Amygdala · Grey Matter

The amygdala is an almond-sized group of nuclei deep in each temporal lobe, in front of the hippocampus, that decides quickly whether something matters emotionally, above all whether it is a threat, and starts the body's response before deliberate thought has caught up.

Amygdala. The amygdala is an almond-sized group of nuclei deep in each temporal lobe, in front of the hippocampus, that decides quickly whether something matters emotionally, above all whether it is a threat, and starts the body's response before deliberate thought has caught up.

Its input side (the basolateral nuclei) receives from every sense, both through the cortex and through a shorter route straight from the thalamus, and learns associations: a tone that has preceded a shock becomes frightening by itself, which is fear conditioning. Its output side (the central nucleus) drives the hypothalamus and the brainstem to raise heart rate, freeze, startle and release stress hormones. It takes context from the hippocampus (the same dog is a threat in the street and not at home) and is regulated by the ventromedial prefrontal cortex.

It responds to more than fear. Rewards, novelty and anything relevant to current goals activate it, so it is better described as a detector of significance.

Its removal takes away the feeling of fear from outside threats. The patient SM, whose amygdalae were destroyed by Urbach-Wiethe disease, handled snakes and showed little fear in a haunted house, yet felt panic when breathing carbon dioxide, a threat signalled from inside the body.

It strengthens memories of emotional events. Stress hormones acting through the amygdala make the hippocampus and cortex store arousing episodes more durably, which is why such episodes are remembered better.

Questions: Why are emotional events remembered better than ordinary ones? An arousing event releases adrenaline and cortisol and activates noradrenaline in the basolateral amygdala, which then strengthens the consolidation of the memory in the hippocampus and the cortex. McGaugh and colleagues showed that blocking noradrenaline receptors in the amygdala removes the advantage of emotional memories, and that stimulating them after learning enhances ordinary ones. People with damage to both amygdalae remember emotional and neutral stories about equally well. The enhancement helps the gist of the event more than every detail, and vivid memories can still be wrong. How can you jump at a shape in the grass before you know it is a stick? Sensory signals reach the amygdala by two routes from the thalamus: a short one that goes straight there, and a longer one through the cortex, which identifies the object properly. In rats LeDoux showed that the direct route alone, from the auditory thalamus, is enough to learn and express fear of a sound, and it is faster because it skips the cortical stages. The short route carries a coarse picture (something long, something moving), enough to start a startle and a racing heart; a moment later the cortical route reports that it is a stick, and the prefrontal cortex calms the alarm. Tractography has since traced a matching pathway in humans. How does a fright become a racing heart and sweaty hands? The central nucleus of the amygdala sends its output to the hypothalamus and the brainstem. The lateral hypothalamus and brainstem centres switch on the sympathetic nervous system within a second or two (faster heart, higher blood pressure, sweating, wide pupils, adrenaline from the adrenal medulla), while the paraventricular nucleus starts the slower hormonal arm, the HPA axis, that brings cortisol minutes later. The hypothalamus generates none of the thought about the threat; it translates the amygdala's verdict into the state of the body. What happens in the brain when frustration builds over a task that keeps failing? Each failure is an error between what was expected and what happened, and the anterior cingulate cortex responds to such errors and to the conflict of trying again. As the failures accumulate the amygdala and the hypothalamus add arousal (a faster heart, tension), and the ventromedial prefrontal cortex, which weighs whether continuing is worth it, registers the reward as being withheld. Frustration is the felt state of that combination, and it can either push effort up or tip into giving up or lashing out, depending on how prefrontal control holds. Which areas are working when you hold back an angry outburst? The provocation drives the amygdala, which pushes toward an immediate response through the hypothalamus and brainstem (heart rate, tension, a raised voice). The anterior cingulate cortex registers the conflict between that impulse and the goal of staying calm, and calls for more control. The dorsolateral prefrontal cortex holds the goal and the reappraisal (it was not meant as an insult), and acts on the amygdala mostly through the ventromedial prefrontal cortex. Whether the outburst comes depends on how strong the alarm is against how well this control is working, and fatigue, stress and alcohol all weaken the control side. What does a person without amygdalae fail to feel, and what can they still feel? The patient SM, whose amygdalae were destroyed by Urbach-Wiethe disease, showed almost no fear of external threats: she handled snakes and spiders, found a haunted house entertaining and reported little fear in frightening situations, while feeling other emotions normally. Yet when she and two other patients with the same lesion inhaled air with 35 % carbon dioxide, they felt intense panic, more than most healthy volunteers. The amygdala is therefore needed to turn threats from the world into fear, while fear from the body's own alarm (suffocation sensed by the brainstem) can arise without it. How does telling yourself it is only a film calm a frightened amygdala? The amygdala and the prefrontal cortex talk in both directions: the amygdala tells the cortex that something is dangerous, and the ventromedial prefrontal cortex, which projects back onto inhibitory cells of the amygdala, can turn its output down. Reappraisal (reinterpreting what is seen, such as a scene being fiction) recruits the lateral prefrontal cortex, which works through the ventromedial part to reduce the amygdala's response, and imaging shows amygdala activity falling as the reappraisal succeeds. The same loop runs the other way when a worrying thought feeds the alarm, which is why thinking can either calm or amplify an emotion. What happens in the brain when you recognise a friend's face in a crowd? The visual cortex extracts edges and features, and the fusiform face area on the underside of the temporal lobe (larger on the right) assembles them into a face and matches it to the faces it knows. The anterior temporal lobe retrieves who the person is, the hippocampus brings back where and when you last met, and the amygdala and reward circuits add the warmth or unease that belongs to that person. Damage to the fusiform region can leave someone seeing a face clearly and recognising nobody (prosopagnosia), and recognising friends by their voice instead.