A simple analogy for the nervous system is a ladder. When neuroception perceives safety, the person is at the top of the ladder, corresponding with the prefrontal cortex and ventral vagal activation. As allostatic load builds, activation moves down the ladder, first to the flight rung, then to the fight rung, and finally to the freeze rung. As danger subsides, they move back up the ladder (Dana & Porges, 2018).
Distress is processed on the high road if not associated with an immediate danger through neuroception. While a gradual increase in stress can activate flight, fight, and freeze responses, it is a slower process as the stress builds up to allostatic overload. The high road provides opportunities for the prefrontal cortex to bring the growing distress into consciousness and apply coping skills to maintain executive and ventral vagal activation.
When neuroception detects extreme and immediate danger, the high road shuts down instantaneously, and a much shorter road activates to increase the chance for survival. While the amygdala sits toward the end of the high road, it plays a leading role on the low road. Instead of gradually releasing stress hormones, the amygdala immediately drops the person out of executive functioning and the vagal brake releases fully. It moves down the ladder to the sympathetic nervous system. The sympathetic nervous system floods the body with cortisol and adrenaline, activating the flight and, if needed, fight response (Goleman, 2006; Ogden et al., 2006; Siegel, 2011).
In most situations, sympathetic activation will continue until the threat has passed and can lead to unusual strength and stamina levels. Sympathetic reactions lack executive functioning, as systems such as the prefrontal cortex cease to receive energy, thus maximizing the energy sent to the muscles and senses (Siegel, 2011).
If the flight-or-fight responses fail, the last option is to shut down. The dorsal vagus slows breathing and heart rate, and HRV decreases. During an intense shutdown or freeze response, the person can dissociate or lose connection to reality. This response limits the physical or psychological pain felt during traumatic events but makes recalling or making sense of the experience afterward tricky or impossible.
The third survival response, shutting down, might become the default response of people who feel trapped and struggle to escape the danger. They have little physical, social, or economic opportunity or power to flee or fight back in these situations. The only way they endure is by shutting down, and in extreme cases, dissociating from the situation to survive physically and psychologically (Ogden et al., 2006).
Let’s examine one last part of the brain that is essential for both high and low roads. On the low road, the hippocampus takes on a calming role. Once neuroception senses that the threat no longer exists, the hippocampus helps quiet the amygdala, allowing the high road to reactivate, and cognitive and social functioning return. A well-functioning hippocampus makes it possible to have a brief low-short road response, including fear or anger, but it does not allow these emotions to continue well into the future. As part of the high road, the hippocampus works with the amygdala to provide an emotional context to the stimulus. Also, the hippocampus plays a significant role in creating new memories.
The nervous system and the mind it supports are complex. HRV measures all this complexity and summarizes it in a simple score. A high HRV score means people can handle stress well and are ready to bring their best emotional and cognitive selves to work. A low score means they perform suboptimally within relationships and intellectual challenges.
Using HRV to measure improvements in nervous system functioning will continue as a theme of this book. Hopefully, this brief introduction to the brain and nervous system gives you an understanding of the science behind HRV. In the next chapter, we will explore how to interpret HRV scores and what they tell us about the states and traits of those we are trying to help.