In this blog post, we will explore how humans react to danger when the amygdala, the part of the brain responsible for fear, is damaged.
Let’s imagine a situation where a person who is afraid of mice is suddenly confronted by one. Before they can make a rational decision, they will instinctively scream and run away. Why is that? The visual information of seeing a mouse is first transmitted to the thalamus in the brain, and the information transmitted to the thalamus is then transmitted to the amygdala and the visual cortex. However, the amygdala, which induces immediate physical reactions as a survival instinct, processes information faster than the visual cortex, which is responsible for rational thinking, so the person screams first. In this way, the amygdala enables us to react quickly to frightening situations.
When a person faces a frightening situation, the amygdala activates the sympathetic nervous system. When the sympathetic nervous system is activated, the adrenal glands begin to secrete the hormone adrenaline. Adrenaline entering the bloodstream causes blood vessels to dilate, heart rate to increase, and the amount of blood pumped out by the heart with each beat (stroke volume) to increase. In addition, the breathing rate increases, allowing more oxygen than usual to enter the body. These physiological changes supply the muscles with more oxygen and heat than usual, enabling the body to respond more quickly to frightening situations.
However, fear responses are not limited to physical changes. Fear also has a powerful psychological impact. In frightening situations, the brain’s cognitive functions also undergo changes. For example, when we feel fear, our thinking becomes simpler and we focus on immediate survival responses. This causes us to make decisions and act more quickly than usual in frightening situations.
However, when faced with a frightening situation so intense that it causes fainting, the sympathetic nervous system becomes overly activated, causing the heart rate and cardiac output to increase excessively. When the heart is overstimulated in this way, the parasympathetic nervous system is activated to stabilize the body. This lowers the heart rate and blood pressure, and causes the pulse to slow down. The normal pulse rate for an adult is around 60 to 80 beats per minute, but if it falls below that, the blood flow to the brain becomes insufficient or is momentarily interrupted. This is why people faint when they experience extreme fear.
On the other hand, fear can also be learned. The amygdala in our brain is related to fear learning, which can be confirmed through fear conditioning experiments. Fear conditioning is the process of associating a neutral stimulus that does not elicit a specific response with an unconditional stimulus that elicits an unpleasant or painful response. Professor Kapf of the University of Vermont conducted the following experiment based on the observation that rabbits show an immediate fear response when presented with an electric shock that causes an unpleasant or painful response. He applied a weak electric shock to the rabbits’ paws every time a specific sound was played, but did not apply any shock when a different sound was played. The rabbits that were conditioned in this way began to show a fear response when they heard the specific sound alone, just as they did when the electric shock was applied. The rabbits’ fear response was confirmed by the fact that their heart rates increased when they heard the specific sound and that their amygdala reacted.
However, in rabbits with damaged amygdalae, the fear response disappeared or weakened. The same is true for humans. In a rare disease called Urbach-Witte syndrome, calcium deposits cause the amygdalae and other parts of the temporal lobes to lose their function. Patients with this disease are known to have difficulty recognizing feelings of fear. Those who are unable to feel fear properly become insensitive to dangers in their daily lives, which increases their likelihood of accidents. This case shows that although fear is an emotion essential for survival, if its mechanism does not function properly, it can actually threaten survival.
