The Polyvagal Theory and Horses: An Introduction

**March 24, 2022 Note: As research and our understanding of things evolve, so too does what we teach in EQUUSOMA®. The following is an excerpt of my book manuscript from 2019, which has since been updated in the current version of the manuscript, which will be in press with Routledge in 2023. We have commissioned specific illustrations that we now use in the EQUUSOMA® training and that will also be in the book. We’re in the process of updating our illustrations, including one that we just re-released today. In the meantime, I hope you enjoy the material below (as well as my 2018 post about the polyvagal theory in relation to horses licking and chewing), which is but an introduction to a rather complex and nuanced topic.


This post has been a long time coming.  The single reason as to why this has not gotten written yet is because I’ve been focusing my writing efforts on the book I have in the works (which includes this topic), that writing a separate blog post on the same subject felt like a repetitive task.  However, it’s an important and complicated topic, well worth trying to explain in accessible terms. And there’s also no rule saying I can’t share material from my book in a blog post. And so, without further ado…

The Original Two-Branch Model

Before discussing the polyvagal theory and why it is foundational in understanding mammalian relationships and trauma recovery, it is worth looking back at our understanding of the nervous system as it stood until recently. Traditionally, the autonomic nervous system (ANS) has been taught as having two distinct branches – the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). This two-branch model has become ubiquitous and mainstream, and is still used by therapists, sports psychologists, coaches, riding instructors, equine behaviourists and other horse professionals. This perspective of the nervous system talks about sympathetic arousal, commonly known as the stress response (the body’s “gas pedal”), which involves the body’s ability to generate energy in order to meet the physiological or emotional demands of a particular situation. It is associated with a sense of capability and agency in taking effective action.

The SNS does not only refer to negative stressors that are more on the unpleasant end of the spectrum (such as uncomfortable situations or experiences of danger or life threat), but also more pleasant experiences of arousal (such as excitement, healthy sexuality, play and physical activity). Sympathetic arousal is also involved in eustress, experiences of arousal that are viewed as a positive or goal-oriented challenge inciting vigor, purposefulness and hope (such as more active or strenuous exercise, preparing for a major presentation, and so on). It is important to note that everyone experiences stressors differently; what is pleasant for one may be unpleasant for another.

Benson, Beary and Carol (1974) coined the term “relaxation response” to refer to the parasympathetic deactivation process that supports the organism to return to a resting state following exertion (the “brakes”). The brakes are released again in response to stress, danger or threat so that the SNS can kick in and mobilize energy required to meet the demands of a situation, and are re-engaged when the stressor has passed once more.  The nervous system was thought to respond and calibrate moment to moment like this on a regular basis. A common expression from this model is “you can’t be stressed and relaxed at the same time”, meaning that finding balance involves coming out of a stress response and into a relaxation response.  Some who hold this view, both in the human trauma field and the horse training field, sometimes interpret any signs of stress as being negative and evidence of distress, pain or confusion, and encourage avoiding things that increase stress, prioritizing “down-regulation” as desired and ideal.  While this is not entirely untrue, it nonetheless is based on a limited understanding of stressors and disregards how stress can be useful and how excessive use of the parasympathetic nervous system can also be damaging. It also does not take into consideration the role of the vagus nerve in arousal modulation, relationships, and health issues.

From Two to Three

Neuroscientist Dr. Stephen Porges has conducted groundbreaking work on understanding the autonomic nervous system (ANS) in mammals.  Among his contributions was the proposition that the ANS has three branches as opposed to two.  Rather than one branch being on while the other was off, the three branches act more like dials, each on to varying degrees at the same time, and responding in a hierarchical manner in response to changing environmental conditions, moderated by the vagus nerve. This is actually much less complicated than it sounds.

To begin, mammals have what Porges refers to as neuroception, or the ability to detect safety, danger or life threat. Depending on what internal (interoception) and external (exteroception) cues an organism picks up on, corresponding adaptive neural adaptations (or responses) will kick in. Typically, we’ll respond to danger or life threat by using our most recently developed strategies first, and gradually move down the line to using evolutionarily older, more primitive responses if we’re unsuccessful. Let’s start in reverse, with oldest first.

  1. Dorsal vagal complex (DVC): The DVC is thought to be the most primitive branch of the parasympathetic nervous system (PNS) and is responsible for immobilization. This response to potential dangers, intrusions, shock, toxins, overwhelm, abandonment or other painful experiences stems back not only to reptiles but can also be witnessed in amoebas and even in protoplasm, the basis for all life (Seifriz, 1954). The DVC is responsible for what is most commonly referred to as the freeze response (tonic or rigid immobility), but also is involved in fold (collapsed immobility), faint (vasovagal syncope), and feigning death. Fragmentation (or dissociation) is also common when organisms are dominated by the DVC, as in “if I can’t get my body to safety, or if it’s not safe being me, then I’ll just leave my body”. Or, as actor Woody Allen famously quipped, “I’m not afraid of death – I just don’t want to be there when it happens.” Dorsal dominance means that the dorsal vagus “dial” is on high (shutdown and energy conservation, when the brakes are slammed on really hard). However, the parasympathetic nervous system is also involved in relaxation or settling; this would be when the dorsal vagus “dial” is on low (also known as “rest and digest” – immobility states where the brakes are applied much more gently, that are involved when meditating, sleeping, nursing, etc.).
  2. Sympathetic nervous system (SNS): This response is responsible for mobilization. Arousal of various kinds stems from the SNS, including what one might consider to be pleasurable or enjoyable (fun) such as play, sex, excitement, going for a walk or jog, exercise, anticipating the visit of a loved one or preparing a meal, and so on. The SNS is also involved in run of the mill physical activity, such as getting out of bed, doing chores, making dinner, and so on. Of course, the SNS is also responsible for survival activation, such as fight or flight. This is the proverbial gas pedal and is also associated with the tensing and tightening associated with preparing for activity and self-protective movements, ranging from posturing, intimidation, pushing, punching, kicking, to moving or running away, and so on. It is important to note that not all SNS arousal is survival activation; all activation is, however, arousal. SNS functions like a dial as well, from low to high, and it is moderated by both branches of the PNS (the dorsal brake and the ventral brake).
  3. Ventral vagal complex (VVC): Social engagement is the second branch of the parasympathetic nervous system. Similar to low tone DVC, the VVC is a gentler brake system; it slows the heart down and supports connection (“freedom, friends and forage” are the most possible when dominated by a low tone DVC or a VVC state — this does not mean that there is no SNS onboard, however. For instance, horses can be playing with their friends at liberty in an open pasture. The VVC “dial” provides the brakes that tempers the SNS just enough that play is fun and does not flip into fight or flight activation).  The ventral vagal branch of the PNS also “allows social interactions to regulate physiology and promotes health, growth and restoration” (Porges, 2018b).Furthermore, when anticipating danger or threat, mammals may turn to social engagement strategies for soothing and protection (finding resources: AKA ‘tend and befriend’, consisting of attachment cry, clinging, proximity seeking, bunching, cohesion, protesting separation) or to defuse tension and aggression and prevent harm (fawn: also known as “tend and befriend”, including appeasement behaviours such as reasoning, caretaking, placating, complying, and so on). There are preparatory movements involved in this category of response pattern as well, such as turning towards and reaching for others.Should social engagement prove successful, and the “other” is seen as receptive or supportive or begins to calm down, our own nervous system calms as well and we are able to remain in social connection. However, should social engagement strategies not be effective, fighting or fleeing become the next resort and, should these also not be available or possible, that leaves shutdown as the final option.  This is known as dissolution, which was first described by Jackson (1884), who stated that “the higher nervous arrangements inhibit the lower, and thus, when the higher are suddenly rendered functionless, the lower rise in activity”.

Hierarchical ANS Responses

Illustration by Carolyn Buck Reynolds. All rights reserved. Updated in 2021.

It is difficult to envision these three branches functioning to various degrees at the same time in the above graph. Graphs are imperfect attempts at conveying complex concepts in simplified ways, but are still useful nonetheless.  My best attempt at showing how the three states operate in overlapping ways (as opposed to being completely mutually exclusive) was in using the shaded gradients. For instance, when we are in a state of ventral vagal dominance (the VVC branch of the PNS), there is usually no dorsal vagal activity on board (the other branch of the PNS is off); however, there can be varying degrees of sympathetic arousal present while in a ventral vagal state. The example of play is one of them. The VVC being on helps mitigate the potentially negative impact of SNS arousal which, if experienced without a sense of safe connection in relationship, can escalate to higher levels and potential shutdown through the dorsal vagal branch of the PNS as the only remaining option. This aligns with attachment theory, in that co-regulation through social engagement (when two nervous systems are in sync in the window of tolerance) is an important component of moderating SNS arousal. Attunement and co-regulation helps our bodies experience that “what goes up must (and will) come down” when the right conditions are in place.

This graph also depicts the concept of “as they go in, so they come out“. Dr. Peter Levine, developer of the Somatic Experiencing approach to trauma resolution, describes how this term was originally used in reference to military veterans who went into a freeze response following intense fight or flight activation. These veterans eventually thawed out of freeze and came back down through the same SNS charge that they experienced prior to shutting down, exhibiting sudden aggression, combativeness, or flight responses following a period of apparent stability (Levine, 2010). In other words, when the nervous system begins to experience the neuroception of safety, it can begin to “let down” and move through all the bound activation that was held in the queue in order to regain balance. This process can also occur when we come in and out of anesthetic for surgery, and is common in survivors of childhood abuse, partner abuse, and other traumatic events.

Numerous videos depict animals coming in and out of the dorsal vagal shutdown in this way as well. A few of them can be found here.  Others can be found saved in my playlist of educational videos on the EQUUSOMA YouTube channel. In horses, it is most commonly seen when a horse has experienced training methods or abuse that resulted in learned helplessness and submission. When the right conditions are in place and the horse is able to “thaw out of freeze”, the incomplete survival responses (fight or flight activation) that are lying just under that dorsal state often emerge in dramatic ways as a horse suddenly becomes more opinionated, flighty, or aggressive after being seemingly docile and “bombproof” for a long time.  Because the parasympathetic NS has two branches (two brake systems), it is easy to confuse one for the other: dissociation, submission, stoicism, and shutdown may “look calm” on the surface, but are really a sign of the high tone dorsal brake being on to try to control the undercurrents of overwhelm or flooding. “True calm” comes from the low tone dorsal brake (rest and digest) or the ventral vagal brake (social engagement), which are at the bottom of the graph and occur following deactivation or “discharge” of the higher states of arousal or activation.

Thresholds of Intensity

This does not mean to imply that the sequence always takes place the way the graph shows. Dorsal vagal shutdown does not always occur just because a mammal is experiencing higher SNS arousal or activation. The curve can peak and drop at various levels of intensity without ever reaching the high tone DVC state of shutdown.

©Hoskinson Consulting Inc. d/b/a Organic Intelligence. All rights reserved. Used with permission. Model developed by Steve Hoskinson for Somatic Experiencing training.

Similarly, this does not always mean that the goal is to limit any kind of SNS arousal, because not all SNS arousal is detrimental to the nervous system. If that was the case, then trauma therapy (in humans, at least, but I would argue for other mammals as well) would consist of nothing more than teaching grounding skills to manage bound activation in the nervous system that is stuck in the queue, never having a chance to be released. Somatic Experiencing, as a polyvagal-informed approach to trauma recovery grounded in mammalian neuroscience, emphasizes how grounding skills and other techniques to prevent or limit activation is only one part of the picture. The over-arching goal is not to continually avoid stress, as this results in a narrow window of tolerance, but to grow the window and one’s resilience by learning that the nervous system can experience fluctuations without going into shutdown.

Someone recently told me “the reason I use positive reinforcement is because I recognize that domesticated horses experience a lot of stress that is out of their control, and so I don’t want to add more stress to their existing load through training methods that use aversives” (as in negative reinforcement, or pressure-release). There is a certain amount of validity to this perspective. Similar to trauma therapy with humans, if the conditions are not optimal (as in, the conditions are not safe and supporting regulation and coherence in the nervous system), then it is not the time to do the harder work of trauma processing (which can be more activating).  However, when the conditions are “good enough” (there is enough safety and stability both internally and externally), then dipping into the deeper stuff can be attempted, to help de-activate the bound charge that is still held in the body and to grow the window of tolerance.

Bear in mind, that there is a difference between therapies that encourage flooding and therapies that focus on the titrated renegotiation of little incremental amounts of survival activation to grow the window of tolerance. Just because the conditions are right doesn’t mean that flooding is ever appropriate (if your goal is to induce compliance, then that’s another story). Titration is still crucial to not send someone into overwhelm, which is simply a re-enactment of retraumatization. Similarly, there are horse training techniques that encourage flooding (most often methods that mis-use negative reinforcement to dominate and induce learned helplessness) and those that are more nuanced and attend to these thresholds of intensity in order to help grow the window of tolerance. Both negative and positive reinforcement can be done with this perspective in mind.

There is a big difference between negative reinforcement done to dominate and shut down (sending the horse into the “red zone” of the graph above), and negative reinforcement that is done while attending to thresholds, nervous system states, and that uses connection and the social engagement system (ventral vagal branch of the PNS) to moderate arousal so that it is not detrimental. We grow the window of tolerance by recognizing that what goes up will come down, and by helping the nervous system tolerate incrementally bigger thresholds without getting stuck in fight or flight activation or shutdown.

© Fed Up Fred – The Three Thresholds

The Fed Up Fred illustration on thresholds explains this in more simple terms. Not all negative reinforcement (-R) is above threshold. Negative reinforcement that is below threshold is not necessarily detrimental to the horse. Negative reinforcement that goes above threshold (i.e., into a high degree of SNS without a ventral brake, or into high tone DVC shutdown) is problematic. What the polyvagal theory and Somatic Experiencing can add to this picture is the idea that it’s maybe not as simple as “show the desired behaviour and you will be allowed to escape or avoid the scary thing”. That implies tolerating and overriding.  Instead, the goal is to experience a pendulation of the nervous system in response to a tiny stimulus (a pendulation is the full arousal and settling of the ANS) to recognize “oh, this isn’t so bad”, before the window of tolerance grows a little to allow a little more — without stimulus stacking and resorting to escape or avoidance to cope. This, in turn, builds confidence and resilience. It is the difference between reliving and what Dr. Peter Levine calls renegotiation:

“Renegotiation is not about simply reliving a traumatic experience.  It is, rather, the gradual and titrated revisiting of various sensory-motor elements comprising a particular trauma.  Renegotiation occurs primarily by accessing procedural memories associated with the two dysregulated states of the autonomic nervous system (hyper/hypo-arousal) and then restoring and completingthe associated active responses. As this progresses, the client moves towards equilibrium, relaxed alertness, and here-and-now orientation.”

–Levine (2015, p. 44)

Of course, Somatic Experiencing is not a recognized training method for horses. Although it was developed for human trauma recovery, it nonetheless draws on mammalian neuroscience, so there is some validity to the notion that it (or elements of it) could be used with horses as well, albeit in adapted ways. My sample size of using it with my own horses as well as with horses I work with in equine-assisted trauma recovery is not sufficient to claim evidence-based practice, of course! Nor am I an equine behaviourist. Similarly, Dr. Stephen Porges is not a trauma therapist; however, his ideas have been enthusiastically adopted by the trauma therapy community because of how useful they are in predicting what we see happening and in helping guide interventions in a more effective way. Similarly, I see the potential for the polyvagal theory being a useful framework for the equine sciences, equestrian, and equine-assisted interventions fields.

Poly-what? Why?

This brings us back to that infamous word. The reason this is called the polyvagal (or multiple vagi) theory is that the tenth cranial nerve (known as the vagus, or “vagabond”) is also the longest and wanders far and wide, linking up with a number of different parts of the face and body. The parasympathetic nervous system’s two branches (DVC and VVC) come on depending on which part of the vagus is firing at any given point in response to internal and external conditions. For instance:

  • The ventral vagus links the muscles of the face and neck, larynx and pharynx (involved in speech), and heart (“supra-diaphragmatic” or above the respiratory diaphragm). These parts of the body are involved in social communication and connection. When this part of the vagus is online or dominating, the face is engaged, eyes are soft or show aliveness and variability of expression, vocalizations or talking are feasible, and heart rate slows to a comfortable pace that allows a sense of comfort or ease in connection. Presence is possible. This is why the ventral vagus is known as the social engagement system. A nervous system that is dominated by a ventral vagal state has a very different facial expression than one that is dominated by a high level of SNS or a high level of DVC. There are even differences in facial expression between humans and horses that are in a high level of SNS arousal that is tempered by the ventral vagal brake (as in play) and humans and horses that are in a high level of SNS arousal tipping into survival activation (as in fighting).As seen in the diagram below, the vagus is intimately linked with the other cranial nerves, including those that innervate the middle ear. When detecting the neuroception of danger or life threat, for instance, the middle ear will focus on sound frequencies like low grumbles, growls, stampeding hooves, equipment noises, and so on, making it difficult to pay attention to the vocal frequencies of the human voice. It is one of the reasons why in arguments it can difficult to really hear what someone else is saying.
Image from Quiring, D.P. (1950). Functional Anatomy of the Vertebrates.
  • The dorsal vagus links the larynx and pharynx with the heart and the gut (operates largely “sub-diaphragmatic” or below the respiratory diaphragm). When a high tone dorsal vagal state is dominating, the parts of the body involved in social communication and connection go offline because the ventral vagus (above the diaphragm) is off; the face has a flat expression, eyes may appear vacant or empty, vocalizations or talking are not possible or difficult to make.  A high tone dorsal state of shutdown is useful for survival, but is not intended to be the main brake system – think of it like driving a car using your hand brake whenever you want to slow down. The amount of wear and tear would be significant and your car would become run down very quickly.  Indeed, human and non-human animals that have become dominated by this shutdown state as a response to threat or overwhelm often have issues in the parts of the body that are innervated by the dorsal vagus – namely heart, gastrointestinal, and feeding or eating-related issues or disorders. Instead, we want to slow down using the low tone dorsal brake (a foot brake that supports restfulness) or the ventral brake (which supports comfort in connection).


Illustration by Frank Henry Netter


Copyright John Chitty, Colorado School of Energy Studies

In the last of the images, the ventral vagus is mapped in blue, the dorsal vagus is mapped in red, and the sympathetic nervous system is in gold. Imagine these three colours superimposed over the full body diagram of the horse above, and you get a reasonable comparison. In short, the neuroception of safety supports the ventral vagus to come back online. It also supports the state of low tone dorsal vagal “rest and digest”. What can shift a nervous system from sensing “safety” to sensing “threat” can be incredibly subtle. So often we assume “bad behaviour” when in fact what we are seeing is a nervous system response to danger.

Nervous System Dials

Technically, it is possible to “be stressed and relaxed at the same time” – if we look at things through a polyvagal lens, where the PNS can be used to temper the experience of the SNS. To give a better sense of the idea of how the 3 branches of the ANS function together as fluctuating dials, consider the following examples:

  • Petting a cat. This is an enjoyable experience until the cat appears to suddenly turn on you and sinks its teeth and claws into your hand. The original experience likely was calming and involved a certain amount of social engagement (low DVC, moderate VVC, low SNS), but as the stroking became overstimulating, VVC would have decreased to low, DVC might increase a little as the cat disconnected, and SNS would have started to increase to higher levels, culminating in the attack.  This is similar to when one person tickles another and it’s still within one’s window of tolerance; DVC is low, and there is a good amount of VVC online to temper the SNS response. However, should the tickler fail to recognize the other person’s threshold and continue even though the person is overstimulated, the game can quickly result in a drop in VVC engagement, increasing SNS, and an increase in DVC (shouting and/or pushing, or shutdown and withdrawing when it is clear the tickler isn’t getting the message).
  • This is similar to the question of sex. Sex that is connected and intimate (not just the mechanical act) typically would involve low tone DVC, moderate to high VVC, and low to moderate SNS, depending on how vigorous or charged the encounter was (sex that was purely physical with no connection would involve low VVC). However, when sex becomes rape, the dials would change to high tone DVC (shutdown, dissociation), low VVC (disconnection), and high SNS (fear).
  • Savasana at the end of a yoga practice (the quiet period before going home, where people lie on their mats and rest): this would typically involve low amounts of DVC, low VVC and low SNS. For some individuals, however, the stillness of savasana can be activating, in which case savasana would more likely consist of potentially higher tone DVC, low VVC (no one to socially engage with to co-regulate), and high SNS.
  • A kitten, squirrel, or mouse being carried by the scruff of the neck by its mother. This would involve high DVC (collapsed immobility; floppy tone), low to moderate VVC, and low SNS.  Because of the absence of fear, the experience of letting go in this way might feel neutral, calming or even pleasurable; when the baby is put down again, muscle tone returns and the animal continues to play, explore and engage the way it did before being picked up.  However, a gazelle held in the jaws of a predator might seem like a similar experience, but the dials here would be different: likely high DVC, low VVC, but high SNS (fear).

Consider the tragic media report of the horse that died from fright as a result of an unexpected fireworks display nearby (Bellis & Hawkins, 2018). Interestingly, there had been a planned fireworks display in the neighbourhood the previous week, and the horse’s owner had brought him indoors and stayed with him to help him remain calm (VVC or social engagement to mitigate the effects of stress). The horse recovered from that situation without issue. However, the owner had not been made aware that there would be another fireworks display on that fateful night a week later, and so the horse, named Solo, had been turned out alone without the ability to calm himself through ventral vagal engagement with other herd members or even a human nervous system to support co-regulation. And so, in the face of intense panic, distress, and inability to get away from the noise (high SNS mobilized to flee), the only way for the nervous system to slow down was to resort to the emergency hand brake (DVC), which has a high cost to the body: the horse’s gut had twisted (a possible sign of dorsal dominance following high stress) following running at top speed in his paddock for a prolonged period, and was in so much pain from the shock that he had to be put down. There was no ventral brake available to modulate the intensity of the terror to reduce the impact of the stimulus to the nervous system.

As devastating as this example is, it highlights how the parasympathetic nervous system (PNS), typically thought of as the body’s relaxation response, is not always restorative in function. It is for this reason, and the others outlined above, that moving away from the earlier understanding of the nervous system as having two branches is not only helpful but necessary to understanding human-horse behaviour dynamics in a more nuanced way. The polyvagal model proposed by Dr. Porges offers a framework to help make sense of interactions and guide ways we might intervene differently, whether we are horse lovers or owners, riding coaches, equestrians, equine-assisted therapy and learning professionals, equine behaviourists, horse trainers, or other animal professionals. This will be elaborated on throughout the book… (in progress!)

Thanks for your patience, everyone. The book is coming. In the meantime, hopefully this provides a clearer understanding of not only the “what” of the polyvagal theory, but also the “why” (its relevance as a lens or framework through which to approach horses and horse-human interactions). There is more to it, but this is just a starting place to pique your curiosity.