The human brain is one of the most fascinating organs in our bodies. It is made up of more than 100 billion neurons (nerve cells) that send signals across the brain and body. The brain controls our cognition, senses, movement, and much more. It uses both electrical and chemical signals to communicate information and regulate our daily functions, including voluntary and involuntary behaviour.
The brain is also the centre of our emotions. The amygdala is an almond-shaped collection of nuclei found in the temporal lobe of the brain. It is thought to play important roles in processing emotion, and most notably, in processing fear.
So, how does the brain process fear? And when does the amygdala do its job? Whenever we are exposed to a dangerous stimulus, our brain acts quickly. According to a paper published by Kerry Ressler in Biol Psychiatry, positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) show that the amygdala is very active as soon as a fearful stimulus is presented to participants.
The stimulus triggers a response in the amygdala, which includes releasing cortisol and signals to areas of the brain like the hypothalamus. This activates our “fight-or-flight” response, which makes our heart race, dilates our pupils, increases our blood pressure, and slows down digestion. Blood flow to our skeletal muscles also increases to prepare us to escape from danger.
A study by Arne Ohman and colleagues at the Karolinska Institute also suggests that cortical processing (conscious processing in the cerebral cortex) is not needed for the activation of the amygdala. This means that the information about fearful stimuli can be sent from the thalamus, the brain’s relay centre, to the amygdala even before we are consciously aware of the fear. This allows us to react before we have time to think about the threat we need to face.
While our threat detection can be critical for our survival, it can also help us enjoy the thrills of activities such as sky-diving, roller-coasters, and horror movies. Lauri Nummenmaa and colleagues at the University of Turku in Finland had participants watch a horror movie while measuring their neural activity using an fMRI. Results show that brain regions were “in continuous talk-back with sensory regions throughout the movie, as if the sensory regions were preparing response networks as a scary event was becoming increasingly likely.” This shows a complex interaction between the amygdala and other cortical networks in the brain, suggesting that our fear circuit is not limited to one area of the brain.
Cold Spring Harbor Laboratory Professor, Bo Li, conducted recent research on mice that delved deeper into the fear processing circuit of the brain. His team found that the amygdala’s role in the brain is broader and more complex than we thought. They found that it communicates with areas of the brain responsible for reward-based learning, punishment responses, memory formation, movement, and positive stimuli processing. Their research shows that an area responsible for the regulation of voluntary movement, the “globus pallidus,” is closely linked to the amygdala. According to the same study, when they “interfered with signaling between the amygdala and the globus pallidus in the brains of mice, they found that the animals failed to learn that a particular sound cue signaled an unpleasant sensation.”
Overall, research on the amygdala and its functions is still growing, as it is a complex but vital structure. While it is most commonly known for its role in processing fear, it has also been shown to be involved in consolidating memories, processing pleasant/unpleasant emotions, and learning based on rewards and punishment. Many of its activities remain unknown and need further studies and replication to confirm these hypotheses. However, you can be sure to hold your amygdala responsible next time you scream during a horror movie.