Physiological Entrainment: A Key Mind–Body Mechanism for Cognitive, Motor and Affective Functioning, and Well-Being

Figures

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  Figure 1. Physics of Entrainment. The upper panel (a), “Non-Synchronized Signals”, shows two oscillatory signals that are not synchronized. The blue signal has a frequency of 5 Hz, while the red signal has a frequency of 7 Hz. In the context of neural entrainment, non-synchronized signals indicate neural activities that are neither phase- nor frequency-aligned. These oscillations occur independently, with peaks and troughs appearing at different times, demonstrating a lack of coordination between the underlying neural pools. Such asynchrony suggests that the neural pools are either not entrained to the same external stimulus, or are not engaged in the same neural processing, resulting in uncoordinated oscillatory activity. The lower panel (b) “Phase-locked Signals” shows two phase-locked signals. The phase difference between two oscillators (or spike trains) remains constant over time (at 5 Hz for both signals). In this case, the individual spikes or oscillations do not necessarily occur at the same time, but the relationship between their phases is stable. For example, if two neurons fire spikes at different times but maintain a consistent phase difference, they are said to be…

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  Figure 2. EEG frequency-tagging method. Panels (a) “Audio time-domain” and (b) “EEG time domain” display the time-domain representations of a 3-Hz auditory rhythm, while panels (c) “Audio frequency-domain” and (d) “EEG frequency-domain” display the frequency-domain representations of a 3-Hz auditory rhythm. The audio tone signal envelope in (a) illustrates a regular oscillation at 3 Hz, while (c) shows the corresponding audio peak of 3 Hz in the frequency spectrum. Panels (b) and (d) depict the EEG responses from a listener (simulated data for portrayal purposes) whose neural activity is entrained with this rhythmic input. In panel (b), the EEG trace reflects synchronized oscillations at 3 Hz, while (d) demonstrates a prominent 3-Hz peak in the neural frequency spectrum, confirming phase-locking between the auditory stimulus and brain oscillations. This approach exemplifies neural rhythm processing using the EEG frequency-tagging method, through which brain activity synchronizes to rhythmic stimuli. The method quantifies the degree of neural entrainment by examining peaks in the EEG spectrum, particularly at frequencies corresponding to rhythmic inputs.

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  Figure 3. Coherence index. This figure depicts two graphs that compare two signals over time and shows how they are related to different frequencies. The upper panel (a) “Comparison of Signals Frequencies Across Time” displays both signals (red signal and blue signal) over a 2-s period. These signals fluctuate in a similar way and appear to share a common rhythm at around 10 Hz (i.e., 10 peaks per second). However, some random noise causes small differences between them, such as minor shifts in peaks and troughs. Overall, the two signals appear to follow a similar pattern, despite the noise. The bottom panel (b) “Spectral Coherence Across Frequencies” represents the coherence between signals, which measures the coupling of these signals at different frequencies. Coherence values range from 0 (no relationship) to 1 (a perfect match). There is a clear peak at around 10 Hz, showing that the two signals are highly related to this frequency. Beyond this, the coherence decreases, meaning that the signals do not align well at other frequencies. This suggests that their coupling is strongest at around 10 Hz, while, at other frequencies, they behave more independently. In summary, the two signals…

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  Figure 4. Coherence and entrainment. This figure shows the level of entrainment across three physiological systems of the human body: heart rate variability, blood pressure, and respiration rhythms over a 10-min period. Around the 300 s. mark, this participant engaged in the “freeze-frame positive emotion refocusing technique”, which is designed to induce a state of coherence. This technique aligns the physiological systems, bringing them into entrainment in such a way that their rhythmic cycles synchronize. The bottom graph panel depicts the frequency spectra before and after the entrainment process. The bottom-right graph marked with “Baseline” and “Entrainment” shows that heart rate variability, blood pressure, and respiration have synchronized with the same frequency (figure taken from Ref. [82], reproduced with the permission of the HeartMath Institute; https://www.heartmath.org/).