The amygdala is a small almond-shaped structure deep in the temporal lobe. It is central to the processing of emotion, especially fear and aggression, and to the emotional coloring of memory.

Sensory information reaches the amygdala by both a fast subcortical route and a slower cortical route. The fast route allows rapid defensive responses before the stimulus is fully identified; the slower route permits more nuanced appraisal. Damage to the amygdala can produce a striking loss of fear and an inability to recognize fear in others’ faces. Overactivity is implicated in anxiety disorders and in the intrusive memories of post-traumatic stress.

The amygdala does not work in isolation. It interacts with the hippocampus to give emotional events their privileged place in memory, with the prefrontal cortex to regulate emotional responses, and with the brainstem to trigger the bodily signs of emotion. It responds not only to threats but also to positive rewards and to social cues such as facial expressions.

When a sudden sound makes the heart race before the mind has named the source, the amygdala has already done its work.