The Amygdala

The amygdala complex is located in the anterior part of the temporal lobe, directly in front of the tail of the caudate nucleus and the inferior horn of the lateral ventricle. However, the complex also includes a piece of cerebral cortex. The amygdala is therefore both: cortex and nucleus - and a transition zone in between.

The amygdala complex is generally divided into three different areas: Firstly, the centromedial core group, including the nuclei centralis and medialis - both descendants of the striatum. Then there is the basolateral complex, with the nuclei nucleus lateralis, nucleus basalis – which also splits into a small-cell inner part and a large-cell lateral part – and nucleus basolateralis. And thirdly, the cortical core group with the cortical nucleus.

This sounds complex and it becomes even more complex because each individual amygdala nucleus has its own neuropeptides and circuits. However, they cooperate closely with each other: numerous nerve fibers with relatively short axons run between the nuclei.

The superficial part of the amygdala primarily influences autonomic functions of the body – such as breathing and circulation – via a strong connection with the brain stem and adapts them to the respective situation. This core area ensures, for example, that our heart beats up to our throat when we are afraid. A particularly thick, macroscopically visible bundle of nerves also runs from here to the hypothalamus in the diencephalon: the stria terminalis. It crosses the thalamus and reaches its destination in a wide arc. The hypothalamus, as the center of the autonomic nervous system, thus learns when it needs to stimulate adrenaline production in the adrenal glands: After all, the anxiety must have a reason for which the body is better prepared. The US psychologist and neuroscientist Joseph LeDoux put it this way: "As soon as you are in danger, you react. Evolution thinks for you."

Last but not least, the medial nucleus is connected to the olfactory cortex areas for odor perception – a phylogenetic legacy from the time when predators revealed themselves through their scent.

The basolateral, phylogenetically younger core group, on the other hand, receives information from the posterior thalamus – which is concerned with reflexes – as well as practically all sensory cortex areas and thus the five senses of smell, taste, sight, hearing and touch. The basolateral core group also transmits back to these cortical areas. It is additionally connected to the prefrontal cortex via the thalamus. This integrates sensory signals with memory content and emotional judgements. In case of doubt, it can inhibit the activity of the amygdala via its lowest section – the orbitofrontal cortex.

If we suddenly see a spider and are startled, we literally flinch. Responsible for this is a nerve connection between the amygdala complex – like the nucleus centralis – and the basal ganglia – connecting the amygdala to the motor system.

Experiments show just how diverse the functions of the amygdala are: if you stimulate the amygdala complex in test animals with an electrode, the reaction depends on which area you hit: if it is the superficial nucleus, the animal will start to smack its lips or make chewing or licking movements. Salivation is also stimulated. If, on the other hand, the electrode stimulates the deep part of the amygdala, the animal raises its head, its pupils dilate, and it looks around attentively. With stronger stimuli, the increased attention turns into fear or anger.

The amygdala acts primarily as an emotional amplifier. The reaction to stimulation in humans therefore also depends on the mood of the test subject at the time. They also report having memory-like hallucinations when stimulating the amygdala, or a déjà vu experience, i.e. the feeling of having experienced a situation before.

The best way to understand the amygdala is to look at what happens when it is missing – for example in monkeys in which the amygdala has been deliberately destroyed in both hemispheres of the brain. As a result, the animals appear more emotionless than before. Above all, however, they lack any aggressive or defensive behavior. The monkeys show not a trace of fear – not even when they encounter a real danger, such as a snake. They do perceive the external stimulus of the snake, but without the amygdala complex, the corresponding startle reflex is absent.

And that's not all: without the amygdala, the animals have difficulty learning emotional associations, such as linking a certain object with a reward or a punishment. In addition, they no longer seek contact with other monkeys and are therefore soon isolated in the group.

The situation is very similar in humans. The British psychiatrist Robin Jacobsen described a patient in whom the amygdala complex had been surgically removed on both sides due to illness. As a result, the person had difficulty recognizing faces and, above all, correctly interpreting the facial expressions of the person opposite. As a result, the person's social behavior was also severely impaired.

Urbach-Wiethe syndrome, a rare hereditary disease in which, among other things, the amygdala calcifies, causes a similar disorder. Sufferers are also severely restricted in their emotional and social life. They cannot assign any meaning to the word ‘fear’.

The amygdala complex also plays a role in memory, or more precisely, emotional memory. Normally, we can remember a situation better if strong emotions were involved – especially fear or anxiety. However, people with a damaged amygdala complex do not show this effect: they remember repulsive, neutral and pleasant scenes – for example in a film – equally well.

Published on September 22, 2011
Updated on December 22, 2023