Fearless brain-damaged patients are terrified of suffocation.
People who lack a brain structure thought to be critical for fear can still experience the emotion.
The patient known as S.M. has not experienced fear since she was a child, and has fascinated brain researchers for many years. S.M. is diagnosed with Urbach-Wiethe disease, a genetic condition that causes a brain structure called the amygdala to gradually harden up and shrivel away. This small, almond-shaped bundle of neurons, located deep within the brain on the inner surface of the temporal lobe, plays an important role in emotions, and is thought to be an essential component of the brain’s fear circuit.
Now, though, the researchers have found something that can drive S.M. into a panic, and made her feel some kind of fear for the first time in decades: a whiff of carbon dioxide.
The study clearly shows that the amygdala is not needed for the fearful response to carbon dioxide, or even for sensing the gas in the first place. It seems to be far more important for responses to threats from the outside world. The stimuli signalling a threat of suffocation – an increase in blood acidity – come from inside the body instead.