Second, the vagus nerve is the primary brake on our heart rate. Without activation of the vagus nerve, your heart would fire on average at about 115 beats per minute, instead of the more typical 72 beats per minute. The vagus nerve helps slow the heart rate down. When we are angry or fearful, our heart races, literally jumping five to ten beats per minute, distributing blood to various muscle groups, preparing the body for fight or flight. The vagus nerve does the opposite, reducing our heart rate to a more peaceful pace, enhancing the likelihood of gentle contact in close proximity with others.
Third, the vagus nerve is directly connected to rich networks of oxytocin receptors, those neuropeptides intimately involved in the experience of trust and love. As the vagus nerve fires, stimulating affiliative vocalizations and calmer cardiovascular physiology, presumably it triggers the release of oxytocin, sending signals of warmth, trust, and devotion throughout the brain and body and, ultimately, to other people.
Finally, the vagus nerve is unique to mammals. Reptilian autonomic nervous systems share the oldest portion of the vagus nerve with us, what is known as the dorsal vagal complex, responsible for immobilization behavior: for example, the shock response when physically traumatized; more speculatively, shame-related behavior when socially humiliated. Reptiles’ autonomic nervous systems also include the sympathetic region of the autonomic nervous system involved in fight/flight behavior. But as caretaking began to define a new class of species—mammals—a region of the nervous system, the vagus nerve, emerged evolutionarily to help support this new category of behavior.
Historians of science have rated Charles Darwin as off-the-charts in terms of kindness and warmth relative to other groundbreaking scientists (he was the only passenger on the Beagle about whom not a negative word was said, and was friend to captain and ship hands alike). As Darwin wrote in his Down House, amid the noisy, loving spectacle of his ten children, he most certainly felt those sensations of expansive warmth in the chest associated with the vagus nerve. The humming of the vagus nerve may have led Darwin to his often neglected thesis that sympathy and the maternal instincts are the centerpiece of human social evolution, that they bring the good in others to completion and are a foundation of high jen ratios. Some hundred and thirty years later a new science has yielded similar insights.
NERVES OF COMPASSION
Steve Porges’s wild-eyed claims about the vagus nerve would have inspired William James. James was the progenitor of the notion that our emotions originate in patterned responses in the autonomic nervous system, which lies below the brain stem and coordinates basic tasks like the distribution of blood, digestion, sexual response, and breathing. What could be more compelling proof that our emotions are embodied in peripheral physiological response, “reverberations of the viscera,” in James’s Victorian language, than the notion that that loftiest of human emotions—compassion—has its own bundle of nerves located deep within the chest?
Walter Cannon, a student of William James’s, was not so convinced by his advisor’s provocative armchair musings. The responses of the autonomic nervous system, Cannon countered, do not carry enough specific meaning to account for the many distinctions people make in their emotional experience. Patterned changes in heart rate, breathing, goose bumps, pupil dilation, cotton mouth, and sweaty palms could never give rise to nuances in experiences of gratitude, reverence, compassion, pity, love, devotion, desire, and pride.
On top of that, Cannon continued, the autonomic responses of emotion are simply too slow to account for the rapidity with which we experience emotion or move from one emotion to another. The autonomic nervous system typically produces measurable responses within fifteen to thirty seconds after the emotion-eliciting event. Clearly our emotional experience arises more rapidly. The blush, for example, peaks at about fifteen seconds after the embarrassing event; our experience of embarrassment, in contrast, arises immediately upon the recognition of the mistake we have made. In Cannon’s eyes, the autonomic nervous system is too slow-moving a system to account for the meteoric emergence and nimble shifting of our emotional experiences.
Finally, and perhaps most persuasively, we are relatively insensitive to the changes in the autonomic nervous system—heart rate increases, sweaty palms, vasoconstriction in the veins of your arms or legs, blushing, or activity in your intestines. Cannon noted, for example, that people actually feel little when their intestines are cut or burned. Slightly less dramatic empirical studies have found that when people are asked to guess whether their heart rate has increased or decreased, they most typically fare little better than chance. Even if the autonomic nervous system generated emotion-specific responses, it is not clear that we would perceive these bodily changes with our conscious minds. It is even less clear to assume that these dimly perceived bodily sensations would weave their way into our emotional experience. It would be foolhardy, by implication, to seek to locate all the nuances of compassion—the sense of undeserved harm, the feeling of concern and common humanity, the urge to help—in something so diffusely distributed in the peripheral nervous system as the vagus nerve.
Undaunted, my student Chris Oveis has risked his career on the very hypothesis that the vagus nerve is a bundle of caretaking nerves. He did so by starting in an obvious place—suffering. Humans are wired to respond to harm from the first moments of life. One-day-old infants cry in response to another infant’s cries of distress but not their own. Many two-year-old children, upon seeing another cry, will engage in the purest forms of comfort, offering their toys and gestures of visible concern to the person suffering. Pictures of sad faces presented so fast participants don’t even know what they’ve seen trigger activation in the amygdala.
So we asked first whether the exposure to harm would trigger activation in the vagus nerve, and whether an emotion that revolves around the inclination to distance oneself from weak others—pride—would not. In the compassion condition, participants viewed images of malnourished children, suffering during wartime, and infants in distress, images that fit the Aristotelian notion of the purest elicitors of compassion: another’s suffering that is extreme and undeserved. Participants in the pride condition viewed slides that would arouse the pride of our UC Berkeley undergraduates—images of landmarks on the campus, pictures of Cal sporting events, and, perhaps the most inspiring of all, a picture of the Cal mascot, Oski the bear.
As participants viewed 2.5 minutes of slides, we measured activity of the vagus nerve with electrodes attached to the chest and a band placed around the abdomen to measure breathing. These measures yield an index called respiratory sinus arrhythmia (RSA), which has been developed in the past fifteen years to capture activation in the vagus nerve. RSA works as follows. When we inhale, the vagus nerve is inhibited, and heart rate speeds up. When we exhale, the vagus nerve is activated, and heart rate slows down (which is why so many breathing practices prioritize exhaling and are soothing to the soul—and a source, perhaps, of compassion). The vagus nerve controls how breathing influences fluctuations in heart rate. We measure the strength of the vagus nerve response, therefore, by capturing how heart rate variability is linked to cyclical changes in respiration.
The first finding of importance from Chris’s study was that brief exposure to images of harm triggered activation of the vagus nerve more so than the images that made participants proud. Perhaps more convincingly, participants’ experiences of compassion and pride were, as James would have hypothesized, quite sensitive to fluctuations in the activity of the vagus nerve. Participants’ reports of their feelings of compassion increased as their vagus nerve activity increased; participants’ self-reports of pride decreased as their vagus nerve activity increased. With increasing vagus nerve response, participants’ orientation shifted toward one of care rather than attention to what is strong about the self.