Introduction
I fell in love with neurons and brains watching a presentation on neural networks in college. During my junior year, I took a neuroscience course, and was awed and sometimes frustrated by the complexity of how nervous systems worked, even the ones of simple animals. It’s been a long time since my doctoral studies in neuroscience, and I’ve traveled a long way to become a teacher of mindfulness, self-compassion and embodiment. But in those 35 years of time, I’m still, really, a neuroscientist at heart, and I want the work that I do to be informed by what we know of the nervous system.
One of the most salient questions about how we work as human beings is how our minds and bodies interact. We want to understand how they work together to help us handle danger, regulate our emotions, and interact with each other socially. We want to understand why, for example, we sometimes freeze when we get triggered, or fly into anger or fear. I continue to be interested in the ways in which our body’s unconscious physiological responses are central to our emotional experience, and I want to understand how our emotional experience can creating avenues for healing and regulation of our emotional lives.
We know a lot about how our behavioral responses to danger and threat affect our minds and bodies, and, importantly, how chronic stress can create long term physiological and psychological effects. What we don’t know a lot about is the very specific mechanisms of these in our brains and bodies.
Science, frankly, is complicated. It is littered with results, theories and laws that are later either proven, revised, or debunked. Any solid scientific conclusions involve years of research by many scientists. They involve many, sometimes conflicting primary research papers which can be difficult to understand by most people who don’t work in the specific field in which they were written, let alone non-scientists.
The study of human neuroscience is a particularly complex undertaking. Our ability to examine and experiment on human beings is, for very good reasons, rather limited. So, our understandings of our brains come from a wide variety of indirect avenues. We look at brains from deceased people, which gives a once-in-time detailed anatomical snapshot but doesn’t tell us much about function. We study animals of all sorts: from invertebrates to primates. The results of our research from animal studies have no guarantees that we specifically function in the same exact way. We can confirm some animal studies using behavioral and psychological research on humans, which can tell us a lot about behavior, but not a lot about the brain. Brain imaging studies, especially the newer techniques, like functional MRI, or EEG tomography, are helpful as well, but have a much lower resolution in terms of detailed brain structure and activity than animal studies or anatomy of brains after death. There are also studies using transcutaneous (over the skin) stimulation, but the precision of exactly what is being stimulated is sometimes a question.
Stephen Porges’ Polyvagal Theory has captured the interest of many people who are not neuroscientists. It tries to explain the ways in which our Peripheral Nervous System (PNS) interacts with our Central Nervous System (CNS) in times of threat, stress, and social engagement. It attempts to help us understand better how our responses to threat interrupt and interact with our ability to engage with other human beings in social settings. It has been used in many fields to provide a jumping off point for therapies relating to chronic stress, trauma, and dissociation. It’s relatively simple to understand and seems to encompass our behavioral observations.
In the Polyvagal Theory there are three different hierarchical levels in the nervous system: The Social Engagement System, the Sympathetic Nervous System, and the Parasympathetic Nervous system. These systems act differently when “neuroception” has told us whether we are in a state of threat or safety. In a state of threat, we first try the Social Engagement System: negotiation, appeasement, etc. If that doesn’t work, we move to fight or flight via the Sympathetic Nervous System. If the situation becomes life threatening, we move to freeze — immobilization, faint, etc. via the Parasympathetic Nervous System. If we are in a state of safety, our Sympathetic system allows for mobilization — dancing, sport, etc., and our Parasympathetic allows for rest and sleep. In the safe state, our Social Engagement system allows for play, sexuality, etc.
This larger framework isn’t in a form that neuroscience can validate experimentally, and it’s based on years of research by psychologists and neuroscientists before this theory emerged. That larger framework is what most people in the therapeutic/embodiment world call “Polyvagal Theory” but these understandings largely predate Polyvagal Theory.
What’s really “new” (Polyvagal theory is 20 years old or so, so “new” is relative) is that Stephen Porges has added the hierarchical suppositions and suggested underlying neurophysiological mechanisms to that framework that are indeed possible to validate. Scientifically, the Polyvagal Theory isn’t experimentally verified, and doesn’t really help to explain anything we hadn’t already been able to understand. It both oversimplifies and overemphasizes the role of the vagus nerve in terms of its involvement in threat and social engagement. There is evidence against many of its core suppositions. There have been critiques of the Polyvagal Theory in the neuroscience literature for many years, but these have not made their way out of that field into the open. Here are a few of the most important of those critiques.
“Polyvagal = Bivagal” and evolutionary age
The term “Polyvagal” is a misnomer, although “poly” does sound sexier than “bi”. There are only two efferent (that is, moving outward from the central nervous system (CNS) to the body, effecting what happens in the body) systems: dorsal (back) and ventral (front). In Polyvagal Theory (PVT), the dorsal, unmyelinated (slow), system emerges from the dorsal motor nucleus (part of the medulla, located in the brain stem) of the vagus, which is largely responsible for gut innervation. Porges’ PVT associates this system, supposedly evolutionarily older, with immobilization and dissociation (freeze/faint.)
The second efferent system is the myelinated (fast) ventral vagus, which is supposed to be evolutionarily newer, and has the ability to down-regulate (that is, inhibit, slow down) immobilization as well as fight/flight responses. It is associated with the nucleus ambiguous (also in the medulla). In PVT, it regulates the heart and lungs, and allows for social engagement, down-regulating in “safe” environments.
A core supposition of Polyvagal Theory is that there is a phylogenetic (that is, evolutionary) shift between reptiles, who only use the “older” dorsal vagus, and mammals, who use the “newer” ventral vagus in addition, which allows for a “face-heart” connection that is involved in social engagement, and allows social interactions, environment, and visceral state to influence one another.
The problem is, there is no evidence of this evolutionary (phylogenetic) shift. You find myelinated ventral vagus nerves in cartilaginous fish (like lungfish and sharks), which are evolutionarily older than reptiles.
Of course, it is true that there has been evolutionary change in terms of the use of facial expressions in social engagement. That said, if you see below, the vagus has little to do with control of the face. Interestingly enough, stimulation of the vagus nerve seems to, in some situations, modify recognition of the facial expressions of others, but it doesn’t seem to modify production of facial expressions.
The nucleus ambiguus is ambiguous
In PVT, responses to the environment fall into three categories: life-threatening (extreme danger), danger, and safety. And they are related to the social engagement system, allowing for facial expressions, vocal expression, etc. — inhibiting them when the environment is dangerous or life-threatening, disinhibiting them when the environment is safe. The nucleus ambiguus, where the “phylogenetically newer” (not correct, as explained above) ventral vagus originates, does have cardioinhibitory neurons (for down-regulation of fight/flight,) and also does have neurons for laryngeal, pharyngeal and esophageal muscles (for vocalization, etc.) but does not control facial expression nor hearing. The facial nucleus, which does control facial expression, does not affect the nucleus ambiguus.
Other cranial nerves have more to do with coordination and control of the facial muscles than the vagus nerve.
Heart regulation is way complicated
A core part of PVT is that the dorsal motor nucleus, which is where the dorsal vagal nerve originates, downregulates fight/flight by slowing the heart rate. There is some evidence suggesting that the dorsal vagus can potentially influence the heart, but the control of the heart is influenced by many systems, and the dorsal vagus connection to heart rate is not supported.
Neuroception isn’t simple, either
An important part of the Polyvagal Theory is the idea that we evaluate our environment, and determine whether it is safe, dangerous, or life threatening via a process called “neuroception”. Neuroception is defined in PVT as “…a neural process that enables humans and other mammals to engage in social behaviors by distinguishing safe from dangerous contexts.”
The problem is, “neuroception” is very complex, and involves several categories of psychological phenomena, including fear, threat perception, social behavior, and emotional regulation. Each of these has been studied in some depth by many researchers, and a large number of brain structures unrelated to the vagus nerve are known to be involved. PVT does not explain any of these with any precision.
Conclusion
There are other neuroanatomical and neurophysiological critiques of PVT, but I won’t detail them here, as they are not as easy to explain. Basically, the most important core suppositions of the Polyvagal Theory don’t seem to have much in the way of evidence to support them, and some evidence against them. Further, and I think perhaps most importantly, PVT doesn’t help to scientifically explain what we already know, nor does it provide any new key neuroscientific knowledge.
One of the things that is great about science is that it does, in very clear methodical ways, help us understand the world. However, it is not simple, and it takes a lot of work to translate what scientists do and know to folks who aren’t scientists. I can understand why the Polyvagal Theory has become so popular in non-scientific circles, even though most neuroscientists have been dismissing it for years.
The basic framework described by the Polyvagal Theory, describing differences in our physiological responses and availability of behavior based on perception of threat, and that chronic activation of our threat system leads to psychological and physiological damage and harm, is important and useful for therapeutic applications and embodiment work. But none of that was particularly new to PVT. The Polyvagal theory did have a way of tying it all together in a neat, relatively easy to understand bow. But the structure underneath that bow doesn’t have any scientific foundation.
The problem with this is that it means that there are things we’ll miss if we take Polyvagal Theory as The Way Things Are. Avenues of understanding we might not embark on, questions we won’t ask, different theories that are harder to understand but more scientifically valid (there actually is at least one, called Neurovisceral Integration) will be dismissed, or at least not talked about in therapeutic or embodiment circles. We don’t always need (and will not always find) neuroscientific evidence for why certain therapies or embodiment practices work. But we shouldn’t let theories with scientific evidence against them drive the way we think about our brains. This is complex stuff, and we have to be willing to embrace the complexity and understand that there may never be a theory that neatly ties this together, because human brains are messy, and understanding human brains is even messier.
