Why do we feel fear? For years, a part of the brain called the amygdala has been implicated in this emotional response. This region links memories with emotional responses, one of which might be fear. A patient (known as S.M.) with dysfunctional amygdalae on both sides of her brain has been known to show no fear in response to various fear-inducing stimuli, including life-threatening traumatic events.

Everyday Gas Induces Fear in the Brain

Another stimulus that is known to evoke fear is carbon dioxide. Inhaling this gas turns on a protein which in turn plays a role in fear and anxiety (how this protein works in inducing fear remains unknown). How would patients with damaged amygdalae react to this stimulus? A team of researchers at the University of Iowa tried to find out.

The first image is a scan from a normal patient and the next three are from patients with damaged amgydalae. The area marked in red shows the lesions present in their brains. [Image Source: Iowa Neurological Patient Registry at the University of Iowa]

‘Fearless’ Patients Show Fear

To their surprise, they found that the 3 people with lesion in their amygdalae (let’s call them patients) showed a greater degree of panic than a group of patients with normal amygdalae. The patients described having experienced emotions they had never felt before, with their descriptions residing well under the category of ‘fear’. Clearly, these results show that the amygdala is not an absolute necessity for fear. However, anticipatory responses to the inhalation, such as an increased heart rate before inhalation, were shown to be significantly increased in controls when compared to patients.

These results led the authors to believe that the carbon dioxide activated a previously unused pathway in patients with damaged amygdalae. One possibility is that most stimuli that normally induce fear are external—perceived visually or auditorily—, whereas inhalation of carbon dioxide represents a physiological, internal, change that does not need processing by the amygdala to generate fear. Another conclusion that the authors came to was that the amygdala might, to some degree, inhibit fear, since the degree of panic attacks was milder in the control group.

Fear is an important survival mechanism, and this experiment gives important clues to its origin. You can read about this research here.