evidence linking localized cellular changes with behavior has been rare.

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