Nothing to fear but asphyxiation?

Think of the scariest movie you’ve ever seen (for me it’s The Ring). How did you feel when the group of teenagers popped in that video, or the girl climbed out of the TV? When the phone rang and the killer was on the other end? Or when the babysitter was home alone and a shadow passed across the screen? Even though you know it’s just a movie, you still experience that knot in your stomach, pounding heart, sweaty palms and building anxiety that comes with a real stressful or frightening encounter.

These visceral, gut reactions are physiological fear responses our brain and body automatically initiate when in a perceived threatening situation. These experiences are thought to be subserved by the amygdala, an old and deeply rooted part of the brain that is essential in processing emotion, particularly fear. This is partly through connections the amygdala has to the sympathetic nervous system, which controls our basic ‘fight-or-flight’ reactions to danger–preparing us to either stand and fight or flee as fast as we can.

However, some people don’t experience this sensation of fear. Individuals who have undergone damage to the amygdala, either through a stroke or head injury, or from the rare genetic condition Urbach-Wiethe disease, report an inability to feel this emotion. One famous example of this absence is in the patient SM, who reported no feelings of fear when faced with snakes, spiders, horror films, or haunted houses. Even after being threatened with a real life knife attack, SM had no experience of fear sensations. However, there was one thing that was able to instill in her these feelings of anxiety and terror–asphyxiation.

Researchers at the University of Iowa have been studying SM over the last decade to try to find something, anything, that would scare her. After exhausting all the typical psychological stressors to no avail, they decided to try a physical stressor that can elicit the same reactions. Published last month in Nature Neuroscience, the researchers had SM and two other people with similar amygdala lesions inhale carbon dioxide for several seconds, cutting off their oxygen flow and essentially suffocating them. This experience typically causes panic attacks and fear responses in people, including extreme distress, pounding heart, and an immediate desire to escape the situation. All three participants–none of whom had previously experienced fear–had these exact same panicky reactions to the CO2. In fact, when compared with normal healthy individuals, the amygdala patients had significantly greater fear responses, both physically and psychologically, than those with intact amygdalas.

So what’s behind this phenomenon? The researchers believe that these panic reactions are distinct from learned fear responses, such as phobias of snakes or spiders. Instead, there appears to be a unique pathway involved in panic from inherent physiological stressors that passes through the amygdala. In fact, this response may be inhibited in the amygdala, as the control participants experienced less dramatic reactions to the carbon dioxide than the amygdala patients. However, learned fears or perceived outside dangers rely on the amygdala to integrate these scary sensory situations—such as seeing someone with a gun—as a threat. Thus, those with amygdala lesions do not learn and incorporate the proper fear associations with these triggers, but they do still have the capacity to experience these dramatic panic responses to internal physical stressors.

So the next time you’re watching a scary movie, you could try reminding yourself that it’s not real, or you could try hyperventilating—it may actually reduce your panic (assuming your amygdala is still intact).*


*I do not actually recommend this as a fear-coping mechanism.

2 thoughts on “Nothing to fear but asphyxiation?

    • I think one of the main sites of this response are the orexin cells in the hypothalamus. They are pH sensing cells, and can detect the level of dissolved carbon dioxide in the blood. When the pH drops (due to the production of carbonic acid) they increase their activity and dramatically increase the output of all the catecholamine systems. It’s an archaic system found in a great number of animals, acting at the brainstem level. Primarily it increases respiratory rate and heart rate during times of high blood CO2, such as during exercise. Activation of this system also causes a panic response when CO2 levels become critical, resulting in flight.

      Interestingly, because the orexin system is only sensitive to CO2, a simple drop in oxygen (hypoxia) is not sufficient to generate this behavioural response. If you filter all of the carbon dioxide out of the system and slowly reduce the oxygen levels, this only leaves behind nitrogen. This results in a dangerous state of hypoxia which goes completely undetected, because there is no CO2 to be sensed. The result of breathing in pure nitrogen is rapid brain damage and ultimately death, without a moment of panic. This phenomenon is known as inert gas asphyxiation.

      Perhaps the amygdala is exerting some sort of top-down control over this system, and this is the core of at least some components of the fear response? This would explain why these patients can still generate a fear response in the presence of CO2, but can’t initiate a top-down amygdala fear response?

      P.S. Loving the blog 😀

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