In a nutshell: The autonomic responses associated with fear and hormonal secretions is the function of the amygdala. Scientific studies of the amygdala have led to the discovery of the location of neurons in the amygdala that are responsible for fear conditioning. Fear conditioning is an associative learning process by which we learn through repeated experiences to fear something.
There was an article in Current Biology, a case study of patient called SM — a rare human patient with focal bilateral amygdala lesions. Scientists admit that it was the first investigation of the induction and experience of fear in such a patient.
To provoke fear in SM, she was exposed to a number fear-inducing situations: live snakes and spiders, a tour of a haunted house, emotionally evocative films — you name it, the woman had it all.
Patient SM failed to exhibit fear behaviors, and demonstrated an overall impoverished experience of fear. Her response indicated NO activation of the sympathetic division of the peripheral nervous system: no accelerated heart rate, no dilated pupils, no increase in metabolic rate, and no increase in blood flow to the muscles. All of the above normally triggers a state of fear, and the human amygdala is responsible for it.
The SM woman with damaged amygdala was afraid of nothing and, after 3 months of experiments, reported in multiple questionnaires that shе was, in fact, fearless. Everything else about her emotional makeup was quite ordinary. SM’s neuropsychological profile has been stable for the past two decades. She performed perfectly within the normal range on standardized tests of IQ, memory, language etc. Conclusion: if there is no triggering mechanism in the brain then there is no state of fear and, consequently, no fear itself.
Interesting that the ancient Incas, as archaeological evidence suggests, might have had the rudimentary knowledge of neurosurgery. Here is an ancient Peruvian skull with evidence of trepanation (a surgical intervention in which a hole is drilled or scraped into the human skull.)
Inca warriors were famously fearless, and many fallen soldiers had holes drilled in their heads… Could their surgeons (or priests or barbers) have figured out where inside the warriors’ heads hides their fear and gone straight for amygdala with an ancient drill to create a monstrously fearless army?
Another interesting article on the subject of fear and amygdala: The Amygdala Is Not The Fear Centre, Scientist Now Claim!