There are a handful of things that make me really uneasy. The mere thought of getting onto a roller coaster, especially the kind that slowly ascends hundreds of feet before dropping uncontrollably at high speeds, makes me want to let out a childish high-pitched squeal. Even the “cool” elevators that glide on the outside of buildings, which might give you a beautiful panoramic view, only leave me feeling sick.
For many people, multi-legged creepy-crawlers set them on edge. Other people avoid confined spaces. The deep and sometimes irrational feelings elicited by some things and situations are beyond a simple “dislike.” They are intense fears, called phobias, which manifest feelings of panic and anxiety. While my first visit to Six Flags was also my last, I was left to ponder how so many aren’t fazed by the imposing sight of these rides. In fact, some people don’t seem to fear much of anything. What’s behind this superhuman quality of fearlessness?
Nothing may be more accurate than the cliché “it’s all in your head” when it comes to fear. The amygdala, a bilateral structure in center of the brain, has long been implicated in mediating fear responses. It is also able to detect unusually high CO2 levels, stimulating breathing and inducing fear and panic, when it’s inhaled.
Researchers from the University of Iowa recently evaluated patients, who had rare bilateral amygdala lesions (both amygdalae were damaged). These patients are never afraid in life-threatening or traumatic situations, but Feinstein and colleagues wanted to see what happened when they were exposed to CO2 gas. When control patients (who had normal amygdalae) inhaled CO2, most reported only low levels of fear. For amygdala-lesioned patients, however, this was a distressing experience! For the first time since childhood, they experienced a panic attack after inhaling CO2! They reported intense fear and found the CO2 to be aversive, causing them to gasp for air. Many of them even tried to escape the challenge by removing their inhalation masks.
Stress wasn’t the only side-effect. CO2 exposure also induced physiological effects in patients with damaged amygdalae, including higher breathing and heart rates. Control patients displayed a gradual increase in these physiological responses, as they observed experimenters preparing to administer the inhalation challenge. These anticipation responses were absent in amygdala-lesioned participants, who reproducibly panicked after being exposed to subsequent CO2 trials.
These results suggest that the amygdala is essential in detecting danger in the environment and prepare the appropriate physiological responses to potential threats. Contrary to the notion that the amygdala is required for fear, these results indicate that fear and panic can still be elicited by CO2 in amygdala-lesion patients!
How can amygdala-lesioned patients display fear and panic after CO2 inhalation and not when faced with another obvious life-threatening events? We can look to superheroes for an answer. They are never afraid of villains or the scariest dangerous scenarios, which are exteroceptive in nature, requiring visual and auditory inputs. But some avoids some specific substances, like kryptonite, at all costs, and – like CO2 – its detection may be interoceptive in nature, requiring internal chemoreceptors that cause an array of physiological responses.
Feinstein J.S., Buzza C., Hurlemann R., Follmer R.L., Dahdaleh N.S., Coryell W.H., Welsh M.J., Tranel D. & Wemmie J.A. (2013). Fear and panic in humans with bilateral amygdala damage, Nature Neuroscience, DOI: 10.1038/nn.3323