When our nervous system detects safety, our metabolic demands adjust. Stress responses that are associated with fight and flight, such as increases in heart rate and cortisol mediated by the sympathetic nervous system and hypothalamic-pituitary-adrenal axis, are dampened.
Similarly, a neuroception of safety keeps us from entering physiological states that are characterized by massive drops in blood pressure and heart rate, fainting, and apnea—states that would support “freezing” and “shutdown” behaviors.
How does the nervous system know when the environment is safe, dangerous, or life threatening? … Specific areas of the brain detect and evaluate features, such as body and face movements and vocalizations that contribute to an impression of safety or trustworthiness. … This process of identifying familiar and trustworthy people and evaluating the intentions of others based on “biological movements” of face and limbs seems to be located in the temporal lobe of the cortex.
If neuroception identifies a person as safe, then a neural circuit actively inhibits areas of the brain that organize the defensive strategies of fight, flight, and freeze. Slight changes in the biological movements that we see can shift a neuroception from “safe” to “dangerous.” When this shift occurs, the neural systems associated with prosocial behavior are disrupted and the neural systems associated with defensive strategies are triggered.
In the presence of a safe person, then, the active inhibition of the brain areas that control defense strategies provides an opportunity for social behavior to occur spontaneously. Thus, the appearance of a friend or caregiver would subdue the neural circuits in the brain that regulate defensive strategies.
And as a consequence, closeness, physical contact, and other social engagement behaviors become possible. In contrast, when situations appear risky, the brain circuits that regulate defense strategies are activated. Social approaches are met with aggressive behavior or withdrawal.
(Stephen Porges, Neuroception)