How is it that we perceive a threat and shift our attention to deal with it? Computers have been helpful in providing a metaphor for how our brain and nervous system work. This is consistent with how neuroscience conceptualizes the mind. For instance, Daniel Siegel, MD, author of The Developing Mind and The Mindful Brain, defines the mind as “an embodied and relational regulatory process.”
However, there is a major difference with the computer: We are not neutral processors of information. Evolutionarily, the processing came as an ever-increasing fine-tuning of our ability to respond to a threat. This is very pragmatic, action-oriented. We did not evolve to process scholarly reports assessing the environment. We evolved to take action instantly. And emotions are a big driver.
So the processing of threat is mediated by the nervous system, especially the autonomic nervous system.

When you took Science in school, the Autonomic Nervous System was probably described as having two branches: The Sympathetic nervous system, which gives us the energy for fight/flight. And the Parasympathetic system, mediated by the vagus nerve, which helps us slow down, and helps digestion and rest.
In classical theory, these were the two forces balancing each other. It was assumed that the “window of mindfulness” was when the two were essentially balancing each other out.
More recently, neuroscientist Stephen Porges developed what he called the Polyvagal Theory, which identified a third circuit. The vagus nerve is divided into two parts, the ventral and dorsal:
– The dorsal part is the more primitive part of the nerve. It essentially does what, in the old model, the Parasympathetic was described as doing.
– The ventral part is associated with mindful engagement.
In other words: There is a specific circuit corresponding to mindful engagement.
The following charts provide a visual model for Relational Mindfulness and the Autonomic Nervous System (ANS).

Diagram of the states of the Autonomous Nervous System (download as PDF)
For more details, see:
– Polyvagal Theory 5-minute video
– Window of Tolerance & Polyvagal Theory Diagram
– Conversation with Stephen Porges about the Polyvagal Theory