The amygdala, a small region of the brain, is not necessarily required for fear and panic in people, according to a study published online this week in Nature Neuroscience. These results are surprising, because decades of research in humans and animals had suggested that the amygdala has an influential role in fear.
To show this, Jon Wemmie and colleagues tested three rare patients with damage to their amygdala, who do not experience fear. Inhaling carbon dioxide (CO2) stimulates breathing and can provoke fear and even trigger panic attacks, as was experienced by the three patients in this study. For one of the patients, it was the first time since childhood that she had experienced fear. Previous work, with this patient and others with similar damage, had suggested that damage to the amygdala resulted in a lack of fear in response to a variety of fear-provoking stimuli, even life-threatening traumatic events. The current findings from Wemmie and colleagues indicate that there are conditions in which the amygdala is not necessary for the expression of fear.
The authors do not know why CO2 can provoke fear without the amygdala when other stimuli cannot. However, most things that provoke fear are sensed by the visual and auditory pathways, which project to the amygdala. In contrast, high levels of CO2 are sensed by receptors in the brainstem and can cause a range of physiological changes, which could activate several different brain areas in addition to the amygdala.