Dr. Stephen Porges on Heart Rate, the Vagus, and the Polyvagal Theory (Interview)
After graduate school, the question that kept pulling at me was a simple one to ask and a hard one to answer: what is the mechanism? My early work ran in stages. I had to build ways to actually quantify physiology. Then I wanted to see how that physiology connected to behavior. After that came the matter of individual differences, why one person's readings differed from another's. And the last stage, the one that turned into something closer to a journey than a study, was figuring out what any of it meant. You can collect biomarkers all day, but a measurement by itself only points at something. A lot of psychology and psychiatry still stops at correlates. I wanted to know what the measures were actually tied to underneath. That curiosity is what led me to the neural mechanisms that regulate heart rate, and those mechanisms became the Polyvagal Theory.
Eric asked me to say something about how the theory touches ideas around social bonding. Before that, though, it helps to back up and explain what the Polyvagal Theory even is.
The theory grew out of a contradiction I kept running into. I was studying human newborns, and human fetuses during delivery, trying to come up with a way to track well-being. My instruments were heart rate and heart rate variability. Healthy babies, I found, had plenty of heart rate variability. I had been assuming that variability came through a cranial nerve called the vagus, so more communication between brain and heart meant more variability, which meant a healthier infant. That part fit.
What didn't fit was the danger. One of the serious risks during delivery and in the newborn period is clinical bradycardia, a sharp slowing of the heart. The medical community also called that a vagal response. So the same nerve was supposedly doing two opposite jobs. How could the vagus be both the thing that protects us from harm and the signature of the harm itself? That question refused to leave me alone, and chasing it meant years of both experiments and reading.
What came out of it was the recognition that we don't have one vagal system, we have two. One of them is old. We inherited it from more primitive vertebrates, the kind of response you see in reptiles. Under threat or fear, a reptile can go still, shut down, drop into something close to inanimate. That is a vagal response too: the heart slows, metabolic demand falls, and a reptile can stay underwater for a couple of hours on almost nothing. Mammals can't take that option. We burn through oxygen and don't have the luxury of switching off. So mammals also carry a newer vagus, a more recently evolved branch, and that one is wired differently.