Brainwave entrainment, also referred to as brainwave synchronization or neural entrainment, refers to the observation that brainwaves (large-scale electrical oscillations in the brain) will naturally synchronize to the rhythm of periodic external stimuli, such as flickering lights, speech, music, or tactile stimuli.
As different conscious states can be associated with different dominant brainwave frequencies, it is hypothesized that brainwave entrainment might induce a desired state. Researchers have found, for instance, that acoustic entrainment of delta waves in slow wave sleep had the functional effect of improving memory in healthy subjects.
History The establishment of the concept of brainwave entrainment is based on several key insights. A relationship between neural activity and external stimuli has been a subject of investigation since Berger's findings (the inventor of EEG) in the late 1920s. He registered event-related desynchronization of the ongoing alpha rhythm, evidenced by an alpha-amplitude decline after stimulation.Arieli A.; Sterkin A.; Grinvald A.; Aertsen A. (1996). "Dynamics of ongoing activity: explanation of the large variability in evoked cortical responses". Science 273, 1868–1871.
The notion of entrainment was introduced in science in the middle of the 20th century as an activation of a timing cue for a biological rhythm, being an inherited circadian oscillator. This biochemical oscillator in animals cycles with a stable phase and is synchronized with solar time. The entrainment concept describes animals' capacity to synchronize with one another or with external stimuli. In 1971, Professor Aschoff and his colleagues pioneered the study of entrainment in biological systems. Their experiments reported that social cues synchronize human circadian rhythms. Since physiology largely reflects neural activity, this discovery demonstrates neural entrainment (or brainwave entrainment ) and shows that neural entrainment to external periodic stimuli is an essential mechanism in physiology.Obleser, J.; Kayser, C. (2019). "Neural entrainment and attentional selection in the listening brain". Trends in cognitive sciences, 23(11), 913-926.
In 1988, two physiological researchers, Leon Glass and Michael Mackay, contributed to understanding dynamic processes in physiology by explaining "dynamic diseases" that arise from disruptions in the synchronization of vital functions. They explored central theoretical questions about physiological rhythms from the perspectives of biology, medicine, physics, and mathematics.Glass, L.; Mackey, M. C. (1988). From clocks to chaos: The rhythms of life. Princeton University Press. This knowledge provided insights into the development of complex systems theory, explaining how two or more independent, autonomous oscillators with differing rhythms or frequencies, when situated in proximity and able to interact for long enough, influence each other to a degree dependent on the coupling force.
In 1999, Professor Thaut and colleagues demonstrated, in numerous experiments, that acoustic oscillations could entrain movement patterns in patients with neurological disorders, establishing the use of neural entrainment in medicine.
In 2018, Professor McCraty found that one's electrocardiogram signal was registered in another person's electroencephalogram and elsewhere on the other person's body. While this signal was most potent when people were in contact, it was still detectable when subjects were in proximity but not connected.McCraty, R.; Atkinson, M.; Tomasino, D. (2018). "Tiller WA. The electricity of touch: Detection and measurement of cardiac energy exchange between people". In: Brain and Values. London, UK: Psychology Press; 2018. pp. 359-379. Professor Gordon and colleagues reported the empirical evidence of similar heartbeats' inter-beat intervals (measured by electrocardiograms) of group members due to an external rhythmical oscillator. Quantitative studies have supported the claim that people's heart rates synchronize when they are in close physical proximity to each other.
In 2022, Professor Pejstrup Agger and colleagues invented a novel treatment for Alzheimer's disease, brainwave entrainment using invisible spectral flicker at 40 Hz.
In 2023, Professor Thomas J. Nichting from Eindhoven University of Technology showed evidence of maternal-fetal cardiac coupling, which refers to entrainment of the fetal heart to maternal heart rhythms.
The interaction between the nervous system and the heart is an emerging area of study. In 2024, Latvian professor Igor Val Danilov proposed a theory on the evolutionary basis of brainwave entrainment to external oscillations. He developed a mother-fetus neurocognitive model, which provides neurophysiological insights into how cognition begins through fetal neural entrainment to the natural oscillations (coined natural neurostimulation) produced by the mother's body.
In 2024, researchers from MIT, led by Professor TaeHyun Kim, found that daily exposure to light and sound at 40 hertz may protect cancer patients from chemotherapy-induced damage, memory impairment, and other cognitive effects.
Origin Brainwave entrainment to the rhythm of periodic external stimuli largely relies on evolutionary mechanisms. A hypothesis of natural neurostimulation explains brainwave entrainment as evolutionarily inherited processes for the mother-fetus nervous system synchronization during gestation, ensuring fetal neural growth and the onset of cognitive functions. According to the prevailing view in cognitive science, the manifestation of cognitive functions is associated with the temporal coordination of neural oscillations across different brain regions.Grover, S.; Nguyen, J.A.; Reinhart, R.M. (2021). "Synchronizing brain rhythms to improve cognition". Annu Rev Med. 2021; 72: 29-43. Most brain activities are related to events in which large sets of neurons cooperate, displaying oscillations in four frequency orders, from the infra-low (<0.01 Hz) to ultra-fast (200 Hz) oscillations: delta (0.5–4 Hz), theta (4–8 Hz), alpha activity (8–13 Hz), beta (13–30 Hz), and gamma (30–200 Hz) frequency bands. High-frequency oscillations are associated with the temporal coordination of local cell ensembles. Research reported that they are localized in time and space, reflecting the synchronous activation of smaller ensembles of cells.Hasenstaub, A.; Shu, Y.; Haider, B.; Kraushaar, U.; Duque, A.; McCormick, D.A. (2005. "Inhibitory postsynaptic potentials carry synchronized frequency information in active cortical networks". Neuron. 2005; 47: 423-435. Considering the propagation of high-frequency waves in tissues over a distance of only a few millimetres,Jia, X., Kohn, A. (2011). "Gamma rhythms in the brain". PLoS biology, 9(4), e1001045. https://doi.org/10.1371/journal.pbio.1001045 the hypothesis of natural neurostimulation explains why high-frequency oscillations from anatomically non-connected nervous system zones become synchronized.
Based on a review of numerous neuroscience studies, Professor Vinck and colleagues highlighted four hypotheses on a mechanism of temporal coordination:Vinck, M.; Uran, C.; Spyropoulos, G.; Onorato, I.; Broggini, A.C.; Schneider, M. et al. (2023). "Principles of large-scale neural interactions". Neuron. 2023; 111: 987-1002.
(1) Oscillatory synchronization (communication-through-coherence);Bastos, A.M.; Lundqvist, M.; Waite, A.S.; Kopell, N.; Miller, E.K. (2020). "Layer and rhythm specificity for predictive routing". Proc Natl Acad Sci. 2020; 117: 31459-31469.
(2) communication-through-resonance;Izhikevich, E.M.; Desai, N.S.; Walcott, E.C.; Hoppensteadt, F.C. (2003). "Bursts as a unit of neural information: Selective communication via resonance". Trends Neurosci. 2003; 26: 161-167.
(3) nonlinear integration;Cohen, U.; Chung, S.; Lee, D.D.; Sompolinsky, H. (2020). "Separability and geometry of object manifolds in deep neural networks". Nat Commun. 2020; 11: 746. and
(4) linear signal transmission (coherence-through-communication).Schneider, M.; Broggini, A.C.; Dann, B.; Tzanou, A.; Uran, C.; Sheshadri, S.; et al. (2021). "A mechanism for inter-areal coherence through communication based on connectivity and oscillatory power". Neuron. 2021; 109: 4050-4067.
Among them, the idea of brainwave interactions for network oscillatory synchronization is the most influential.Bressler, S.L.; Kelso, J.S. (2001). "Cortical coordination dynamics and cognition". Trends Cogn Sci. 2001; 5: 26-36. Then, Latvian professor Igor Val Danilov extended these findings. He argues that, in temporal coordination, low-frequency heart oscillations can harmonize high-frequency brain oscillations across the nervous system due to the law of interference.Val Danilov, I. (2023). "Shared Intentionality Before Birth: Emulating a Model of Mother-Fetus Communication for Developing Human-Machine Systems". In: Arai, K. (eds) Intelligent Systems and Applications. Intelli-Sys 2023. Lecture Notes in Networks and Systems, vol 824. Springer, Cham. https://doi.org/10.1007/978-3-031-47715-7_5 Indeed, in physics, when incoherent waves are superimposed, the intensity of the resulting wave is equal to the sum of the powers of the superimposed waves. The energy of the resulting oscillations of each point of the medium is similar to the sum of the energies of its oscillations due to all incoherent waves. So, interference occurs when two oscillations combine: low-frequency oscillations modulate the high ones and propagate them at the distance of their own propagation. In neuroscience, this wave interference, where the phase of the underlying slow rhythm modulates the power of faster oscillations, is often called nested oscillations. Due to the low-frequency waves' ability to propagate over long distances in human tissues, heartbeats-nested high-frequency brain oscillations from anatomically non-connected nervous system zones become synchronized. That is, interference between heart oscillations and high-frequency brain oscillations is crucial for temporal coordination, integrated neuronal processing, and cognition.Val Danilov, I. (2023). "Low-Frequency Oscillations for Nonlocal Neuronal Coupling in Shared Intentionality Before and After Birth: Toward the Origin of Perception". OBM Neurobiology 2023; 7(4): 192; doi:10.21926/obm.neurobiol.2304192. https://www.lidsen.com/journals/neurobiology/neurobiology-07-04-192 During pregnancy, the mother's heart oscillations consolidate neuronal activity across both organisms, which coexist in the same ecological context, i.e., voice stimuli from the mother's environment. The fetal environment includes both physicochemical interactions with the mother's body and sounds from the mother's environment that can reach the fetus's auditory system. In this manner, physical interactions between the mother and fetus stimulate the fetal sentience. Due to neural synchrony, particular reactions of the mature nervous system become a template for the naive organism in the early stages of perception development, and subsequently, cognition.