Head-Heart Entrainment: A Preliminary Survey
R. McCraty, W. A. Tiller, M. Atkinson

Paper presented at the Key West Brain-Mind, Applied Neurophysiology, EEG Biofeedback 4th Annual Advanced Colloquium, Key West, FL, Feb. 1996.

Abstract
Introduction
Experimental Protocol
Results
Discussion
References
Introduction
A number of physiological systems have been identified as exhibiting nonlinear oscillatory interaction phenomena; these include the sinoatrial node in the heart [1], the respiratory system [2], synchronization of nonlinear biochemical systems [3], coupling in embryonic heart cells [4] and the digestive system [5]. Although these physiological systems clearly perform different functions, for each system the concept of synchronization or entrainment underlies its dynamic behavior. Early modeling of these systems involved a linear approach; however, many of the observed data do not fit a linear model. Therefore, in recent years, nonlinear modeling has been employed [6]. There are important differences between linear and nonlinear systems. For example, when two linear oscillatory systems operating at different frequencies are coupled, beating occurs and the resulting output contains frequencies which directly correspond only to the modes of oscillation of the individual systems (fundamental, sum, and difference frequencies). However, when the oscillators are nonlinear, the coupling causes the two oscillators to lock into a common frequency. As the frequency difference between the two nonlinear systems is reduced, a point is reached where the combined system output suddenly consists of only a single dominant frequency. This is called "frequency selective entrainment." Another nonlinear phenomenon, called "frequency-pulling," can arise when the amplitude of one of the oscillators is insufficient to cause full entrainment but is capable of pulling or displacing the frequency of the other oscillator. This has been observed in a number of physiologic systems, e.g., electromyograph signals from the smooth muscle of the arterial system and the gut as well as the interaction between respiration and heart rate variability. This is a complex subject which cannot be covered in the space allotted here and interested readers are referred to the book by Grossman, et al. [7].

In this study we are reporting on a similar entrainment phenomenon occurring between heart rate variability (HRV), respiration rate and very low frequency (VLF) electroencephalograph (EEG) recordings. Previous studies have demonstrated a special relationship between respiration rate and high frequency EEG [8].

In our previous studies of the relationship between HRV, respiration rate, blood pressure waves and other somatic systems, we reported that when one is sincerely experiencing positive emotional states, there is a tendency for systems to naturally entrain and, further, that this entrainment process can be facilitated by specific techniques that shift conscious attention to the area around the heart (Freeze-Frame) [9-12] and/or by specifically designed music [13]. An example of entrainment between HRV, pulse transit time (PTT) and respiration rate is shown in Figure 1. A shift in perception and a greatly increased intuitive awareness has also been subjectively observed in many subjects when entrainment occurs.

In all of these interesting considerations, one question that compels attention is, "Is there a 0.1Hz oscillator in the brain?" Another is, "Is any VLF signal in the brainwaves associated with electromagnetic (EM) radiation from the heart or is it mainly nerve linked?" Still another is, "Are there pacemaker cells in the brain that like to function systematically with ~0.1 Hz oscillations and, if not, where does this stimulus come from?" Finally, the issue of the perceptual changes needs to be understood.

In order to facilitate understanding of the data reported herein, a brief review of the interactions of the heart and brain is necessary for the unfamiliar reader. Figure 2a is a simplified block diagram illustrating the two-way communication and feedback system between the heart and brain. The sympathetic and parasympathetic branches of the autonomic nervous system (ANS) influence the sinus node of the heart and vascular systems thereby modulating heart rate and blood pressure. Changes in heart rate and blood pressure are fed back to the brain via the baroreceptor system. The baroreceptor input to the brain has numerous effects on brain function. For example, when one’s mental or emotional state is causing the sympathetic system to be overdriven in stressful situations, the baroreceptor feedback inhibits sympathetic outflow, increases parasympathetic neuronal activity [7], causes brain wave slowing [14] and cortical inhibition [15] in order to protect the overall system. This feedback loop or connection between the heart and brain stem is thought to be responsible for the well known 10 second rhythm or 0.1 Hz frequency peak seen in the HRV power spectrum [6]. The baroreceptor input to the brain is also responsible for the 0.1 Hz rhythms seen in reticular neuronal activity in the brain stem and the cardiac rhythms seen in the sympathetic outflow to the body [7]. The reticular neuronal network is a multifunctional system controlling respiration, cardiovascular and somatomotor systems, as well as the degree of cortical activity [7].

Figure 2b illustrates the power spectrum of a 5-minute HRV waveform. The HRV power spectrum has been divided into three frequency ranges for our studies, LF (0.01 to 0.05 Hz), MF (0.05 to 0.15 Hz) and HF (0.15 to 0.5 Hz) [16]. The MF region can be used to discriminate the power in the baroreceptor feedback loop, which is responsible for beat-to-beat blood pressure control [17]. Power in the MF region can be due to parasympathetic or sympathetic outflow from the brain or due to a mixture of both [16, 18]. Power in the MF region of the HRV spectrum has also been correlated to nerve traffic in the baroreceptor feedback loop back to the brain [19]. The HF region is associated with only parasympathetic activity while the LF region is associated with mostly sympathetic activity.

We have developed a simple method termed the "Freeze-Frame method" (FF) [12] for consistently producing beneficial shifts in parasympathetic tone and sympathovagal balance. This has been tested in both normal individuals [20] and in subjects with a number of pathological states [21]. The technique has been successfully employed in a number of applications to reduce stress and emotional reactivity. The application of the FF technique readily leads to ordered states of heart function, one of which is the entrainment mode which has been described in detail elsewhere [9].

In the entrainment mode of heart function, power in the MF region of the HRV frequency spectrum is greatly increased indicating increased neural traffic in the baroreceptor system. The entrainment between the HRV waveform and the low frequency portion of the EEG reported here was observed in subjects who have been practicing the FF technique for some time and, thus, can consistently demonstrate an increased ratio of time in the heart entrainment mode and shift to this mode at will. It appears that, as one develops the ability to maintain the HRV entrainment mode, the brain is also brought into entrainment with the heart rhythms. Subjective statements, from several hundred newly trained practitioners, indicate that a significant shift in perception and awareness is associated with using this technique. Therefore, the present study set out to explore the electrophysiological correlations between heart and brain functions.

Prior to starting the practice of FF, the subjects, whose data is shown herein, did not demonstrate either significant LF activity in the EEG, an ordered mode of heart function or entrainment between any biological oscillators of the body.