Hypothesis: Pulmonary Afferent Activity Patterns During Slow, Deep Breathing Contribute to the Neural Induction of Physiological Relaxation

1Respiration is a regulable variable under voluntary control with access to central autonomic circuits linked to the modulation of cognition and behavior. Relaxation techniques involving slow, deep breathing have shown promise in a variety of cardiorespiratory and stress-related disorders including chronic heart failure, hypertension, anxiety, and depression. Despite an abundance of mind-body practices that involve slow, deep respiration as a fundamental component, our knowledge of the optimal parameters and underlying mechanisms of deep breathing for therapeutic benefit remains incomplete. Our own work and several recent reviews have sought to synthesize research on the psychophysiology of slow breathing, highlighting the impressive degree of cardiorespiratory synchrony resulting from these practices. Here we explore how slow breathing may modify cognition by promoting relaxation (reduced stress reactivity) and alert functional states (enhanced attention and memory consolidation). Expanding on the recent identification of brainstem neurons linked to reductions in behavioral arousal, we emphasize respiratory-based changes in afferent encoding that include recruitment of pulmonary slowly-adapting mechanoreceptors and their putative interactions with central autonomic circuitry. We also describe olfactory-hippocampal pathways that may mediate the selective effects of slow nasal breathing on memory function.

2Our neurophysiological model for the mechanisms mediating effects of slow, deep breathing proposes a central role for vagal myelinated lung afferent pathways through brainstem relay nuclei. It is important to keep in mind that these pathways do not function in isolation but instead are integrated within a larger network of sensory and motor circuits that contribute to the overall cognitive experience of slow breathing, including descending motor drive from higher-level brain centers (e.g., ). Voluntary slow breathing may engage respiratory motor networks, since an individual can sense the motor effort required to inhale larger volumes. However, there is also evidence that voluntary breathing might not be essential to realize the basic physiological benefits of these techniques; spontaneous slow breathing without cognitive motor intent may be sufficient. Furthermore, along with pulmonary afferents, sensory elements mediating effects of slow, deep breathing likely encompass upper airway afferents including those in the nose, pharynx and larynx. Since breathing practices often involve large volume inhalation through the nose with slow controlled exhalation through the mouth, sensory afferent input from these pathways may essentially guide cognitive centers during breathing exercises. Sensorimotor systems of the mouth are likely also integrated to provide an expiratory resistance controlling the rate of airflow from the lungs. Although the contributions of these different sensory systems to the cognitive effects of slow, deep breathing remain to be elucidated, here we primarily focus on contributions from vagal pulmonary afferents and olfactory afferents that have recently been implicated in the context of nasal slow breathing.

3In the broadest sense, slow, deep breaths are those occurring more slowly than the typical rate of 12–15 breaths per minute in normal adults. Several yogic techniques target the specific frequency of six breaths per minute (0.1 Hz). This frequency maximizes heart rate variability (HRV) and dramatically enhances cardiorespiratory synchronization, the phenomenon whereby the heart rate increases during inhalation and decreases during exhalation. This phenomenon is known clinically as respiratory sinus arrhythmia (RSA), and is greatly exaggerated at slower respiratory frequencies as the difference between peak and trough heart rates is maximized. Peak-to-trough differences in heart rate are the simplest measure of HRV; spectral analysis of heart rate variations based on ECG recordings is another commonly used quantitative method to calculate HRV and reflects the same trend. Reduced HRV is a clinically relevant feature of stress-related disorders including anxiety, depression, and epilepsy.

4Respiratory ventilation consists of two components: tidal volume – the volume of air displaced during each breath – and breathing frequency, or ‘respiratory rate.’ Under resting conditions, these two parameters vary inversely in order to satisfy relatively stable metabolic demands for oxygen (O2) consumption. Therefore, slower breaths are also necessarily deeper. From here on we occasionally use the terms ‘slow’ and ‘deep’ interchangeably, assuming consistent ventilation to fulfill relatively stable metabolic needs. Tidal volume and respiratory rate are known to vary with behavioral and emotional state. For example, a restful state may be characterized by high tidal volume-low respiratory rate, whereas low tidal volume-high respiratory rate is a characteristic feature of an anxious state. Similarly, respiratory rate increases during stress and is reduced during restful slow wave sleep. High basal respiratory rates also predict negative cardiopulmonary outcomes. This suggests that basal respiratory rate is an index of stress and may reflect its detrimental impact on physiology and behavior.

5Slow breathing exercises such as those in pranayama yoga improve stress-related physiology, including autonomic imbalance, cardiopulmonary and neuroendocrine function, and mood. These techniques can increase HRV by a factor of two or more and drastically reduce self-reported depression. Such physiological and behavioral benefits are believed to be associated with a shift away from sympathetic dominance toward a net increase in parasympathetic (vagal) tone, thereby reducing stress effects. The adaptive changes in autonomic function associated with sustained slow breathing are consistent with the ‘relaxation response’, a state of deep rest that changes the physical and emotional responses to stressors.

6Despite evidence of slow, deep breathing’s therapeutic benefit in disorders of autonomic imbalance, very few studies have focused on isolating slow breathing from attentional or emotional regulatory elements of training. A recent systematic review of slow breathing techniques analyzed the results of 15 studies, concluding that despite some disparities, breath control at low frequencies (<10 breaths/minute) results in decreased anxiety and arousal and increased relaxation. However, very few studies have shed light on underlying mechanisms or neural substrates. One recent neuroimaging study found that breath-by-breath increases in tidal volume enhanced activity within a dorsal medullary region encompassing the nucleus of the solitary tract (NTS). Another study has found that two distinct meditative practices induce cardiorespiratory synchronization at a respiratory frequency of 0.1 Hz. Even inexperienced Zen meditators ‘lock in’ at 0.1 Hz. Intriguingly, religious and artistic practices can also facilitate 0.1 Hz cardiorespiratory synchronization, and may have evolved to promote a feeling of wellness that individuals would associate with such practices. For example, the timing of repetition of the Ave Maria in rosary prayer and of yoga mantras cycles at ∼0.1 Hz and produces cardiorespiratory synchronization. Certain forms of rhythmic poetry recitation (e.g., hexameter verse from ancient Greek literature) similarly lead to this phase-locking, as do specific music phrases, as in Verdi’s famous arias. In sum, respiration rate and depth (tidal volume) may covertly control our state of well-being, as the few studies that have investigated the impact of specific respiratory frequencies have found an impressive synchronization of systemic function at the characteristic frequency of 0.1 Hz.

7Slow breathing exercises have been proposed as novel therapeutics in a variety of disease states, such as in epilepsy, where stress is the most common trigger of seizures and there are measurable changes in autonomic balance. Biofeedback approaches have also been devised to increase RSA, a key cardiorespiratory variable enhanced during slow breathing techniques. These approaches, aiming to mimic the heightened cardiorespiratory synchronization achieved during 0.1 Hz breathing and ranging from wearable sensors and patches to mobile phone applications, have proven effective in disorders as diverse as asthma, PTSD, and depression.

8It is important to consider slow breathing-induced changes in cardiorespiratory circuit recruitment that could trigger the relaxation response. One intriguing possibility is that a unique profile of sensory afferents activated during deep breathing (Figure 1) projects along specific pathways that optimize patterns of sympathetic and parasympathetic output (Figure 2). Cardiovascular and respiratory afferents activated by breathing are known to project to second-order relay neurons in brainstem autonomic regions, most notably the NTS via the vagus nerve. The NTS, a relay station for all vagal afferent input, integrates these signals and sends divergent synaptic connections to a variety of brain regions, especially those involved in central autonomic regulation. The central autonomic network is a system of cortical, subcortical, and brainstem structures involved in internal regulation and linked to stress-related disorders, including the amygdala, hypothalamus, NTS, and ventrolateral medulla. Candidate afferents and projection systems, along with the specific physiological events proposed to link slow breathing to a decreased stress response (relaxation) are described below.

9Sustained deep breathing repeatedly activates pulmonary stretch receptors in the lungs and bronchi. Pulmonary stretch receptors are innervated by fast-conducting myelinated afferent fibers sensitive to both the static and dynamic aspects of lung volume and transmural pressure. Normal breathing preferentially activates rapidly-adapting receptor afferents (RARs). Deep inhalations also activate slowly-adapting pulmonary receptor afferents (SARs). SARs are activated by exaggerated stretching of the lungs. SARs end inspiration and facilitate expiration as part of the Hering-Breuer reflex (HBR). However, SARs and the HBR may not play a role in setting typical eupneic breathing patterns in adults including in humans. Unusually large transpulmonary pressures, such as those achieved during deep breathing, may be required to activate SAR pathways through the NTS during adulthood. While the reflex circuit mediating the HBR to prevent lung over-inflation (via the vagus to inhibit central inspiratory drive) is fairly well characterized, our model for breathing-dependent relaxation proposes a physiological purpose for SAR pathways through the NTS with anatomical and functional connectivity to central autonomic networks. The HBR may operate during more extreme inspiratory depths to ensure their constraint within an appropriate physiological range. RARs have phasic responses to large lung inflations and deflations, whereas SARs respond to large hyperinflations (e.g., 30 cm H2O) throughout the duration of the stimulus (Figure 1). Therefore, deep breaths may preferentially engage SAR-dependent pathways by effectively recruiting both low- and high-threshold SARs that remain active throughout the extended inspiratory phase of slow breathing. In contrast, RARs would become silent during long inspirations, reducing their excitatory impact on CNS circuitry. Simultaneously, slow and deep breathing leads to cyclic variations in intrathoracic pressure, impacting venous return to the heart, cardiac output, and consequently blood pressure. These effects result in high-amplitude oscillations in arterial blood pressure known as respiratory arterial pressure waves (Figure 1). Stretch-sensitive peripheral baroreceptors located predominantly in the aortic arch and carotid sinus respond to these changes in arterial blood pressure synchronized to the cardiac and respiratory cycle, contributing to blood pressure and heart rate oscillations that have periods from seconds to minutes. All of these afferent mechanoreceptor populations send vagal sensory input to the NTS in the brainstem (Figure 2).

10Baroreceptor afferent activation typically initiates a negative feedback cycle whose final output is cardiovagal cholinergic efferents from the nucleus ambiguus innervating the sinoatrial node of the heart. In contrast, SARs act on second-order ventrolateral NTS neurons to directly or indirectly inhibit cardioinhibitory neurons of the nucleus ambiguus. Through these second-order neurons SARs inhibit parasympathetic vagal outflow to the heart, leading to an increase in heart rate. Conversely, inhibition of SARs following cessation of deep inhalation likely results in strong rebound reciprocal changes that accentuate parasympathetic effects during exhalation, an effect that is consistent with SAR excitation evoking a tachycardia that is gradually masked by a reflex bradycardia. While RAR and baroreceptor afferent projections to the NTS could impinge on multisynaptic central circuits to inhibit the sympathetic nervous system, SAR pathways would instead activate sympathetic outflow during inhalation. This contradicts the notion of slow breathing as a purely parasympathetic state and suggests the importance of autonomic patterning with the breathing cycle.

11One powerful way that deep breathing could dampen stress-related hyperactivity of the central autonomic circuitry is through vagal modulation. Increased vagal tone is necessary for cardiovascular recovery from psychological stress, while decreased vagal reflexes have been associated with increased risk of cardiovascular disease and death after myocardial infarction. Activating cardiorespiratory vagal afferents – including SARs and baroreceptors – could inhibit CNS activity through at least two distinct mechanisms:

12SARs synapse with GABAergic interneurons that postsynaptically inhibit third-order NTS neurons projecting to the amygdala, hypothalamus, and other downstream targets (Figure 2);

13Cardiorespiratory vagal afferents could presynaptically inhibit local excitatory input to the NTS via primary afferent depolarization, initiating widespread central autonomic circuitry inhibition.

14Presynaptic inhibition is “more powerful than postsynaptic inhibition in depressing the central excitatory actions of almost all primary afferent fibers”. Despite its important role in the CNS, primary afferent presynaptic inhibition has rarely been studied in cardiorespiratory vagal afferents. All low-threshold cardiorespiratory afferents from this study produced presynaptic inhibition of respiratory afferents, with the magnitude of inhibition increasing with increasing intratracheal pressure. Because both low- and high-threshold SARs respond preferentially to the inspiratory phase of deep breaths, while peripheral baroreceptors respond to the expiratory phase, presynaptic inhibition may be a physiologically effective means of alternating between sympathetic and parasympathetic afferent drive throughout the ventilatory cycle. Supporting this proposal, afferent stimulation evoked a decrease in blood pressure that was closely correlated with the magnitude of presynaptic inhibition.

15Arterial blood pressure varies during respiratory cycles as a consequence of changing intrathoracic pressures (; Figure 1). Baroreceptor afferents detect elevated blood pressure during exhalation and, through direct projections to several subregions of the NTS known to innervate cardioinhibitory regions of the nucleus ambiguus, stimulate parasympathetic outflow to the heart to lower blood pressure; reduced baroreceptor activation during inhalation exerts opposite effects. Baroreceptor input has a strong and powerful effect on HRV with each cycle of the breath via these effects on cardiovagal parasympathetic output. Therefore, at lower respiratory frequencies and for deeper breaths, changes in blood pressure will greatly accentuate changes in heart rate, leading to a wide range of observed heart rate values during respiration and consequently amplifying HRV. The arterial baroreflex exhibits positive feedback properties at a frequency of 0.1 Hz (0.4 Hz in rats and mice), resulting in self-sustained oscillations in arterial blood pressure called ‘Mayer waves’, and leading to high-amplitude, vagally mediated heart rate oscillations at 0.1 Hz. As elaborated below, voluntarily breathing at a frequency around 0.1 Hz (six breaths per minute) could entrain blood pressure and heart rate to this same resonant frequency. Both the frequency of Mayer waves and the respiratory rate producing maximal HRV are surprisingly invariable within a given species.

16All systems have at least one preferred frequency in which oscillations occur at a greater amplitude. These are their resonant frequencies, and independent systems with common resonant frequencies can be mutually reinforcing when coupled. Biologically speaking, resonance is the most energetically and thus metabolically efficient way to increase signal power. In this manner, correlated oscillations of cardiac and respiratory rhythms at one frequency (referred to as their coherence) can lead to mutually reinforcing signal amplification. While the origin of Mayer waves remains incompletely understood, baroreceptor resonance patterns are known to contribute. Studies using baroreceptor denervation or blockage with alpha-adrenergic agents implicate peripheral baroreceptors in the corresponding 0.1 Hz rhythm. It is possible that an intrinsic central oscillator in the caudal medulla closely interacts with baroreceptor and pulmonary input in the NTS to change HRV and alter autonomic activity. This leads to the idea that breathing at six breaths per minute may maximize HRV by promoting baroreceptor resonance effects to enhance inherent physiological rhythms. The end result would be increased cardiorespiratory synchronization and, via baroreceptor cardiovagal signaling pathways, shifted autonomic balance in the parasympathetic direction.

17Several distinct meditation and yoga techniques profoundly enhance cardiorespiratory synchronization around the characteristic frequency of 0.1 Hz. This entrainment of cardiac variability to the respiratory rhythm – so that heart rate increases during inhalation and decreases during exhalation – is referred to clinically as RSA, and is greatly exaggerated at slower respiratory frequencies, such that incremental reductions in respiratory rate result in non-linear increases in RSA. This suggests that a lower respiratory rate will lead to a corresponding increase in RSA, regardless of whether it is possible to reach the frequencies corresponding to peak RSA.

18Central drive from the respiratory central pattern generator (CPG) exerts an important influence on the heart and thus RSA, and recent evidence suggests that the core respiratory CPG is embedded within an anatomically distributed pattern-generating network including the NTS. Pulmonary afferent pathways through the NTS are likely one of several important factors contributing to the increase in cardiac oscillation amplitude (visualized by RSA) at the breathing frequency of 0.1 Hz, although their role has been assigned varying degrees of importance ranging from minor to obligatory. The primary factor appears to be the synchronization of respiratory and cardiac oscillations without any delay (unlike during normal breathing), and out of phase by 180° with blood pressure, allowing the baroreflex to be ideally activated. The presence of Mayer waves at 0.1 Hz also reflects baroreceptor resonance effects at this same frequency that contribute to the increase in RSA amplitude. Thus, the powerful modulation of RSA observed during slow and deep breathing is a complex and multifaceted phenomenon that remains only partially understood.

19RSA appears to be a reliable physiological index of respiratory rate’s impact on autonomic function. It is decreased in individuals with depression, anxiety, and panic disorder, and enhanced in physically active individuals. Several therapeutically relevant meditation techniques that involve slowed breathing also enhance RSA, and increases are associated with improved parasympathetic function and wellness. In analogy with repeated exercise promoting physical fitness, voluntary and repeated lowering of respiratory rate may lead to autonomic fitness and promote well-being.

20Cardiorespiratory synchronization around 0.1 Hz may enhance ventilation-perfusion matching. During inhalation, expansion of the chest cavity and the resulting intrathoracic negative pressure increase leads to expansion of blood vessel diameter, effectively increasing perfusion of freshly inhaled O2 to vital tissues. Conversely, exhalation corresponds to the ebb in perfusion. Studies have confirmed that pulmonary gas exchange is improved with enhanced RSA, supporting an active physiological role. The net result is decreased energy expenditure (eliminating unnecessary heartbeats during exhalation) and increased metabolic efficiency. This is in line with a study of the relaxation response that found associated changes in systemic gene expression, particularly in markers of primary metabolism and cellular stress responses.

21In the previous section, we described afferent pathways and physiological processes recruited by slow breathing. Here, we develop a neurophysiological model (Figure 2) that relates afferent systems to brainstem nuclei and central autonomic networks to explain how a state of calm alertness may ensue.

22As the major relay station for vagal afferents arriving from the periphery, the NTS forms an early part of the central autonomic network. Deep breathing-induced pulmonary and baroreceptor afferent activity patterns may lead to the recruitment of specific classes of NTS, hypothalamic and amygdalar neurons identifiable by their anatomical topography (see Figure 2). The overall profile of recruitment could represent a “neural signature” for emotional engagement and wellness. Specifically, NTS neurons appear to engage brain circuitry involved in emotion and internal regulation, with particularly dense projections to subcortical regions including the central nucleus of the amygdala (CeA) and paraventricular nucleus of the hypothalamus (PVN). In animal studies, acute social stress increases Fos expression in the CeA and PVN, while depressing HRV and increasing circulating catecholamines. Given the CeA’s key role in activating sympathetic autonomic output pathways and centrally coordinating the fear network, deep breathing may exert its anxiolytic effects by opposing stress-induced recruitment of the CeA. Reduced firing in amygdala neurons could contribute to physiological calming. Furthermore, in cats with experimentally elevated blood pressures and respiratory rates, inactivation of the CeA lowers both values. This supports a bidirectional relationship between the amygdala and NTS, with amygdala-associated emotional state driving increases in respiratory rate and respiratory rate in turn driving amygdala activity via NTS projections.

23Emerging evidence suggests that slow breathing may engage distinct neuronal networks from those employed during normal breathing. A collection of neurons in the brainstem known as the pre-Bötzinger complex (pre-BötC) plays an important role in respiratory rhythm generation. Recent studies in the mouse suggest that removal of a small, molecularly identifiable, neuronal subpopulation within the pre-BötC (Cdh9/Dbx1 pre-BötC neurons) reduces arousal and leads to a calm behavioral phenotype characterized by increased bouts of stillness and alterations in grooming and sniffing, despite leaving normal breathing largely unaltered. The excitatory neurons studied were found to directly project to noradrenergic neurons of the locus coeruleus (LC), a brainstem region implicated in arousal, attention, and responsivity to stress. Through its extensive efferent network, the LC potently regulates systemic function and behavioral state. Therefore, this pathway may represent an important circuit regulating the balance between calm and arousal behaviors. Despite the clear evolutionary importance of preparing an organism to deal with conditions evoking rapid or impaired breathing, over-activation of this pathway could be detrimental. Via mechanisms described below, slow breathing could optimize circuit function and reduce behavioral arousal. The hypothesis was recently put forth that respiration and attention comprise a coupled system via the LC, suggesting a dual role for the LC in slow breathing-induced reductions in arousal and enhancement of flexible attentional states.

24A recent paper demonstrated that activation of a functional subpopulation of noradrenergic neurons (Hoxb1-NE) prevalent within the A2 cell group – centered within intermediate and caudal levels of the NTS – attenuated behavioral responses to acute stress. Using neuroimaging, the authors found that Hoxb1-NE neuron activation reduced neuronal activity in several stress-related brain regions including the amygdala and LC contrary to the generally recognized role of norepinephrine (NE) in stress circuit activation and well-known arousing effects of widespread NE release from the LC. These Hoxb1-NE neurons provide substantial input to the LC and peri-LC dendritic field, suggesting they may directly modulate LC activity. Furthermore, medullary Hoxb1-NE neurons project to the CeA. This suggests that SAR, RAR, and baroreceptor afferent pathways – via a common relay through the NTS subregions housing Hoxb1-NE neurons – may regulate a shared NTS outflow, providing inhibitory noradrenergic control of the central autonomic network (Figure 2). As the CeA is the primary output or effector region of the amygdala and regulates the release of cortisol through the PVN, and the noradrenergic projections of the LC swarm the entire forebrain, coincident modulation of both pathways may powerfully influence behavioral state. During slow breaths, the balance of these effects could lead to a state of alert relaxation.

25Pulmonary afferent systems also project via divergent NTS pathways to central respiratory neurons including those in the pre-BötC and Bötzinger (BötC) regions. Correspondingly, the inherent rhythmicity of pre-BötC circuits normally requires dynamic adjustment through network interactions, and these excitatory and inhibitory circuits are controlled by convergent inputs, including from the NTS. The combined effects of a tonic excitatory drive to the NTS and activation of RARs during normal breathing could promote basal states of arousal through Cdh9/Dbx1 pre-BötC-LC arousal pathways. Conversely, via rhythmic activation of SARs, slow and deep breaths could recruit inhibitory NTS circuitry to restrain activation of pre-BötC-LC arousal pathways. However, whether pulmonary afferent and Cdh9/Dbx1 neuronal circuits are integrated at the level of pre-BötC or represent two alternative pathways (e.g., sensory reflex-dependent vs. central oscillator-derived) for reducing behavioral arousal remains to be determined. In comparison, it is well-established that SARs monosynaptically activate pump (P-) cells that project to medial and commissural NTS subnuclei to inhibit RAR-cells. These same medial and commissural NTS subregions provide the majority of its noradrenergic input to the CeA and PVN. Therefore, SAR pulmonary afferent input through the NTS, potentially converging with central respiratory networks, could effectively control activation of subsequent circuits such that these pathways are inhibited during inspiration and disinhibited during expiration. Following SAR-mediated inhibition of medial NTS RAR-cells during deep inhalation, reciprocal inhibition and afterhyperpolarization of SARs during prolonged expiration could result in a profound disinhibitory state that recruits efferent NTS circuitry to dampen forebrain activity.

26In the Section “Relating the Basic Neurophysiology of Slow, Deep Breathing to Modulation of Known Stress Pathways” we explored the impact of slow breathing on central autonomic regulation and developed a model to account for its effects on stress circuitry. In this section we address whether slow breathing changes higher-order brain activity. Proposed mechanisms include:

27Nasopharyngeal breathing activates olfactory afferents that recruit specific hippocampal rhythms in circuitry involved in memory processes.

28Cardiorespiratory afferent pathways project via the NTS to downstream brain regions including the LC, amygdala and hypothalamus (see previous section; Figure 2).

29Neural activity synchronizes to the respiratory cycle, entraining slow brain rhythms around 0.1 Hz throughout stress- and memory-related cortical circuitry (Figure 3).

30Slow brain rhythms resonate with the default mode network to promote altered states of consciousness associated with meditation.

31Slow breathing drives mechanical, pulsatile changes in blood and cerebrospinal fluid (CSF) flow that non-specifically activate mechanosensitive afferent pathways to evoke widespread, global brain synchrony (i.e., broad recruitment of neural circuits).

32We provide further detail elaborating on these possibilities below.

33Slow nasal respiration and slow cortical rhythms can also enhance memory, and these effects are thought to be mediated via olfactory bulb-driven neural oscillations that travel to the piriform cortex and onward to the hippocampus. Indeed, nasal respiration entrains hippocampal oscillatory activity and enhances memory performance in humans, and the modulatory effects of hippocampal rhythms may coordinate with thalamic and cortical activity. Respiration could thereby initiate neural activity patterns that reverberate to the whole brain, seen as respiratory rhythmic slow waves. It remains unknown whether the widespread prevalence of these slow waves reflects conventional network transmission or is due to volume transmission; the latter effect may implicate nitric oxide signaling. Recent studies in rodents and humans have directly linked slow nasal respiration to slow brain rhythms. In rodents these are dissociable from other low-frequency rhythms, and modulate local gamma activity in the medial prefrontal cortex, organizing prefrontal network activity.

34We have outlined basic physiological changes linking 0.1 Hz breathing to reduced stress circuit activation, examined how slow nasal breathing confers selective benefits on memory circuits, and summarized recent research linking these changes to respiratory-entrained slow brain rhythms. These combined changes could account for a global, ‘meditative’ state characterized by cognitive and behavioral relaxation, alertness, and improved memory. Proposed neural signatures for voluntary slow breathing’s diverse effects are shown in Figure 3.

35Nasal pathways through the olfactory bulb and SAR pathways through the NTS may represent two separate mechanisms for the generation of slow cortical rhythms. The combined activity of both could be required for full cortical synchrony. For instance, tracheotomy can weaken respiratory-related oscillations localized to the hippocampus and olfactory cortex in rodents. Comparison of functional cortical differences in response to nasal vs. oral breath pacing in humans could be studied using electroencephalography (EEG). Mechanistically, we hypothesize that the vagal afferent activity induced by slow breathing could entrain EEG activity and recruit slow cortical rhythms. Neuronal oscillations in the amygdala and LC have not been studied to the same extent as neocortical and hippocampal oscillations. However, slow, respiratory-coupled oscillations have recently been discovered in the CeA of mice, with human studies further demonstrating respiratory-based entrainment of amygdala circuits. These phenomena may have previously escaped detection due to the absence of respiration monitoring or similarity with other brain oscillations. In rats respiration entrains rhythmic LC activity, which can form temporary resonant states with frontal cortical oscillatory activity. Moreover, firing of LC neurons presumably provides neuromodulatory input that contributes to EEG slow waves during sleep, to promote plasticity within these circuits.

36The impact of slow breathing on cortical rhythms does not appear restricted to 0.1 Hz, as both spontaneous breathing and respiratory-entrained neural oscillations typically occur at frequencies > 0.1 Hz. Intracranial EEG recordings from human epilepsy patients have revealed that cortical oscillatory activity entrains with respiration at slightly higher frequencies (0.16–0.33 Hz), with synchronized electrical activity observed in limbic-related brain areas, including the amygdala and hippocampus. Nonetheless, the unique frequency of 0.1 Hz or its harmonics could be crucial for realizing the full cognitive benefits of slow breathing. For instance, a recent study in humans found that mechanically simulating breathing at 0.05 Hz – a subharmonic of 0.1 Hz – slowed cerebral rhythms, enhancing delta-theta EEG activity throughout default mode network (DMN) structures and inducing altered states of consciousness akin to respiration-based meditative practices.

37The DMN is a network of brain regions that show highly correlated activity during rest and reduced activation during attention-demanding tasks, and contains the medial prefrontal and posterior cingulate cortices (both known to be involved in central autonomic regulation) as core regions. The DMN is characterized by blood-oxygen-level-dependent (BOLD) fMRI signal fluctuations around 0.1 Hz. Although the DMN average frequency of 0.1 Hz synchronizes activity in widespread regions of the brain and is highly correlated with respiration, the precise impact of respiratory fluctuations on the DMN is poorly understood and large amplitude 0.1 Hz hemodynamic oscillations observed in human fMRIs indicative of cortical resting states are rarely considered. It has been hypothesized that, via neural activity oscillations around 0.1 Hz possibly attributed to SARs, respiratory impulses during slow breathing may synchronize with the DMN. Since slow, local propagating hemodynamic waves appear related to vasomotion and perfusion of active functional areas, respiratory-linked neuronal oscillations around 0.1 Hz could couple the demands of neural activity to perfusion and improve brain tissue capacity, enhancing physiological function. Studies investigating the link between respiration and brain activity have suggested that respiratory rhythms may be an organizing principle of cortical oscillations in the human brain. Indeed, slow inhalations at 0.1 Hz have been found to increase EEG spectral power in the delta band and may enhance synchronization in thalamocortical circuits. The DMN in turn could support higher processing associated with awareness, memory, and cognitive performance, and may be involved in consciousness. Future studies should determine the contributions of respiration to these phenomena and test nasal vs. oral breathing, as well as different rates and modalities of paced breathing or mechanical stimulation, to determine whether 0.1 Hz optimally tunes physiological function.

38While slow wave oscillations have been linked to the respiratory rhythm, there is also compelling evidence of cerebral blood flow fluctuations associated with cardiovascular variability and reductions in cerebral blood flow during slow wave sleep. Respiratory activity, which is fairly irregular during waking conditions, tends to stabilize during deep sleep when respiratory rate is precisely correlated with heart rate, enhancing cardiorespiratory synchronization. Recent research has also discovered that inspiration crucially regulates the flow of CSF. CSF flow is driven by arterial pulsations and may be impaired in disease states. This raises the possibility that a traveling fluid wave through the ventricles could activate central mechanoreceptors to complement the direct effects of slow breathing on neural circuits. Future experiments should track CSF flow rate and correlate it with temporal neural activity to validate this proposition, and determine whether 0.1 Hz respiration optimizes the flow of CSF.

39We have thus far described the various components of our hypothesis connecting slow breathing to the modulation of afferent systems, central autonomic networks, and cortical pathways. We have discussed several mechanisms linking 0.1 Hz breathing to systemic physiological function including how slow breathing can enhance RSA via increased baroreflex gain and its impact on optimizing ventilation-perfusion matching, and engaging adaptive systems to promote autonomic balance and relaxation. We conclude by discussing the components of our model (Figures 2, 3) that require further testing, with special emphasis on (i) afferent pathways and proposed neurocircuitry, and (ii) respiration-entrained slow cortical rhythms. Basic mechanistic research will help identify new therapeutic targets, while clinical studies should optimize respiratory strategies for different patient populations, in order to fully unlock the potential benefits of slow breathing.

40Whether animal studies on vagal afferent activity patterns generalize to human slow, deep breathing practices remains uncertain and in need of further study, although there is evidence of similar pulmonary reflex mechanisms. The subsection “Human Subjects Research on Slow, Deep Breathing” summarized recent human studies on 0.1 Hz respiration. An important next step is novel approaches using animal models and/or targeted respiratory feedback. Clinical studies have had difficulty ruling out extraneous variables (e.g., placebo effects and expectancy biases developed during training) as contributors to observed changes, and formulating proper control groups. An intriguing possibility is the development of animal models that mimic the pure physiology of slow breaths while ignoring additional cognitive components associated with meditation. A review by Ley concluded that “breathing behavior is amenable to the principles of Pavlovian and operant conditioning”. Operant conditioning protocols have had some success altering respiration in rats and humans. Our laboratory recently devised a yoked-control operant paradigm to investigate slow breathing as a technique to reduce stress reactivity. Rats learned to decrease their resting respiratory rates, an effect that was maintained between sessions and corresponded with a reduction in anxiety-like behavior. Future studies that enhance the ecological validity of operant conditioning procedures could optimize physiological learning. Additional studies on controlled animal respiration have employed classical conditioning paradigms, or externally induced (“forced”) respiration paradigms using anesthetized and mechanically ventilated preparations, intubated rats, or hypoxic/hypercapnic animals. The application of these approaches in animals to study slow breathing could represent a new frontier in translational research, with animal experiments identifying specific physiological events that link slow breathing to decreased stress responses (relaxation) for subsequent complementary studies in humans that determine the impact of slow breathing as a component of different meditative practices.

41Due to the overwhelming prevalence of stress-related disorders in the general population (e.g., ) and the ongoing deficiency of technological and pharmacological tools to ameliorate these conditions, there is an urgent need for studies that mechanistically associate slow breathing with relaxation. Studies should test whether activation of pulmonary afferent pathways leads to physiological effects consistent with our proposed model. For example, although the NTS projects extensively to respiratory control centers, how exactly this circuitry may influence pre-BötC-LC “calming” pathways is unknown. SAR afferents project to a region of the NTS containing substance P (SP), and terminate on a class of second-order neurons (P-cells) whose projections include the BötC, leading to the hypothesis that SAR-activated SPergic P-cells projecting to the BötC may inhibit pre-BötC-LC pathways. Using targeted approaches to selectively control populations of neurons in the NTS or pre-BötC, recently attempted using genetic methods, is one elegant technique that could be performed to more definitively ascertain this connection. Also of relevance are the medial and commissural NTS subregions that provide the majority of its noradrenergic input to the CeA and PVN. In contrast to the ventrolateral NTS, these subnuclei coexpress neuropeptide Y (NPY), which is known to promote stress coping and resilience. A pivotal role for these NTS projections in reducing stress reactivity is suggested by the recent study showing that activation of a small subpopulation of genetically identifiable (Hoxb1-NE) noradrenergic neurons originating from multiple regions including the A2 cell region led to attenuated responses to acute stressors. The anxiolytic effects of Hoxb1-NE neuron activation apparently required noradrenergic signaling through α1 and β adrenoceptors. Pharmacological studies could administer noradrenergic agonists and antagonists via local iontophoretic application and examine stress-related physiology at α1- or β-adrenergic receptors in the CeA or PVN. A subset of these calming noradrenergic projections also coexpressed NPY, which the authors concluded may modulate NE transmission to facilitate “anti-stress” responses; the NPY system may represent a promising complementary target for experimental manipulation. Combining such measures with other steps aimed at clarifying biological mechanisms, such as quantifying measures of circuit activation including Fos expression in the CeA, PVN, or NTS, will help determine the underlying neurophysiology of slow breaths. These approaches will ideally be combined with others that enable monitoring of behavioral and cardiorespiratory variables to provide a more complete understanding of the mechanisms linking slow breathing to relaxation-related outcomes.

42Future studies should also test whether oral breathing has the same effects on relaxation as nasal breathing, and evaluate the mechanisms through which specific breathing patterns around 0.1 Hz (e.g., decreased inhalation/exhalation ratio) confer selective benefits. In healthy adults breathing quietly, the normal inhalation/exhalation ratio is approximately 1:1.2 but this decreases during deep breathing, and yogic techniques emphasize a breath pattern of 0.1 Hz with prolonged exhalation (ratio 1:2). During slow and deep exhalations, when the respiratory gate through the NTS is “opened,” maximal activation of the pathways described above may evoke a state of parasympathetic dominance. Humans breathing at 0.1 Hz with prolonged exhalation display reduced physiological and psychological indices of arousal and report increased relaxation as compared to prolonged inspiration or equal ratio breathing. In a similar vein, it will be important to isolate the relative contributions of different afferent populations to the physiological effects of 0.1 Hz breathing. For instance, a recent neuroimaging study of slow breathing found that baroreflex-based cardiovascular responses (blood pressure and HRV) associated with activity within the ventral medulla while increased tidal volume associated with activity in the NTS and dorsal pons (including the LC). Therefore, the effects of high tidal volume breathing on pulmonary afferent activation may be more connected to its impact on modulating arousal, with baroreceptors and associated ventral medullary regions more directly related to cardiorespiratory effects that optimize ventilation to match perfusion [RSA – see subsections “Breath Synchronizes Heart Rate: Respiratory Sinus Arrhythmia (RSA) and Ventilation-Perfusion Matching and Metabolic Benefits”]. Experimental approaches that target specific primary afferent pathways – such as electrophysiological stimulation of vagal pulmonary afferents in animal models – could provide such a distinction.

43The signal coherence provided by cardiorespiratory synchronization around 0.1 Hz could lead to related emergent phenomena. Autonomic neural circuits could entrain cortical activity around 0.1 Hz, generating slow cortical rhythms similar to those seen during deep sleep. Generation of large arterial pressure 0.1 Hz Mayer waves would facilitate tissue perfusion independent of heart function, thereby reducing cardiac burden with consequent reduced metabolic demand, and may support a sense of physical relaxation. The relaxation response would reduce stress reactivity and could facilitate learning and memory.

44While slow breathing induces spatiotemporal patterns of brain activity overlapping with the DMN, certain meditation techniques emphasizing focused attention or effortless awareness, as well as highly pleasant ‘flow’ states, are associated with reductions in DMN activity. Humans at rest alternate between activation of the DMN and an anticorrelated task-positive network, with an average switching frequency of 0.05–0.1 Hz. Respiratory afferent activity patterns recruited by slow breathing could contribute a rhythmic component to ongoing cortical activity, linking cognitive function to the process of breathing. Studies undertaken in animal and human subjects should further investigate slow breathing for its impact on DMN-associated slow brain rhythms and cognition. Although recent work has helped to clarify the relationship between respiration and slow rhythms in humans, studies are needed to establish a direct link with breathing around 0.1 Hz. This frequency may maximize resonant effects, possibly directly or via cerebral blood flow fluctuations associated with cardiovascular variability. Slow cortical oscillations observed during sleep or under deep anesthesia occur at approximately the species-specific frequency of Mayer waves in the cat (0.3 Hz), but little research has explored the connection between vasomotion, Mayer waves, and resting state functional connectivity. Slow breathing could enhance inherent slow rhythms while activating a unique profile of neural networks to explain the subjective experience of alert relaxation vs. deep sleep. The possibility that slow oscillations are a Mayer wave-linked phenomenon, with potential links to the DMN, should be further examined using a combination of electrophysiological and neuroimaging approaches. Studies in meditators breathing at different rates and using different strategies (e.g., voluntary control vs. passive attention) will help us understand the relationship between these global phenomena and how they might be recruited by targeted breathing practices to engage subjective states of relaxation and awareness.

45DN drafted the manuscript with SH providing substantial revisions.

46The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.