Breath of Life: The Respiratory Vagal Stimulation Model of Contemplative Activity

Abstract

Contemplative practices, such as meditation and yoga, are increasingly popular among the general public and as topics of research. Beneficial effects associated with these practices have been found on physical health, mental health and cognitive performance. However, studies and theories that clarify the underlying mechanisms are lacking or scarce. This theoretical review aims to address and compensate this scarcity. We will show that various contemplative activities have in common that breathing is regulated or attentively guided. This respiratory discipline in turn could parsimoniously explain the physical and mental benefits of contemplative activities through changes in autonomic balance. We propose a neurophysiological model that explains how these specific respiration styles could operate, by phasically and tonically stimulating the vagal nerve: respiratory vagal nerve stimulation (rVNS). The vagal nerve, as a proponent of the parasympathetic nervous system (PNS), is the prime candidate in explaining the effects of contemplative practices on health, mental health and cognition. We will discuss implications and limitations of our model.

Keywords: cognition; heart rate variability; meditation; mind-body exercises; mindfulness; respiration; stress; vagus nerve.

Figure 1

Cumulative number of scientific publications on contemplative activity (ContAct), from the 1945–1996 bracket to 2017 per individual year. Obtained from Web of Science in January 2018 using the search terms: “mindfulness” OR “meditation” OR “yoga” OR “tai chi.”

Figure 2

Panel (A) represents an overview of the respiratory vagal nerve stimulation (rVNS) model of ContAct. See the text body for more details. There are two pathways through which respiration style stimulates VN: direct and indirect (biofeedback through afferent projections), shown in (B,C) respectively. (D) The tonic changes in the networks and the long-term effects. Color coding: red = VN, blue = other anatomy, purple = physiology, green = function, dark blue = direct route, orange = indirect route. Arrow-ends represent role: triangle = activating or increasing, circle = deactivating or decreasing, bladed = structural increase, ellipsis = structural decrease, diamond = afferent. Numbers on lines represent the temporal sequence during stimulation and the thickness of lines the phasic relative synaptic weight of the connection as a result. The dashed line represents the hypothetical afferent pathway of the thoracic/abdominal ratio to VN. RR, respiration rate; I/E, inhalation/exhalation ratio; T/A, thoracic/abdominal respiration ratio; HR, heart rate; HRV, heart rate variability; INF, inflammation state; SNS, sympathetic nervous system; HPA, hypothalamic pituitary adrenal axis; CAN, central autonomous network; DMN, default mode network.