Answer: Patient SM had a very rare brain disease that destroyed her amygdala bilaterally. He did was unable to show fear.
Fear is a highly adaptive trait that helps animals survive in the face of a threat to their safety. Fear causes the blood pressure to increase, the heart rate to increase, and the respiration rate to elevate and deepen. Fear triggers the release of norepinephrine in the peripheral nervous system, which is a consequence of sympathetic nervous system activation. The sympathetic nervous system is sometimes called the “fight or flight” response, because the physiological response to a fearful stimulus enables our bodies to respond to the threat optimally.
In animals, the perception of fear is mediated by a small almond-shaped part of the limbic system called the amygdala. The amygdala is part of the older brain, meaning that it's function was important for phylogenetically ancient species. The amygdala communicates with the hypothalamus, which is actually responsible for the release of norepinephrine throughout the body.
However, the amygdala can be damaged, as was observed in Patient SM. She had an extremely rare genetic disorder called Urbach-Wiethe disease, which caused her amygdala specifically to be destroyed. As could bed predicted, with the amygdala missing, she was unable to display fearful responses to threatening or scary stimuli. For example, experimenters showed her video clips from scary / suspenseful movies (Se7en, The Silence of the Lambs), and she only displayed curiosity but not fearful responses. Her heart rate did not fluctuate while observing these stimuli.
The experimenters then provided her with even more scary stimuli, including visiting a haunted house - the Waverly Hills Sanatorium in Louisville, Kentucky. She did not exhibit any fear as she walked through the exhibit, instead wanting to interact with and talk to the “monsters”.
Through Patient SM’s injuries to her amygdala, researchers discovered that damage to the amygdala results in a lack of fear responses. Much of what has been learned about fear and the amygdala was described in a publication called “The Human Amygdala and the Induction and Experience of Fear”.
Other patients who have had their amygdala removed have showed similar symptoms.