The polyvagal theory was put forward by Stephen Porges (2011). It proposes three vagal-mediated adaptive responses for different circumstances, namely safety, danger, and extreme threat to life. These responses are initiated by our perception of what is going on in our environment.

The first of these circuits is the ventral vagal complex (also known as the “smart vagus”), which down-regulates the sympathetic “fight or flight” response so we can be social, engage in bonding, and emotionally self-regulate. This part of our nervous system is implicated in social communication on account of its effect on the muscles of the face, and in self-soothing/calming through inhibition of the defensive limbic system. The ventral vagal complex also influences heart rate: high vagal tone (strength of vagus response) has a suppressant effect on the heart’s natural pacemaker; in the absence of vagal regulation, the heart’s pacemaker rapidly increases heart rate.

The second circuit is the dorsal vagal complex, an unmyelinated circuit that is found in most vertebrates and is associated with primitive survival strategies. This is the branch of the vagus nerve that engages defensive “fight or flight” behaviour in the face of threats.

The third vagal mechanism acts through the dorsal vagal complex, which, in the face of an extreme threat to life, will feign death by producing neurogenic bradycardia (slowing of the heart rate) via the parasympathetic nervous system, manifesting in a freeze or faint. The bradycardia can be paralleled by apnoea—a response that may be adaptive for reptiles, but is potentially lethal for humans.
The initiation of any one of these three vagal responses is based on our perception of threat. This perception Porges calls neuroception—a subconscious neural process that distinguishes whether situations or people are safe, dangerous, or life-threatening.

If there is no perception of threat, then the ventral vagal complex has control and we can be social and relaxed; we have good HRV, and all is well. But we all have different perceptions, or subconscious neuroceptions, about what is threatening and what is not, based on earlier experiences. For an individual who has had mainly experiences of trauma, nothing in the world feels safe, and the default response is characterized by more frequent engagement of the dorsal vagal complex than is necessary to ensure actual safety. Lower HRV has been observed in individuals with post-traumatic stress disorder and borderline personality disorder (Meyer et al., 2016), rendering these people in a psychological state more prone to “fight or flight” than social engagement behaviours (Austin, Riniolo, & Porges, 2007).

Austin, M. A., Riniolo, T. C., & Porges, S. W. (2007). Borderline personality disorder and emotion regulation: Insights from the Polyvagal Theory. Brain and Cognition, 65, 69–76. 10.1016/j.bandc.2006.05.007

Meyer, P. W., Müller, L. E., Zastrow, A., Schmidinger, I., Bohus, M., Herpertz, S. C., & Bertsch, K. (2016). Heart rate variability in patients with post-traumatic stress disorder or borderline personality disorder: Relationship to early life maltreatment. Journal of Neural Transmission,123, 1107–1118. doi:10.1007/s00702-016-1584-8

Porges, S. (2011). The polyvagal theory: Neurophysiological foundations of emotions, attachment, communication, and self-regulation. New York, NY: W. W. Norton.

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