Fear is a primitive human emotion vital for responding to physical and emotional danger. Although fear can be learned, in our evolutionary past, fear was essential as it enabled humans to respond to danger using the fight-or-flight response via the Sympathomedullary Pathway.
An important neuroanatomical structure involved in the process of fear is the amygdala (LeDoux, 2003) which is part of the limbic system. Research has shown that the amygdala is implication in recognition of fear (Adolphs et al. 1995) and feelings of fear (e.g. Ketter et al. 1996). Evidence comes from a human case study (SM) by Damasio et al (1994) who used MRI scanning on a patient with Urbach-Wiethe disease which lead total atrophy of her bilateral amygdala. SM was unable to describe fear in pictures and videos of faces yet she had no difficulties with positive emotions. Therefore damage to the amygdala results in an inability to link facial expressions with the emotion of fear. Using PET scanning, Isenberg et al. (1999) found that when healthy participants saw words that signify threatening situations, amygdala activity increased.
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The lateral nucleus of the amygdala is widely accepted to be a key site of plastic synaptic events that contributes to fear learning (Pare, Quirk & LeDoux, 2004). This is because the Hebbian mechanism where continued or repeated stimuli strengthen neural connections leads to fear conditioning. The central nucleus of the amygdala also plays a role in fear. Coover, Murison and Jellestad (1992) found that animals with central nucleus damage show no fear when confronted with stimuli paired with aversive events. Researchers also found that the central nucleus is involved the acquisition of fear conditioning. Using functional inactivation methods Wilensky et al. (2006) found that inhibition of protein synthesis in the central nucleus impairs fear memory