What are you afraid of?

FearLooking into those several week old containers of leftovers in the fridge? Analyzing data for an experiment you’ve now done a third time looking for a tie-breaker? Walking into that small conference room with your thesis committee hoping to graduate? Each of these things can cause the body to enact the fear response.

A great deal of evidence has implicated the portion of the brain called the amygdala in fear and recall of fear and behaviors associated with it. It is incredibly rare for a human to have damage to their entire amygdala but these cases do exist. One example is Patient SM who has a rare condition called Urbach-Wiethe disease. This disease causes a build-up of calcium in blood vessels which can damage particular parts of the brain, in the case of Patient SM, both portions of her amygdala have been destroyed by these calcifications. This woman exhibits no fear response to normally frightening stimuli like snakes, spiders, haunted houses, or scary movies. She cannot recognize fear in the faces of others. She and a few other patients have been critical to studies of how the amygdala modulates fear in humans. Are these people truly fearless?

While all of the previous studies on these rare individuals looked at the lack of response to common fear stimuli one stimulus has been neglected. Inhalation of carbon dioxide stimulates increased breathing and can cause “air hunger”, fear and panic attacks. A study led by Justin Feinstein at the University of Iowa, examined how patient SM and two additional patients (AM and BG) who also have amygdala lesions from the same disease, respond to a 35% carbon dioxide inhalation challenge.

In a shocking departure from the prediction (that inhaling 35% carbon dioxide should cause no fear reaction), all three patients experienced panic attacks when breathing the 35% carbon dioxide both on the first and second exposures in comparison to 20% of the matched control subjects. Despite showing fear at no other stimulus, carbon dioxide inhalation caused a strong fear response in the absence of a functioning amygdala. This suggests that carbon dioxide inhalation is activating some other fear/panic response pathway that has been undetectable until now.

There is nothing to fear except high concentrations of carbon dioxide?


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