What Is Coherence? also see the discussion of the valsalva wave

In "psycho-physiological" terms, coherence is the state wherein fundamental bodily rhythms are in synergistic harmony. This harmony provides positive reinforcement for both physiological and psychological processes. Evidence of the synergy of these processes is visible via numerous biometrics including heart rhythm, brainwaves, blood flow, arterial pressure, galvanic skin response, etc.

As it turns out, optimal coherence exists during the state of optimal autonomic nervous system balance, i.e. equal sympathetic or "activating" influence, and parasympathetic or "deactivating" influence. Generally, autonomic nervous system balance results in coherence and imbalance results in incoherence. In fact, autonomic nervous system balance while at rest or semi-activity is a literally a function of "breathing coherence", i.e. consistency of breathing frequency, phase, and depth. Why? Because breathing governs blood flow, and blood flow, to a large extent, governs autonomic status.

Optimal breathing results in a phenomenon called the respiratory arterial pressure wave. This wave moves blood from the lungs through the left side of heart to the extremities coincident with exhalation and literally draws blood back through the right side of the heart to the lungs and thoracic cavity coincident with inhalation. During resonance this wave action occurs with every breath. It is conjectured that the arterial pressure wave of deep resonant breathing adds substantially to total blood flow as compared to the respiratory arterial pressure wave of relatively rapid shallow breathing.

Figure 1 below illustrates this process.

Figure 1: Thoracic Cavity - Source & Sink

This respiratory arterial pressure wave occurs when breathing is synchronous and regular in depth. Magnitude of the pressure wave varies with breathing depth - when depth of inhalation and exhalation are optimal, a respiratory arterial pressure wave of maximal magnitude is produced.

The autonomic nervous system senses the presence of this respiratory arterial pressure wave. In fact, the heart rate variability (HRV) cycle is an outcome of autonomic nervous system regulation of instantaneous pressure, instantaneous pressure being a function of the respiratory arterial pressure wave, maximal HRV amplitude corresponding to maximal arterial pressure wave magnitude.

During "resonance", optimal breathing frequency and depth, many body rhythms are operating in synchrony with this wave. This includes brainwaves.

This "coherence" or "harmony" is reflected throughout physiological as well as psychological processes. Figure 2 below presents some of the many "correlates of autonomic balance and imbalance":

IMBALANCE		BALANCE
discomfort	physical comfort	comfort
tense	physical tension	relaxed
anxious	mind state	at ease
muddled	thinking	clear
extreme	sensibilities	tempered
impeded	mind-body communication	free flowing
weak	short term memory	good
reduced	sphere of awareness	expanded
reduced	intuition	increased
reduced	openness to new ideas	increased
defensive	interpersonal communication	accepting
acid	serum pH	balanced
higher	average heartbeat rate	lower
much higher	heart duty cycle (work)	much lower
higher	blood pressure	lower
lower	heart rate variability amplitude	maximal
lower	blood oxygenization/gas exchange efficiency	maximal
faster	breathing rate	slower

FIGURE 2: Psycho-Physiological Correlates of Autonomic Nervous System Balance

The heartbeat is always attempting to synchronize with the breathing cycle (except during REM sleep). This is because the cardiopulmonary system is arranged to take advantage of the pumping action of the thoracic cavity, i.e. it makes sense for the heart and arterial system to assist in moving the blood when it is being offered by the thoracic cavity, not when it is not.

Imagine the problems that would result if the heart attempted to maximize blood flow into the arterial tree during deep inhalation, i.e. when blood flow into the left atrium is in short supply. Or alternatively, if it attempted to throttle blood flow during deep exhalation, when maximal blood volume is being ejected by the thoracic cavity. Well, to be clear, this is what is happening during "incoherence". The heart and vascular system are always attempting to synchronize and adapt to the breathing cycle, even when it is incoherent, thus resulting in cardiopulmonary incoherence.

So, "psycho-physiological coherence" is an outcome of the respiratory arterial pressure wave coherence, which is an outcome of respiratory coherence. Of course, this is the mechanism by which Coherent Breathing facilitates "coherence". By the same token, incoherent breathing results in psycho-physiological incoherence.

Make no mistake - it is the autonomic nervous system that is governing all of this synchrony. Why? Because, this state of "cardiopulmonary resonance" represents the most efficient and effective operation, i.e. optimal homeostasis. And, because governance of blood flow and arterial pressure are a highest priority for the autonomic nervous system.

Please see The New Science of Breath - Coherent Breathing for Autonomic Nervous System Balance, Health, and Well-Being for more information.