also support learning in response to reward. Reward is an essential component of many learning processes, and it is thought to be involved in both declarative and S-R learning (Adcock, Thangavel, et al., 2006; Shohamy, Myers, et al., 2008).
Emotional experiences have powerful influences on memory, particularly on the accuracy and emotional tone of recalled memories in the declarative memory system. Emotional learning depends heavily on the interactions of the amygdala with the physically adjacent hippocampus, as well as with more remote structures that include the striatum and the frontal cortex. The interaction of the amygdala with memory systems imbues memories with the emotional tone experienced during and following the recalled event (McGaugh, 2004). Experimental emulation and manipulation of various emotions in animal models have shown that the interactions between the amygdala and the hippocampus are influenced heavily by the actions of various neurotransmitters and hormones that mediate the effects of emotional experience on the recall of arousing, rewarding, and stressful life events (McGaugh, 2004; Roozendaal, Okuda, et al., 2006).
In addition to declarative, S-R, and working memory systems, the brain supports associative or conditioned learning, as originally described by Pavlov. This form of learning involves the pairing of a stimulus that does not produce an innate behavioral response (the to-be “conditioned stimulus” or “CS,” such as a tone) with a stimulus that does produce an innate behavioral response (the “unconditioned stimulus” or “US,” such as a food odor that produces salivation). After repeated pairings of the CS and the US, the CS alone will elicit the unconditioned response (salivation). Conditioned learning involves numerous brain regions, including the hippocampus and the cerebellum (Thompson, 2005; Daum, Schugens, et al., 1993; Logan and Grafton, 1995).
The obverse of conditioned learning is extinction, in which the unconditioned response to the CS is modulated downward over time. Extinction involves exposing an animal repeatedly to a stimulus that has been previously conditioned to elicit fear, but now in the absence of any aversive event. This will extinguish the fearful, conditioned response. Extinction is therefore an active process and not simply a passive, dissipating process of forgetting (Myers and Davis, 2007; Quirk and Mueller, 2008). Extinction is cue-specific, in that extinction to one CS does not induce or accompany extinction to another CS (Myers and Davis, 2007). When extinction fails, as it can during times of stress, the conditioned behavior can reappear (Akirav and Maroun, 2007). The neural basis of fear extinction is thought to include the amygdala, the hippocampus, and the medial prefrontal cortex (Myers and Davis, 2007; Quirk and Mueller, 2008).
Disturbances in one or more of these various learning and memory systems have been implicated in the pathogenesis of a wide range of disorders.