Meet patient SM: One evening, SM was walking in the park late at night when suddenly a man grabbed her from behind and put a knife to her throat. He threatened that if she wouldn’t give him her money he would slit her throat. SM just laughed and threatened back to chase and beat him, with or without a slit throat.
SM is the only person in the world without any fear mechanism in the brain, her amygdala nuclei are inactive since she was 10 due to a very rare neurological disorder. The amygdala is involved in emotional control processing and the creation of emotional memories associated with fear. It is highly sensitive to stress hormones and is involved in emotional regulation in response to danger.
Scientists have repeatedly tried for years to teach her fear until one experiment led to a breakthrough in the field and first revealed reactions of fear and anxiety.
When they let her inhale carbon dioxide gas, her brain sent danger signals to her body and for the first time in her life, she knew fear!
Bonus fact: When researchers gave subjects with normal amygdala the same treatment they showed lower anxiety levels than SM did. Why? – Like SM they knew they were part of an experiment and have nothing to fear, but unlike SM they had a functional amygdala to regulate their response to danger.