There's a lot of interest in therapeutic and neurobiology and medicine circles these days, and for a few years now , around what Stephen Porges has entitled "Polyvagal Theory." It refers to the vagus nerve (autonomic system) and its net of interacting nerves from brain to heart and heart to brain, and other central body organs.
A lot of people interested in the neurobiology of fear and trauma have taken to this theory and psychotherapists and psychiatrists are particularly interested. I have not given the theory much attention yet. I am always reluctant to get on the new bandwagon of the science of fear researchers and their positivist empiricism that biases their theories of explaining fear (and 'fear'). It is typically too reductionistic in itself as explanation of such a complex phenomena as "fear." The fearologist prefers to think in holistic-integral deep and broad ways before explaining "fear." But if we are to pay attention to all dimensions of our bodies and minds and our history as a species, then neurobiology has something to offer. So, let me quickly say what I hear this theory is about, based on Porges' description in a video interview "Polyvagal Theory: How Your Body Makes the Decision." There are many videos, articles and books on this topic now a days and many professionals and theorists who put their own spin on it as well--not, to say Porges would likely agree with all the interpretations and uses per se.
Porges explains an evolutionary theory of Defenses, that is, of systems that have developed and evolved over hundreds of millions of years in vertebrates, and in particular in mammals. A major shift went on, a kind of transformation of mega-proportions in the operation of the brain and nervous system from reptiles to mammals , he says. We as humans, the theory goes, are still hard-wired like our mammalian cousins in the past with the Defense system being one of the most fundamental of our systems to remain rather strongly intact because of its "proven" adaptive value to survive. That said, the beauty of that system can also be a nightmare.
Dr. Stephen Porges
I have always argued that "fear" is part of DI or Defense Intelligence (part of Emotional Intelligence) . We can utilize it and we can mature it and take more and more conscious charge of it as we evolve ourselves via new levels of consciousness re-setting the operational dynamics of DI, and allowing for more flexibility than mere "reflex"  I would also argue that this developmental evolutionary theory of consciousness (e.g., Ken Wilber's work) gives an essential nuancing to how DI and how "defenses" work--this is not something that I see Porges' neurobiolgical research includes, which is typical of most neuroscience and positivism that don't care much about consciousness (or "mind") soft-structure platforms because they are "invisible" and not so measurable easy like brain-nerve firings (e.g., the vagaus nerve complex). The exception I know to this is Bruce Lipton's work on cell biology and consciousness, including his understanding of the most basic Love vs. Fear dynamics in the ecology of cells and their relationship to the environment.
Nonetheless, as Porges points out in this brief interview, "fear" has its own operational platform of Defense, as if it has its own rapid-fire "mind" (i.e., brain-reflex wiring that is very ancient and powerful and still active in the neuroceptive regions of pre-consciousness autonomic activity, which are really unconscious). He says, this is essential because such defense decisions (instincts) are needed sometimes in an instant when in a particular context of some severe life-threat or potential life-threat. There's no time to think about it, is the argument, and that's why these rapid-fire decisions systems are biologically-based to hi-jack the higher thought platforms of the mid-brain and cerebrum. You may note, that lots of research on the brain's amygdala surfaced in the 1990s and onward telling us of this same basic notion about fear and our defense systems... but, Porges has merely fine-tuned this and looked more at the action of the autonomic system overall and the vagus nerve network in particular.
What the amygdala research emphasized is more the "fight-flight" responsivity of the defense systems we inherited. Where Porges' work focuses on the polyvagal network and its function--quite complex, and I won't do it justice here--but that it primarily moves bi-modally as meta-motivational patterns between "protecting" and "connecting" in the reptilian-mammalian architecture of the body/brain system .
Porges and his followers are interested in the notion of "safety" first  for self/system regulation (and rehabilitation and growth), as far as I can tell. And that is the role of safety from a neurobiollogical (vagal) point of view. They assert that the defense system is hard-wired for not just fight-flight but also "shut down" (or paralysis) process under severe threat--that is, under severe trauma. They call this fear as originary and thus natural and valuable. This shut-down may take place as a kind of "trance" and/or "dissociation" or "fainting." It is very powerful to protect the organism. It is sometimes the "chosen" 3rd of the fear responses mentioned by various authors talking about fear management: in hierarchial order of activation frequency is something like this: (1) flight, (2) fight and (3) freeze (i.e., "shut down" or "shock"). Porges notes that fear-immobilization has been largely ignored traditionally in psychological fear theories and especially ignored in medicine and psychology and therapy theories . He is bringing this out to the forefront, and using his polyvagal theory to help understand more of human's behaviors when distressed, when triggered by painful memories, etc. The point being, is to see that "shut down" which may not look very brave or courageous socially, is incredibly "intelligent," natural and unconscious, and not a typical "rational choice"--but it can be the very best strategy in some situations of extreme violence or damage potential of such. So, the point is we ought not be down on this 3rd option and we ought to respect the natural archaic body and defense systems to enact in our best interest.
However, like all the 3 options of fear-responses, so-called, they can be healthy responses and they can be "dissociated" or "inaccurate" and on "over-drive" creating more problems than one already has. In other words, the fear management/response of any of these 3 responses can be "irrational" and "pathological"--leading to breakdowns of the intelligence of the Defense system. We need to learn about these systems in a good "fear education" (as I call it) so we can work with them and overcome their down-sides and potential pathological enactments. I won't go into polyvagal theory (i.e., bi-modal meta-motivation theories) beyond this brief introduction. Unless people want to explore this on the FM ning. FYI, you may or may not recall that in my work, I have added 2 more fear-response options beyond Porges  and his followers: that is, (4) tend-and-befriend, and (5) fearlessness.
1. Apparently the first article that had impact on the greater scientific community by Porges (a psychiatrist and researcher) came in 1994. I find that an interesting date historically in the research on "fear." It was around this same time that a great wave of interest came from MRI studies of the brain and studies of the neurobiology of fear, pinning down the brain centre called the amygdala as the place of storage and processing of "fear" from ancient times to the present. The mid-1990s was also the time when the first critics of the "culture of fear" were starting to get published and listened to in the social sciences (albeit, they began naming this phenomena a decade earlier to explain cultural, historical and political dynamics of the major role that "fear" plays, beyond individual psychology).
2. This is not new as an insight nor a neurobiological fact, and it has physio-behavioral impacts of great significance in understanding humnan behavior in the past and today. The most explicit evolutionary theory of this deep structures of human motivation patterns came out of the research and theorizing of the "triune brain" concept of at least going back to Paul McLean's work, a very controversial theory that we are made up of an R-complex (Reptilian), M-complex (Mammalian) and H-complex (Human)--and, all three have their roles, and their attempts to dominate the behavior of an organism or system depending on the context. I found this in the late 1970s and was taken by what seemed a very logical brain/physiological (dynamic) explanation of behaviors and values sytems. Polyvagal theory is more or less interested in the same issues, the same hierarchical organization of these three brain platforms, albeit, Porges has his own way of naming and casting them.
3. I am interested in a theory (Howard Gardner's) of multiple intelligences that ought to be part of our general awareness and certainly part of how we design systems that utilize the best of human potential along these lines of mulitple intelligences, e.g., cognitive, affective, relational, musical, existential, etc. The most popularized and "successfully" applied version of this has been the great attention to Daniel Goleman's work on emotional intelligence, which also bloomed in the mid-1990s (see note 1.) Again, there are critics of Goleman's theory and applications, and I myself have critiqued it several times over the decades. Mostly, there is not room in his theory for the deep structures of the evolutionary brain, as in McLean's or Porges' work, and thus, it leaves out the deep motivational phenomenology of Defense Intelligence as a core essential intelligence.
4. My own theorizing on this for decades has been that the developmental levels (and fear management defense systems) increasingly via consciousness evolution are less and less fear-based themselves. Thus, fearlessness as a path is always a path of continual growth (under good conditions) of connectivity and more reflexive thus, less reactive defensivity re: mistrust or protectionism.
5. There is a large universal body of research and phenomenological experience throughout recorded history that notes this bi-modality of human motivation (and a good deal of animal motivation, or even plants). Re: autonomic nervous system, Porges' polyvagal theory brings out the bimodality of parasympathetic nervous system vs. sympathetic nervous system. As well, the simplest (non-technical) expression of it is the only "two emotions" theory--what I have called meta-motivations of Love and Fear. The equivalency is that our basic "drives" as organisms move along a spectrum of Love at one end pulling us to connection, and Fear at the other end pulling us to protection (cf. Porges' theory). Now, with any extra investigation, I am now thinking of all the other theories that are ought there, prior to Porges', like John Bolby on attachment theory (based on two bimodal orientations of the baby: fear of abandonment and fear of enmeshment, of which the first is the pole of protection or defense and its problems, and the second is the pole of connection and its problems). There is also the bimodal theories of "Fear vs. Freedom" as a synthesis of many theories which I have seen brought forward by the Terror Management Theory folks, and, of course the earliest theory along this line (other than the ancient Love vs. Fear philosophies and theologies) is Erik Erikson's first stage of affective development which he named "Trust vs. Mistrust" --and, so there is a great interet in this bi-modal theorizing to explain human behavior, and Porges' theory is just the latest, with more neurobiological evidence that basically "proves" what was already known to be tested and true in human experience by the other critical observers. I'd have to think more about how Indigenous worldview (and "theories") of human motivation may also contribute here. Each of these theories has partial truths to add to the puzzle.
5. My own 'red-flags' go up when I listen to Porges and his followers and their excitement about this theory of "safety" first rhetoric and discourse--which can slide into an ideological protectionism in the name of "connectivity." A tricky thing to tease apart, and we have to use fearanalysis to sort it out re: what is fear-based motivation and what is not. To what degree is fearlessness operating? To what degree is a fear-based structuration operating in even the most positive and benign looking campaigns (even the current 'wellness' craze)? Educators and psychologists are now saying without this polyvagal "calm" (motivation for connectivity overcoming motivation for protection) there is no way children or adults can learn through teaching-connections, or no way compassion and empathy can occur. This is a highly problematic, oversimplified, discourse today, with political ideologies to examine in that such cannot be left only in the analysis of polyvagal neurobiology, no matter how much partial truths the scientists may have at their finger tips in arguing for "safety" first as foundational to basic moral-compassionate connectiing--i.e, what makes us mammalian (sometimes linked to a "feminine" perspective and/or the oxytocin of mothering and bonding processes). I won't go into my critique further here, as it is a long one, suffice it to say it involves how the State (elite) powers have mis-appropriated this "safety" (connectivity) first evolutionary and neurobiological predisposition into various uses to control people/organizations and also to construct "enemies" to be destroyed who may threaten that "safety" first need or motivation. Terror Management Theory (social psychology empirical research) has offered some of the very best analysis on this and how humans behave in inclusive-exclusive dynamics re: safety needs hooked to self-esteem needs at individual and collective platforms. Fear of course (i.e., extreme cousin in the form of terror) cannot be left out of the equation of understanding major ways humans behave.
6. My own early study of ethology (animal behavior in the wilds) gave a lot of concerted attention to the "play dead" shut-down process of the nervous system and as a general behavioral strategy in many species. So, I would say the human psychologists and psychiatrists are much slower on picking up on this in general.
7. Which is not to say that Porges has not taken this into account in terms of co-evolutionary face-to-face interactions, and a notion of "care" that brings about polyvagal homeostasis in mammals, especially humans. The "tend-and-befriend" is a feminist theory of fear-response by Taylor et al. not based on polyvagal theory as far as I can tell.