There's a part of the brain that's known as the "fear centre" -- the amygdala -- which lights up in response to frightening situations. It's an essential part of the body's panic response, and we know this because numerous studies have shown that it lights up when people get scared. It's surprising, then, to see a study today that claims to show that it's still possible to scare people who have a damaged or shrunken amgdala -- that is, you can frighten the fearless.
The study, in Nature Neuroscience, concerns people with a rare recessive genetic condition known as Urbach-Wiethe disease. Fewer than 300 people have been found to have it since its discovery in 1929', and among its many symptoms (many of which involve skin lesions) the amygdala can also harden and shrivel up, impairing its function. It was this phenomenon that neurologists Justin Feinstein, Colin Buzza and Rene Hurlemann sought to explore.
While clips from horror films proved unable to inspire fear in three participants with Urbach-Wiethe disease, the researchers wanted to know if the amygdala's other function -- detecting carbon dioxide -- was also impaired. If the acidity of a person's bloodstream is increased by higher concentrations of carbon dioxide (which could be a signal of possible suffocation) the amygdala can cause someone to feel panic and terror. So, the researchers had the participants breathe in air that was -5 percent carbon dioxide through a mask.
Much to their surprise, all of the participants -- the three with Urbach-Wiethe disease and 12 normal controls -- experienced a feeling of intense fear almost immediately. Even more surprisingly, the three with the damaged amygdalas actually felt more terrified than the control participants, fearing that they were being suffocated.
The implication of this study is that the amygdala isn't as central and necessary to fear responses in humans as we had previously thought -- the study authors write that "results indicate that the amygdala is not required for fear and panic, and make an important distinction between fear triggered by external threats from the environment versus fear triggered internally by CO2".
Buzza told Wired.co.uk: "The occurrence of fear and panic in these patients was a surprising result because research to date has clearly shown the important role the amygdala plays in fear responses. I think it shows that a healthy, functioning amygdala is not required for the experience of fear or panic, but rather that some other brain structures are sufficient to produce such feelings in response to an internal trigger such as rising levels of CO2." "Perhaps as surprisingly and importantly, the fact that the amygdala lesion patients seemed prone to panic (responding at a rate similar to patients with panic disorder), suggests that an intact amygdala might actually inhibit panic. This raises the question of whether some sort of amygdala dysfunction may contribute to panic disorder."
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