Extinction of maladaptive conditioned responses or behaviors is a process of new and active learning, and requires the organism to learn new stimulus–response and action–outcome relationships, and to form new associations between previously hypersalient stimuli (ie, trauma-related cues and contexts) and appropriate cognitive and/or behavioral responses. Much of our knowledge about the neural substrates that underlie extinction processes comes from studies of fear conditioning (Myers and Davis, 2007), but an increasing number of studies have begun to examine the mechanisms underlying extinction of drug-seeking behavior (Cleva and Gass, 2010).

Adult neurogenesis is an ongoing process that occurs in most mammalian species, including humans. This phenomenon occurs primarily in two brain regions: the subgranular layer of the dentate gyrus region of the hippocampus, which gives rise to neurons that migrate and integrate into the granule cell layer (GCL), and the subventricular layer of the lateral ventricles, which supplies newborn neurons to the olfactory system through the rostral migratory stream. Factors that contribute to the birth, differentiation, maturation, migration, and survival of adult-born neurons, as well as the specific function of surviving neurons, are poorly understood. However, adult-born neurons may be involved in certain learning and memory processes.

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