The Amygdala

An almond-sized cluster of nuclei that detects threats and other emotionally important events, drives fear learning, and tags memories with emotion.

The amygdala (Greek for "almond") is a cluster of a dozen or so nuclei just in front of the hippocampus in each temporal lobe.

Detecting what matters

The amygdala receives input from every sense, often via fast routes through the thalamus as well as slower, more detailed routes through the cortex. The neuroscientist Joseph LeDoux called these the "low road" and "high road": the low road lets you flinch at a curved shape on a path before your cortex has decided whether it's a snake or a stick. The amygdala responds to threats, but also to rewards, novelty and social cues such as facial expressions — anything emotionally significant.

Triggering responses

Through its central nucleus, the amygdala connects to the hypothalamus and brainstem, launching the body's defensive responses: freezing, raised heart rate, stress hormones, startle. It also connects to the cortex, influencing attention and decisions.

Fear learning and memory

In fear conditioning, an animal learns that a neutral tone predicts a mild shock; afterwards the tone alone causes freezing. This learning depends on synaptic changes in the amygdala. The amygdala also enhances memories of emotional events through its effects on the hippocampus — one reason you remember where you were during a shocking event.

Life without an amygdala

The patient known as S.M., who has a rare condition (Urbach–Wiethe disease) that destroyed both amygdalae, has been studied for decades. She appears to feel almost no fear in situations that terrify most people — handling snakes, walking through a haunted house — and struggles to recognise fear in faces. Yet, strikingly, she experienced panic when inhaling carbon dioxide, showing some fear arises through other routes. Over-reactive amygdala activity has been linked to anxiety disorders and PTSD.