Brain facts

How can we start fearing something even before consciously realising it?

Following up on last week’s new theme (have a look here if you missed it), today I will introduce to you a new area of the brain: the amygdala.

The amygdala is an almond-shaped cluster of nuclei located deep and medially within the temporal lobes of the brain in complex vertebrates, including humans (see picture below to see the location of the amygdala in the brain). It is considered the integration centre of neuronal processes such as decision making and emotions (like fear, anxiety and aggression). Indeed, it is also active during the comparison of new stimuli with past experiences and during the elaboration of olfactory stimuli. As an example, while the hippocampus (have a look here to figure out the role of hippocampus) “remembers” an event, the amygdala gives the emotional valence to it: the amygdala assigns to each stimulus the right level of attention, it enriches it with emotions and it starts the process of storing the stimulus in our brain as a memory.

Location of the amygdala in the human brain. From commons.wikimedia.org

Only in 1930, researchers started considering the amygdala as a fundamental element in the fear perception. Nowadays, its role is indeed considered much more complex. At first, researchers hypothesised that the amygdala contained a neural circuit associated with fear and fear associated behaviours, but studies conducted on animal models and humans gradually showed that the amygdala is indeed the main area involved in all manifestations of the emotional sphere.

The first studies on amygdala were conducted by Heinrich Klüver and Paul C. Bucy from the University of Chicago, who described the dramatic behavioural changes showed by rhesus macaques whose temporal lobes (containing the amygdala) where removed: some of these behaviours comprised the unusual will of approaching snakes and other dangerous animals. Further researches supported the role of the amygdala as a fear inducing areas, up until the beginning of the 90s, when a research group in Iowa published a case-study on a woman, S.M., with bilateral lesions of the amygdala, due to a rare genetic condition known as Urbach-Wiethe syndrome. This paper was showing how S.M. was unable to recognise fear-related facial expressions. While this case was confirming the already well-considered role of the amygdala being the brain centre involved in fear, the follow up studies showed that this woman was also unable to recognise other facial expressions showing different emotions.

This led scientists to better investigate the role of amygdala and they eventually showed that this area has connections that allow it to reply to the stimuli before they can even reach the new cortex (that basically gives us the conscious perception of what is happening). Indeed, the amygdala is able to analyse every experience, scanning through the events and the perceptions: for example, when the amygdala perceives something as dangerous, it startles like a neural trigger and it sends signal of warning all over the brain, stimulating the release of hormones that start the fight or flight reaction (have a look here to better understand what the fight of flight reaction is). At the same time, it starts to recall every useful information on this fear situation, in order to compare the current experience with a past one.

When the present situation has something in common with a previous one, the amygdala starts to act even before we can get a full understanding of what is actually happening, and this is due to the aforementioned connections that allow us to feel emotions independently from our conscious thoughts and, usually, way before them.

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