The chapter summarizes current knowledge about the brain structures involved in fear in animals and humans. The periaqueductal gray of the brain stem is the most primitive structure known to be involved in defensive and fear-related behavior in animals and fear in humans. The basal ganglia are also involved in defensive and fear-related behavior in animals, but their role in human fear is not clear. As the chapter explains, the amygdala, which is a part of the limbic system, is the neural nexus of fear in the brain, and it appears to be the primary source of fear in mammals, including humans. Fear as we know it may not have existed before the evolution of the amygdala. The amygdala generates fear as part of its function to assess potential threats of physical harm and to warn us about them. The chapter explains how the amygdala, which is said to operate under the “better safe than sorry principle,” tends to over-react to ambiguous stimuli as if they are threats, and therefore, produces fear even when something may not actually pose a threat of harm. This over-reaction can be countered by the prefrontal cortex, which makes it own threat assessments and can suppress the fear generated by the amygdala if it decides the fear is not justified.
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