Researchers have identified a cluster of neurons in mouse brains that encourage fear response to perceived threats. When an adjacent cluster of neurons is activated, boldness and courage behaviors are exhibited.

med.stanford.edu/news/all-news/2018/05/scientists-find-fear-courage-switches-in-brain.html

Salay, L. D., Ishiko, N., & Huberman, A. D. (2018). A midline thalamic circuit determines reactions to visual threat. Nature. doi.org/10.1038/s41586-018-0078-2