The Amygdala and Fear Conditioning in Animals

Much of the scientific interest in the amygdala stems from its established role in fear conditioning, research that has been carried out mostly in rats. Classical Pavlovian fear conditioning is a type of emotional learning in which an emotionally neutral conditioned stimulus (CS), often a tone, is presented in conjunction with an aversive unconditioned stimulus (US), typically a small electric shock to the foot of the animal.

After one or more pairings, the emotionally neutral stimulus (CS) is able to elicit a constellation of species-specific conditioned responses (CRs) that are characteristic of fear, such as freezing or escape behavior, autonomic responses (elevated heart rate and blood pressure), potentiated acoustic startle to aversive acoustic stimuli, and increased neuroendocrine responses (release of stress hormones). Fear conditioning therefore allows new or learned threats to activate ways of responding to threat that have been long established in evolution.

Numerous studies have demonstrated that lesions to the amygdala impair the acquisition and expression of conditioned fear in rats. The basolateral complex of the amygdala is a substrate for sensory convergence from both the cortical and subcortical areas, and it is considered a putative locus for CS-US association during fear conditioning. Thus, its cells encode this emotional learning.

By contrast, the central nucleus of the amygdala projects to brain regions implicated in the generation of fear responses, such as the hypothalamus; it may therefore act as a common output pathway for the generation of fear- conditioned responses. Consistent with this hypothesis, lesions to either the basolateral or central nucleus of the amygdala impair the both the acquisition and expression of conditioned fear.

Figure 1. During fear conditioning convergence of inputs (CS and US) induce long-term potentiation of EPSPs evoked by the CS. a, Schematic; b, graphs. CS, conditioned stimulus; EPSPs, excitatory postsynaptic potentials; US, unconditioned stimulus

The amygdala pathways involved in fear conditioning have also been studied using electrophysiological studies (see Figure 1). During fear conditioning, the convergence of CS and US inputs to the basolateral complex results in sustained enhancement, or long-term potentiation, of EPSPs evoked by the CS (Figure 1). Thus, the amygdala is able to both integrate and associate sensory information and influence the motor and physiological responses associated with fear conditioning.