brainwave entrainment

{{Short description|Neural activity synching to external stimuli}}

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,{{Cite journal|last1=Notbohm|first1=Annika|last2=Kurths|first2=Jürgen|last3=Herrmann|first3=Christoph S.|date=2016|title=Modification of Brain Oscillations via Rhythmic Light Stimulation Provides Evidence for Entrainment but Not for Superposition of Event-Related Responses|journal=Frontiers in Human Neuroscience|volume=10|pages=10|doi=10.3389/fnhum.2016.00010|issn=1662-5161|pmc=4737907|pmid=26869898|doi-access=free}} speech,{{Cite journal|last1=Ding|first1=Nai|last2=Simon|first2=Jonathan Z.|date=2014|title=Cortical entrainment to continuous speech: functional roles and interpretations|journal=Frontiers in Human Neuroscience|volume=8|pages=311|doi=10.3389/fnhum.2014.00311|issn=1662-5161|pmc=4036061|pmid=24904354|doi-access=free}} music,{{Citation|last=Thaut|first=Michael H.|title=Chapter 13 - The discovery of human auditory–motor entrainment and its role in the development of neurologic music therapy|date=2015-01-01|url=https://www.sciencedirect.com/science/article/pii/S0079612314000314|journal=Progress in Brain Research|volume=217|pages=253–266|editor-last=Altenmüller|editor-first=Eckart|series=Music, Neurology, and Neuroscience: Evolution, the Musical Brain, Medical Conditions, and Therapies|publisher=Elsevier|language=en|doi=10.1016/bs.pbr.2014.11.030|pmid=25725919|isbn=9780444635518|access-date=2021-12-01|editor2-last=Finger|editor2-first=Stanley|editor3-last=Boller|editor3-first=François|url-access=subscription}} or tactile stimuli.

As different conscious states can be associated with different dominant brainwave frequencies,{{Cite book|title = Clinical Neurotherapy: Application of Techniques for Treatment|url = https://books.google.com/books?id=j6-NfhUbLfQC&pg=PA177|publisher = Academic Press|date = 2013-10-18|isbn = 9780123972910|first1 = David S.|last1 = Cantor|first2 = James R.|last2 = Evans}} 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.{{Cite journal|last1=Diep|first1=Charmaine|last2=Ftouni|first2=Suzanne|last3=Manousakis|first3=Jessica E|last4=Nicholas|first4=Christian L|last5=Drummond|first5=Sean P A|last6=Anderson|first6=Clare|date=2019-11-06|title=Acoustic slow wave sleep enhancement via a novel, automated device improves executive function in middle-aged men|journal=Sleep|volume=43|issue=1|doi=10.1093/sleep/zsz197|pmid=31691831|issn=0161-8105|doi-access=free}}

Neural oscillation

Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system.{{cite journal |last1=Buzsáki |first1=György |title=neural oscillation {{!}} Definition, Types, & Synchronization |journal=Encyclopædia Britannica |url=https://www.britannica.com/science/brain-wave-physiology |access-date=7 January 2021 |language=en}} Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.Niedermeyer E. and da Silva F.L., Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. Lippincott Williams & Wilkins, 2004.{{cite web| url=https://capitaldistrictneurofeedback.com/ |title= Capital District Neurofeedback }} Saturday, 30 July 2022

The activity of neurons generate electric currents; and the synchronous action of neural ensembles in the cerebral cortex, comprising large numbers of neurons, produce macroscopic oscillations. These phenomena can be monitored and graphically documented by an electroencephalogram (EEG). The EEG representations of those oscillations are typically denoted by the term 'brainwaves' in common parlance.{{cite journal | vauthors = da Silva FL | year = 1991 | title = Neural mechanisms underlying brain waves: from neural membranes to networks | journal = Electroencephalography and Clinical Neurophysiology | volume = 79 | issue = 2| pages = 81–93 | pmid = 1713832 | doi = 10.1016/0013-4694(91)90044-5 }}{{cite journal | vauthors = Cooper R, Winter A, Crow H, Walter WG | year = 1965 | title = Comparison of subcortical, cortical, and scalp activity using chronically indwelling electrodes in man | journal = Electroencephalography and Clinical Neurophysiology | volume = 18 | issue = 3| pages = 217–230 | doi=10.1016/0013-4694(65)90088-x| pmid = 14255050 }}

The technique of recording neural electrical activity within the brain from electrochemical readings taken from the scalp originated with the experiments of Richard Caton in 1875, whose findings were developed into EEG by Hans Berger in the late 1920s.

Neural oscillation and cognitive functions

The functional role of neural oscillations is still not fully understood;{{cite journal |last1=Llinas|first1=R. R. |title=Intrinsic electrical properties of mammalian neurons and CNS function: a historical perspective |journal=Front Cell Neurosci |volume=8 |page=320 |year=2014 |pmid=25408634 |pmc=4219458 |doi=10.3389/fncel.2014.00320|doi-access=free }} however they have been shown to correlate with emotional responses, motor control, and a number of cognitive functions including information transfer, perception, and memory.{{cite journal | author = Fries P | title = A mechanism for cognitive dynamics: neuronal communication through neuronal coherence | journal = Trends in Cognitive Sciences| volume = 9 | issue = 10 | pages = 474–480 | year = 2005 | doi=10.1016/j.tics.2005.08.011| pmid = 16150631 | s2cid = 6275292 }}{{cite journal | vauthors = Fell J, Axmacher N | title = The role of phase synchronization in memory processes | journal = Nature Reviews Neuroscience | volume = 12 | pages = 105–118 | year = 2011 | doi = 10.1038/nrn2979 | pmid = 21248789 | issue = 2 | s2cid = 7422401 }}{{cite journal | doi = 10.1038/nrn1650 | vauthors = Schnitzler A, Gross J | title = Normal and pathological oscillatory communication in the brain | journal = Nature Reviews Neuroscience | volume = 6 | pages = 285–296 | year = 2005 | pmid=15803160 | issue=4 | s2cid = 2749709 }} Specifically, neural oscillations, in particular theta activity, are extensively linked to memory function, and coupling between theta and gamma activity is considered to be vital for memory functions, including episodic memory.{{cite book | author = Buszaki G | title = Rhythms of the brain | year = 2006 | publisher = Oxford University Press}}{{cite journal|last1=Nyhus|first1=E|author2=Curran T|title=Functional role of gamma and theta oscillations in episodic memory|journal=Neuroscience and Biobehavioral Reviews|date=June 2010|volume=34|issue=7|pages=1023–1035|doi=10.1016/j.neubiorev.2009.12.014|pmid=20060015|pmc=2856712}}{{cite journal | doi = 10.1038/nature08860 | vauthors = Rutishauser U, Ross IB, Mamelak AN, Schuman EM | title = Human memory strength is predicted by theta-frequency phase-locking of single neurons | year = 2010 | journal = Nature | volume=464 | issue = 7290 | pages = 903–907 | pmid = 20336071 | bibcode = 2010Natur.464..903R | s2cid = 4417989 | url = https://authors.library.caltech.edu/18277/2/nature08860-s1.pdf }}

Etymology

Entrainment is a concept first identified by the Dutch physicist Christiaan Huygens in 1665 who discovered the phenomenon during an experiment with pendulum clocks: He set them each in motion and found that when he returned the next day, the sway of their pendulums had all synchronized.{{cite journal | vauthors = Pantaleone J | year = 2002 | title = Synchronization of Metronomes | journal = American Journal of Physics | volume = 70 | issue = 10| pages = 992–1000 | doi=10.1119/1.1501118| bibcode = 2002AmJPh..70..992P }}

Such entrainment occurs because small amounts of energy are transferred between the two systems when they are out of phase in such a way as to produce negative feedback. As they assume a more stable phase relationship, the amount of energy gradually reduces to zero, with systems of greater frequency slowing down, and the other speeding up.Bennett, M., Schatz, M. F., Rockwood, H., and Wiesenfeld, K., Huygens's clocks. Proceedings: Mathematics, Physical and Engineering Sciences, 2002, pp563-579.

The concept of entrainment was developed in complex systems theory. The theory explains the way that two or more independent, autonomous oscillators with differing rhythms or frequencies, when situated in proximity where they can interact for long enough, influence each other mutually, to a degree dependent on coupling force. They then adjust until both oscillate with the same frequency.

Examples include the mechanical entrainment or cyclic synchronization of two electric clothes dryers placed in close proximity, and the biological entrainment evident in the synchronized illumination of fireflies.{{cite journal | vauthors = Néda Z, Ravasz E, Brechet Y, Vicsek T, Barabsi AL | year = 2000 | title = Self-organizing process: The sound of many hands clapping | journal = Nature | volume = 403 | issue = 6772| pages = 849–850 | doi=10.1038/35002660| pmid = 10706271 | arxiv = cond-mat/0003001 | bibcode = 2000Natur.403..849N | s2cid = 4354385 }}

The term 'entrainment' has been used to describe a shared tendency of many physical and biological systems to synchronize their periodicity and rhythm through interaction. This tendency has been identified as specifically pertinent to the study of sound and music generally, and acoustic rhythms specifically. The most familiar examples of neuromotor entrainment to acoustic stimuli is observable in spontaneous foot or finger tapping to the rhythmic beat of a song.

Brainwaves, or neural oscillations, share the fundamental constituents with acoustic and optical waves, including frequency, amplitude and periodicity. Consequently, Huygens' discovery precipitated inquiry{{citation needed|date=March 2016}} into whether or not the synchronous electrical activity of cortical neural ensembles might not only alter in response to external acoustic or optical stimuli but also entrain or synchronize their frequency to that of a specific stimulus.Will, U., and Berg, E., "Brainwave synchronization and entrainment to periodic stimuli" Neuroscience Letters, Vol. 424, 2007, pp 55–60.Cade, G. M. and Coxhead, F., The awakened mind, biofeedback and the development of higher states of awareness. New York, NY: Delacorte Press, 1979.Neher, A., "Auditory driving observed with scalp electrodes in normal subjects. Electroencephalography and Clinical Neurophysiology, Vol. 13, 1961, pp 449–451.Zakharova, N. N., and Avdeev, V. M., "Functional changes in the central nervous system during music perception. Zhurnal vysshei nervnoi deiatelnosti imeni IP Pavlova Vol. 32, No. 5, 1981, pp 915-924.

Brainwave entrainment is a colloquialism for 'neural entrainment',Obleser, J., Kayser, C., "Neural Entrainment and Attentional Selection in the Listening Brain, Trends in Cognitive Sciences, Vol. 23, No. 11, 913-926 which is a term used to denote the way in which the aggregate frequency of oscillations produced by the synchronous electrical activity in ensembles of cortical neurons can adjust to synchronize with the periodic vibration of external stimuli, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds, perceived as rhythm, or of a regularly rhythmically intermittent flashing light.