Polyvagal Theory and Hope In Healing from Childhood Trauma

Critics of the polyvagal theory point out that its premises are not supported by empirical, scientific research. Prof. Paul Grossman of University Hospital Basel argues that there is no evidence that the dorsal motor nucleus (DMN) is an evolutionarily more primitive center of brainstem parasympathetic system than the nucleus Ambiguus (NA), and that no evidence supports the claim that sudden decrease in heartrate elicited by extreme emotional circumstances (like trauma-related dissociation) is due to DMN efferent activity to the heart.^[12] In fact, there seems no evidence that such decrease happens in trauma-related dissociation in the first place.

"There is consensus among evolutionary biologists that there are huge species differences among fishes, reptiles (and birds) with respect to whether dorsal brainstorm centers (corresponding to DMN) or ventral centers (corresponding to NA) are responsible for parasympathetic control of heart rate."^[13]

Grossman also points out that even the results of Porge's own study on two species of lizard was flawed due to inappropriate measurement of heart rate variability.^[14]

While the criticism does not address the clinical speculations of the polyvagal theory directly, it contradicts its premises. In particular, it pulls the rug from under the suggestion that there is a phylogenetic hierarchy, where one vagal system is more primitive than the other, and therefore is activated only when the more evolved one fails (as in dissociation, or acute trauma). It has been known for roughly a century that "a differentiation of the visceral efferent column of the vagus nerve into a dorsal motor nucleus and a ventrolateral nucleus (nucleus ambiguus) is first seen in reptiles (Ariens Kappers, ’12; Ariens Kappers et al., ’36; Addens, ‘33)"^[15]. This contradicts the polyvagal description of the dorsal motor nucleus as being "phylogenetically older" than the nucleus ambiguus, or of the latter being unique to mammals. In addition, recent findings in lungfish suggest that myelinated vagus nerve fibres leading from the nucleus ambiguus to the heart long precede the evolution of mammals.^[16][17]

In polyvagal theory the term vagal tone is equated with respiratory sinus arrhythmia (RSA), which is suggested to be linked to dimensions of psychopathology. A number of research studies have evaluated RSA responses across a range of dimensions of psychopathology, but a comprehensive meta-analysis has shown that no clinically meaningful relation can be found between psychopathology and RSA reactivity.^[18] Apart from this, a correlation between vagal tone and stress regulation would not necessarily indicate a causal role of the vagus nerve activity on different stress responses in mammals (see Correlation does not imply causation for more).

By overemphasizing the role of the vagus nerve in deciding between freezing and other fear responses, the theory disregards decades of neuroscientific findings on the origins of the freeze response^[19] and fear responses in general^[20]. While the vagus nerve undoubtedly plays a role in transmitting emotion-related signals between the brain and the rest of the body (a fact established long before the emergence of polyvagal speculations, see Vagusstoff), there is no evidence to suggest that it has any control over whether a freeze response is triggered or not.

From a methodological perspective, many claims of this "theory" do not meet the criteria of a Scientific theory because they are formulated in a manner too vague for empirical testing. For example, the precise functioning of the two proposed distinct "vagal systems" or of the "social engagement system" is not explained^[21], nor is that of the "face-heart connection" supposedly embodied in the ventral branch of the vagus nerve. While other brain areas known to be involved in fear responses (e. g. the amygdala and periaqueductal gray^[19][20]) are mentioned by Porges, he does not integrate them into the decription of his own hypothesized systems. Simply observing an anatomical link between two areas of the body is not sufficient for explaining complex social and emotional behaviours as Porges broadly attempts to do.

In addition, polyvagal theory introduces the term "neuroception" for "a neural process that enables humans and other mammals to engage in social behaviors by distinguishing safe from dangerous contexts" ^[21]. It thus attempts to encompass several categories of psychological phenomena, each one of which constitutes a broad field of research in its own right: fear, threat perception, social behaviour, and emotion regulation. The neural substrates for many of the included phenomena are known at least tentatively, and comprise a large number of brain structures including, but not limited to, the vagus nerve. Polyvagal theory does not explain the mechanism of any of these phenomena with any precision, resulting in an oversimplification rather than an expansion or refinement of existing knowledge.