{{Short description|Neural oscillation in the 25–140Hz range}} {{Distinguish|gamma rays}} right|thumb|Gamma waves|400px A '''gamma wave''' or '''gamma rhythm''' is a pattern of neural oscillation in humans with a frequency between 30 and 100 Hz, the 40 Hz point being of particular interest.<ref name="pmid30040729">{{cite journal| author=McDermott B, Porter E, Hughes D, McGinley B, Lang M, O'Halloran M, Jones M.| title=Gamma Band Neural Stimulation in Humans and the Promise of a New Modality to Prevent and Treat Alzheimer's Disease | journal= Journal of Alzheimer's Disease| year= 2018 | volume= 65 | issue= 2 | pages= 363–392 | pmid=30040729 | doi=10.3233/JAD-180391 | pmc=6130417 }}</ref> Gamma waves with frequencies between 30 and 70 hertz may be classified as low gamma, and those between 70 and 150 hertz as high gamma. Gamma rhythms are correlated with large-scale brain network activity and cognitive phenomena such as working memory, attention, and perceptual grouping, and can be increased in amplitude via meditation<ref name="pmid15534199">{{cite journal| author=Lutz A, Greischar LL, Rawlings NB, Ricard M, Davidson RJ| title=Long-term meditators self-induce high-amplitude gamma synchrony during mental practice | journal=Proceedings of the National Academy of Sciences| year=2004 | volume=101 | issue=46 | pages=16369–73 | pmid=15534199 | doi=10.1073/pnas.0407401101 | pmc=526201 | bibcode=2004PNAS..10116369L | doi-access=free }}</ref> or neurostimulation.<ref name="pmid30040729"/><ref name="pmid29493598">{{cite journal| author=Thomson H| title=How flashing lights and pink noise might banish Alzheimer's, improve memory and more | journal=Nature | year=2018 | volume=555 | issue=7694 | pages=20–22 | pmid=29493598 | doi=10.1038/d41586-018-02391-6 | bibcode=2018Natur.555...20T | doi-access=free }}</ref> Altered gamma activity has been observed in many mood and cognitive disorders such as Alzheimer's disease,<ref name="pmid18607528"/> epilepsy,<ref name="Hughes">{{cite journal | pmid = 18439878 | doi=10.1016/j.yebeh.2008.01.011 | volume=13 | issue=1 | title=Gamma, fast, and ultrafast waves of the brain: their relationships with epilepsy and behavior |date=July 2008 | journal= Epilepsy & Behavior| pages=25–31 | author=Hughes JR| s2cid=19484309 }}</ref> and schizophrenia.<ref name="pmid21556334">{{cite journal| author=Jia X, Kohn A| title=Gamma rhythms in the brain | journal= PLOS Biology| year= 2011 | volume= 9 | issue= 4 | article-number= e1001045 | pmid=21556334 | doi=10.1371/journal.pbio.1001045 | pmc=3084194 | doi-access=free }}</ref>
==Discovery== Gamma waves can be detected by electroencephalography or magnetoencephalography. One of the earliest reports of gamma wave activity was recorded from the visual cortex of awake monkeys.<ref name="pmid14282370">{{cite book|author=HUGHES JR|title=International Review of Neurobiology |chapter=Responses from the Visual Cortex of Unanesthetized Monkeys |series=<!-- International Review of Neurobiology--> |year= 1964 |volume=7 |pages= 99–152 |pmid=14282370 |doi=10.1016/s0074-7742(08)60266-4 |isbn=978-0-12-366807-3}}</ref> Subsequently, significant research activity has concentrated on gamma activity in visual cortex.<ref>{{cite journal |last1 = Adjamian |first1 = P |last2 = Holliday |first2 = IE |last3 = Barnes |first3 = GR |last4 = Hillebrand |first4 = A |last5 = Hadjipapas |first5 = A |last6 = Singh |first6 = KD |year = 2004 |title = Induced stimulus-dependent Gamma oscillations in visual stress |journal = European Journal of Neuroscience |volume = 20 |issue = 2|pages = 587–592 |doi=10.1111/j.1460-9568.2004.03495.x|pmid = 15233769 |s2cid = 17082547 }}</ref><ref>{{cite journal |author1=Hadjipapas A. |author2=Adjamian P |author3=Swettenham J.B. |author4=Holliday I.E. |author5=Barnes G.R. |year = 2007 |title = Stimuli of varying spatial scale induce gamma activity with distinct temporal characteristics in human visual cortex |journal = NeuroImage |volume = 35 |issue = 2|pages = 518–30 |doi=10.1016/j.neuroimage.2007.01.002|pmid=17306988 |s2cid=25198757 }}</ref><ref>{{cite journal |vauthors=Muthukumaraswamy SD, Singh KD |year = 2008 |title = Spatiotemporal frequency tuning of BOLD and gamma band MEG responses compared in primary visual cortex |journal = NeuroImage |volume = 40 |issue = 4|pages = 1552–1560 |doi=10.1016/j.neuroimage.2008.01.052 |pmid=18337125|s2cid = 2166982 }}</ref><ref>{{cite journal |vauthors=Swettenham JB, Muthukumaraswamy SD, Singh KD |year = 2009 |title = Spectral properties of induced and evoked gamma oscillations in human early visual cortex to moving and stationary stimuli |journal = Journal of Neurophysiology |volume = 102 |issue = 2|pages = 1241–1253 |doi=10.1152/jn.91044.2008 |pmid=19515947}}</ref>
Gamma activity has also been detected and studied across premotor, parietal, temporal, and frontal cortical regions.<ref name="Kort16">{{cite journal|last1 = Kort |first1 = N |last2 = Cuesta |first2 = P |last3 = Houde |first3 = JF |last4 = Nagarajan |first4 =SS |year = 2016 |title = Bihemispheric network dynamics coordinating vocal feedback control |journal = Human Brain Mapping |volume = 37 |issue = 4 |pages = 1474–1485|doi = 10.1002/hbm.23114|pmid = 26917046 |pmc = 6867418 }}</ref> Gamma waves constitute a common class of oscillatory activity in neurons belonging to the cortico-basal ganglia-thalamo-cortical loop.<ref name="pmid25460069">{{cite journal|author=McCormick DA, McGinley MJ, Salkoff DB|title=Brain state dependent activity in the cortex and thalamus |journal= Current Opinion in Neurobiology|year= 2015 |volume= 31 |pages= 133–40 |pmid=25460069 |doi=10.1016/j.conb.2014.10.003 |pmc=4375098 }}</ref> Typically, this activity is understood to reflect feedforward connections between distinct brain regions, in contrast to alpha wave feedback across the same regions.<ref name="pmid25205811">{{cite journal|author=van Kerkoerle T, Self MW, Dagnino B, Gariel-Mathis MA, Poort J, van der Togt C, Roelfsema PR|title=Alpha and gamma oscillations characterize feedback and feedforward processing in monkey visual cortex |journal= Proceedings of the National Academy of Sciences|year= 2014 |volume= 111 |issue= 40 |pages= 14332–41 |pmid=25205811 |doi=10.1073/pnas.1402773111 |pmc=4210002 |doi-access=free }}</ref> Gamma oscillations have also been shown to correlate with the firing of single neurons, mostly inhibitory neurons, during all states of the wake-sleep cycle.<ref>{{cite journal |author1=Le Van Quyen M. |author2=Muller L.E. |author3=Telenczuk B. |author4=Halgren E. |author5=Cash S. |author6=Hatsopoulos N. |author7=Dehghani N. |author8=Destexhe A. |year = 2016 |title = High-frequency oscillations in human and monkey neocortex during the wake-sleep cycle |journal = Proceedings of the National Academy of Sciences|volume = 113 |issue = 33 |pages = 9363–8 |doi=10.1073/pnas.1523583113 |pmid=27482084 |pmc= 4995938|bibcode=2016PNAS..113.9363L |doi-access=free }}</ref> Gamma wave activity is most prominent during alert, attentive wakefulness.<ref name="pmid25460069"/> However, the mechanisms and substrates by which gamma activity may help to generate different states of consciousness remain unknown.
===Controversy=== Some researchers contest the validity or meaningfulness of gamma wave activity detected by scalp EEG, because the frequency band of gamma waves overlaps with the electromyographic (EMG) frequency band. Thus, gamma signal recordings could be contaminated by muscle activity.<ref name="pmid23596409">{{cite journal |author=Muthukumaraswamy SD |title=High-frequency brain activity and muscle artifacts in MEG/EEG: a review and recommendations |journal= Frontiers in Human Neuroscience|year= 2013 |volume= 7 |page= 138 |pmid=23596409 |doi=10.3389/fnhum.2013.00138 |pmc=3625857 |doi-access=free }}</ref> Studies utilizing muscle paralysis techniques have confirmed that scalp EEG recordings do contain significant EMG signal,<ref name="pmid17574912">{{cite journal |author=Whitham EM |title=Scalp electrical recording during paralysis: quantitative evidence that EEG frequencies above 20 Hz are contaminated by EMG |journal= Clinical Neurophysiology|volume=118 |issue=8 |pages=1877–88 |date=Aug 2007 |pmid=17574912 |doi=10.1016/j.clinph.2007.04.027 |name-list-style=vanc|author2=Pope KJ |author3=Fitzgibbon SP |display-authors=3 |last4=Lewis |first4=T |last5=Clark |first5=C |last6=Loveless |first6=S |last7=Broberg |first7=M |last8=Wallace |first8=A |last9=Delosangeles |first9=D|s2cid=237761 }}</ref><ref name="pmid18329954">{{cite journal |author=Whitham EM |title=Thinking activates EMG in scalp electrical recordings |journal=Clinical Neurophysiology|volume=119 |issue=5 |pages=1166–75 |date=May 2008 |pmid=18329954 |doi=10.1016/j.clinph.2008.01.024 |name-list-style=vanc|author2=Lewis T |author3=Pope KJ |display-authors=3 |last4=Fitzgibbon |first4=Sean P. |last5=Clark |first5=C. Richard |last6=Loveless |first6=Stephen |last7=Delosangeles |first7=Dylan |last8=Wallace |first8=Angus K. |last9=Broberg |first9=Marita|s2cid=28597711 }}</ref> and these signals can be traced to local motor dynamics such as saccade rate<ref name="pmid18466752">{{cite journal |vauthors=Yuval-Greenberg S, Tomer O, Keren AS, Nelken I, Deouell LY |title=Transient induced gamma-band response in EEG as a manifestation of miniature saccades |journal=Neuron |volume=58 |issue=3 |pages=429–41 |date=May 2008 |pmid=18466752 |doi=10.1016/j.neuron.2008.03.027|doi-access=free }}</ref> or other motor actions involving the head. Advances in signal processing and separation, such as the application of independent component analysis or other techniques based on spatial filtering, have been proposed to reduce the presence of EMG artifacts.<ref name="pmid23596409"/>
In at least some EEG textbooks, users are instructed to put an electrode on an eyelid to catch these, as well as 1 on the heart, & a pair on the sides of the neck, to catch muscle-signal from the body below the neck.
==Function== ===Conscious perception=== [[File:Development-of-grouped-icEEG-for-the-study-of-cognitive-processing-Video1.ogv|thumb|320px|alt="intracranial EEG movie shows increased gamma power in cortical regions associated with object-naming task"|Electrocorticographic movie showing changes in high-frequency broadband gamma activity in specific cortical regions when visual stimuli are presented during a face-/place-naming task]]
Gamma waves may participate in the formation of coherent, unified perception, also known as the problem of combination in the binding problem, due to their apparent synchronization of neural firing rates across distinct brain regions.<ref name=buzsaki/><ref name=pollack>Robert Pollack, [http://www.cse.iitk.ac.in/~amit/books/pollack-1999-missing-moment-how.html The Missing Moment], 1999</ref><ref name=singer>{{cite journal | last1 = Singer | first1 = W. | last2 = Gray | first2 = C.M. | year = 1995 | title = Visual feature integration and the temporal correlation hypothesis | journal = Annual Review of Neuroscience| volume = 18 | pages = 555–586 | doi=10.1146/annurev.ne.18.030195.003011 | pmid=7605074| citeseerx = 10.1.1.308.6735 }}</ref> {{val|40|u=Hz}} gamma waves were first suggested to participate in visual consciousness in 1988,<ref name=gold>{{cite journal| doi = 10.1006/ccog.1999.0399| author = Ian Gold| title = Does 40-Hz oscillation play a role in visual consciousness?| journal = Consciousness and Cognition| volume = 8| issue = 2| pages = 186–195| year = 1999|pmid = 10448001| s2cid = 8703711| doi-access = free}}</ref> e.g. two neurons oscillate synchronously (though they are not directly connected) when a single external object stimulates their respective receptive fields. Subsequent experiments by many others demonstrated this phenomenon in a wide range of visual cognition. In particular, Francis Crick and Christof Koch in 1990<ref>Crick, F., & Koch, C. (1990b). Towards a neurobiological theory of consciousness. Seminars in the Neurosciences v.2, 263-275.</ref> argued that there is a significant relation between the binding problem and the problem of visual consciousness and, as a result, that synchronous 40 Hz oscillations may be causally implicated in visual awareness as well as in visual binding. Later the same authors expressed skepticism over the idea that {{val|40|u=Hz}} oscillations are a sufficient condition for visual awareness.<ref>{{cite journal|author = Crick, F., Koch, C.|title = Framework for consciousness|year = 2003|journal = Nature Neuroscience|volume = 6|issue = 2|doi = 10.1038/nn0203-119 | url= https://zenodo.org/record/852680|pmid=12555104|pages=119–26|s2cid = 13960489}}</ref>
A number of experiments conducted by Rodolfo Llinás supports a hypothesis that the basis for consciousness in awake states and dreaming is {{val|40|u=Hz}} oscillations throughout the cortical mantle in the form of thalamocortical iterative recurrent activity. In two papers entitled "Coherent 40-Hz oscillation characterizes dream state in humans" (Rodolfo Llinás and Urs Ribary, Proc Natl Acad Sci USA 90:2078-2081, 1993) and "Of dreaming and wakefulness" (Llinas & Pare, 1991), Llinás proposes that the conjunction into a single cognitive event could come about by the concurrent summation of specific and nonspecific {{val|40|u=Hz}} activity along the radial dendritic axis of given cortical elements, and that the resonance is modulated by the brainstem and is given content by sensory input in the awake state and intrinsic activity during dreaming. According to Llinás' hypothesis, known as the thalamocortical dialogue hypothesis for consciousness, the {{val|40|u=Hz}} oscillation seen in wakefulness and in dreaming is proposed to be a correlate of cognition, resultant from coherent {{val|40|u=Hz}} resonance between thalamocortical-specific and nonspecific loops. In Llinás & Ribary (1993), the authors propose that the specific loops give the content of cognition, and that a nonspecific loop gives the temporal binding required for the unity of cognitive experience.
A lead article by Andreas K. Engel ''et al''. in the journal ''Consciousness and Cognition'' (1999) that argues for temporal synchrony as the basis for consciousness, defines the gamma wave hypothesis thus:<ref>{{cite journal |author1=Andreas K. Engel |author2=Pascal Fries |author3=Peter Koenig |author4=Michael Brecht |author5=Wolf Singer |title = Temporal Binding, Binocular Rivalry, and Consciousness | journal = Consciousness and Cognition | volume = 8 | issue = 2 | year = 1999 | doi = 10.1006/ccog.1999.0389 |pmid=10447995 | pages=128–151 |citeseerx=10.1.1.207.8191 |s2cid=15376936 }}</ref> :The hypothesis is that synchronization of neuronal discharges can serve for the integration of distributed neurons into cell assemblies and that this process may underlie the selection of perceptually and behaviorally relevant information.
===Attention=== The suggested mechanism is that gamma waves relate to neural consciousness via the mechanism for conscious attention: {{blockquote|The proposed answer lies in a wave that, originating in the thalamus, sweeps the brain from front to back, 40 times per second, drawing different neuronal circuits into synch with the precept [sic], and thereby bringing the precept [sic] into the attentional foreground. If the thalamus is damaged even a little bit, this wave stops, conscious awarenesses do not form, and the patient slips into profound coma.<ref name=pollack/>}}
Thus the claim is that when all these neuronal clusters oscillate together during these transient periods of synchronized firing, they help bring up memories and associations from the visual percept to other notions.<ref>{{Cite journal|last1=Baldauf|first1=D.|last2=Desimone|first2=R.|date=2014-04-25|title=Neural Mechanisms of Object-Based Attention|journal=Science|language=en|volume=344|issue=6182|pages=424–427|doi=10.1126/science.1247003|pmid=24763592|bibcode=2014Sci...344..424B|s2cid=34728448|issn=0036-8075|doi-access=free}}</ref> This brings a distributed matrix of cognitive processes together to generate a coherent, concerted cognitive act, such as perception. This has led to theories that gamma waves are associated with solving the binding problem.<ref name=buzsaki>{{cite book |author=Buzsaki, György |title=Rhythms of the brain |publisher=Oxford |year=2006|chapter=Cycle 9, The Gamma Buzz|isbn=978-0-19-530106-9}}</ref> <!-- Recent studies have shown that recognition of new insights occur when frequencies shift from 20 to 40 Hz. : Pls cite source or drop. -->
Gamma waves are observed as neural synchrony from visual cues in both conscious and subliminal stimuli.<ref>{{cite journal |vauthors=Melloni L, Molina C, Pena M, Torres D, Singer W, Rodriguez E |title=Synchronization of neural activity across cortical areas correlates with conscious perception |journal= Journal of Neuroscience|volume=27 |issue=11 |pages=2858–65 |date=Mar 2007 |pmid=17360907 |doi=10.1523/JNEUROSCI.4623-06.2007 |pmc=6672558 }}</ref><ref>{{cite journal |vauthors=Siegel M, Donner TH, Oostenveld R, Fries P, Engel AK |title=Neuronal synchronization along the dorsal visual pathway reflects the focus of spatial attention |journal=Neuron |volume=60 |issue=4 |pages=709–719 |date=Mar 2008 |doi=10.1016/j.neuron.2008.09.010 |pmid=19038226|doi-access=free |hdl=2066/71012 |hdl-access=free }}</ref><ref>{{cite journal |vauthors=Gregoriou GG, Gotts SJ, Zhou H, Desimone R |title=High-frequency, long-range coupling between prefrontal and visual cortex during attention |journal=Science |volume=324 |issue=5931 |pages=1207–1210 |date=Mar 2009 |doi=10.1126/science.1171402 |bibcode=2009Sci...324.1207G |pmid=19478185 |pmc=2849291}}</ref><ref>{{cite journal |vauthors=Baldauf D, Desimone R |title=Neural mechanisms of object-based attention |journal=Science |volume=344 |issue=6182 |pages=424–427 |date=Mar 2014 |pmid=24763592 |doi=10.1126/science.1247003 |bibcode = 2014Sci...344..424B |s2cid=34728448 |doi-access=free }}</ref> This research also sheds light on how neural synchrony may explain stochastic resonance in the nervous system.<ref>{{cite journal |vauthors=Ward LM, Doesburg SM, Kitajo K, MacLean SE, Roggeveen AB |title=Neural synchrony in stochastic resonance, attention, and consciousness |journal= Canadian Journal of Experimental Psychology |volume=60 |issue=4 |pages=319–26 |date=Dec 2006 |pmid=17285879 |doi=10.1037/cjep2006029 }}</ref>
==Clinical relevance== ===Mood disorders=== Altered gamma wave activity is associated with mood disorders such as major depression or bipolar disorder and may be a potential biomarker to differentiate between unipolar and bipolar disorders. For example, human subjects with high depression scores exhibit differential gamma signaling when performing emotional, spatial, or arithmetic tasks. Increased gamma signaling is also observed in brain regions that participate in the default mode network, which is normally suppressed during tasks requiring significant attention. Rodent models of depression-like behaviors also exhibit deficient gamma rhythms.<ref name="pmid30181587">{{cite journal|author=Fitzgerald PJ, Watson BO|title=Gamma oscillations as a biomarker for major depression: an emerging topic |journal= Translational Psychiatry|year= 2018 |volume= 8 |issue= 1 |article-number= 177 |pmid=30181587 |doi=10.1038/s41398-018-0239-y |pmc=6123432 }}</ref>
===Schizophrenia=== Decreased gamma-wave activity is observed in schizophrenia. Specifically, the amplitude of gamma oscillations is reduced, as is the synchrony of different brain regions involved in tasks such as visual oddball and Gestalt perception. People with schizophrenia perform worse on these behavioral tasks, which relate to perception and continuous recognition memory.<ref name="BowerSciNews">{{cite journal | author=Bruce Bower | title=Synchronized thinking. Brain activity linked to schizophrenia, skillful meditation | journal=Science News | volume=166 | issue=20 | year=2004 | page=310 | doi=10.2307/4015767 | jstor=4015767 }}</ref> The neurobiological basis of gamma dysfunction in schizophrenia is thought to lie with GABAergic interneurons involved in known brain wave rhythm-generating networks.<ref name="pmid20087360">{{cite journal| author=Uhlhaas PJ, Singer W| title=Abnormal neural oscillations and synchrony in schizophrenia| journal=Nature Reviews Neuroscience| year= 2010 | volume= 11 | issue= 2 | pages= 100–13 | pmid=20087360 | doi=10.1038/nrn2774 | s2cid=205505539}}</ref> Antipsychotic treatment, which diminishes some behavioral symptoms of schizophrenia, does not restore gamma synchrony to normal levels.<ref name="BowerSciNews"/>
===Epilepsy=== Gamma oscillations are observed in the majority of seizures<ref name="Hughes"/> and may contribute to their onset in epilepsy. Visual stimuli such as large, high-contrast gratings that are known to trigger seizures in photosensitive epilepsy also drive gamma oscillations in visual cortex.<ref name="pmid28486114">{{cite journal| author=Hermes D, Kasteleijn-Nolst Trenité DGA, Winawer J| title=Gamma oscillations and photosensitive epilepsy | journal= Current Biology| year= 2017 | volume= 27 | issue= 9 | pages= R336–R338 | pmid=28486114 | doi=10.1016/j.cub.2017.03.076 | pmc=5438467 | bibcode=2017CBio...27.R336H }}</ref> During a focal seizure event, maximal gamma rhythm synchrony of interneurons is always observed in the seizure onset zone, and synchrony propagates from the onset zone over the whole epileptogenic zone.<ref name="pmid28839247">{{cite journal| author=Sato Y, Wong SM, Iimura Y, Ochi A, Doesburg SM, Otsubo H| title=Spatiotemporal changes in regularity of gamma oscillations contribute to focal ictogenesis | journal=Scientific Reports| year= 2017 | volume= 7 | issue= 1 | page= 9362 | pmid=28839247 | doi=10.1038/s41598-017-09931-6 | pmc=5570997 | bibcode=2017NatSR...7.9362S }}</ref>
===Alzheimer's disease=== Enhanced gamma band power and lagged gamma responses have been observed in patients with Alzheimer's disease (AD).<ref name="pmid18607528">{{cite journal| author=van Deursen JA, Vuurman EF, Verhey FR, van Kranen-Mastenbroek VH, Riedel WJ| title=Increased EEG gamma band activity in Alzheimer's disease and mild cognitive impairment | journal= Journal of Neural Transmission| year= 2008 | volume= 115 | issue= 9 | pages= 1301–11 | pmid=18607528 | doi=10.1007/s00702-008-0083-y | pmc=2525849 }}</ref><ref name="pmid26937378">{{cite journal| author=Başar E, Emek-Savaş DD, Güntekin B, Yener GG| title=Delay of cognitive gamma responses in Alzheimer's disease | journal=NeuroImage: Clinical | year= 2016 | volume= 11 | pages= 106–115 | pmid=26937378 | doi=10.1016/j.nicl.2016.01.015 | pmc=4753813 }}</ref> Interestingly, the tg APP-PS1 mouse model of AD exhibits decreased gamma oscillation power in the lateral entorhinal cortex, which transmits various sensory inputs to the hippocampus and thus participates in memory processes analogous to those affected by human AD.<ref name="pmid27833535">{{cite journal| author=Klein AS, Donoso JR, Kempter R, Schmitz D, Beed P| title=Early Cortical Changes in Gamma Oscillations in Alzheimer's Disease| journal= Frontiers in Systems Neuroscience| year= 2016 | volume= 10 | page= 83 | pmid=27833535 | doi=10.3389/fnsys.2016.00083 | pmc=5080538 | doi-access=free}}</ref> Decreased hippocampal slow gamma power has also been observed in the 3xTg mouse model of AD.<ref name="pmid30121451"/>
Gamma stimulation may have therapeutic potential for AD and other neurodegenerative diseases. Optogenetic stimulation of fast-spiking interneurons in the gamma-wave frequency range was first demonstrated in mice in 2009.<ref>{{Cite journal|doi=10.1038/nature08002|title=Driving fast-spiking cells induces gamma rhythm and controls sensory responses|year=2009|last1=Cardin|first1=Jessica A.|last2=Carlén|first2=Marie|last3=Meletis|first3=Konstantinos|last4=Knoblich|first4=Ulf|last5=Zhang|first5=Feng|last6=Deisseroth|first6=Karl|last7=Tsai|first7=Li-Huei|last8=Moore|first8=Christopher I.|journal=Nature|volume=459|issue=7247|pages=663–667|pmid=19396156|pmc=3655711|bibcode=2009Natur.459..663C}}</ref> Entrainment or synchronization of hippocampal gamma oscillations and spiking to 40 Hz via non-invasive stimuli in the gamma-frequency band, such as flashing lights or pulses of sound,<ref name="pmid29493598"/> reduces amyloid beta load and activates microglia in the well-established 5XFAD mouse model of AD.<ref>{{cite journal|last1=Iaccarino|first1=Hannah F.|last2=Singer|first2=Annabelle C.|last3=Martorell|first3=Anthony J.|last4=Rudenko|first4=Andrii|last5=Gao|first5=Fan|last6=Gillingham|first6=Tyler Z.|last7=Mathys|first7=Hansruedi|last8=Seo|first8=Jinsoo|last9=Kritskiy|first9=Oleg|last10=Abdurrob|first10=Fatema|last11=Adaikkan|first11=Chinnakkaruppan|last12=Canter|first12=Rebecca G.|last13=Rueda|first13=Richard|last14=Brown|first14=Emery N.|last15=Boyden|first15=Edward S.|last16=Tsai|first16=Li-Huei|title=Gamma frequency entrainment attenuates amyloid load and modifies microglia|journal=Nature|date=7 December 2016|volume=540|issue=7632|pages=230–235|doi=10.1038/nature20587|pmid=27929004|pmc=5656389|bibcode=2016Natur.540..230I}}</ref> Subsequent human clinical trials of gamma band stimulation have shown mild cognitive improvements in AD patients who have been exposed to light, sound, or tactile stimuli in the 40 Hz range.<ref name="pmid30040729"/> However, the precise molecular and cellular mechanisms by which gamma band stimulation ameliorates AD pathology is unknown.
===Fragile X syndrome=== Hypersensitivity and memory deficits due to Fragile X syndrome may be linked to gamma rhythm abnormalities in the sensory cortex and hippocampus. For example, decreased synchrony of gamma oscillations has been observed in the auditory cortex of FXS patients. The FMR1 knockout rat model of FXS exhibits an increased ratio of slow (~25–50 Hz) to fast (~55–100 Hz) gamma waves.<ref name="pmid30121451">{{cite journal| author=Mably AJ, Colgin LL| title=Gamma oscillations in cognitive disorders | journal= Current Opinion in Neurobiology| year= 2018 | volume= 52 |pages= 182–187 | pmid=30121451 | doi=10.1016/j.conb.2018.07.009 | pmc=6139067 }}</ref>
==Other functions==
=== Meditation === High-amplitude gamma wave synchrony can be self-induced via meditation. Long-term practitioners of meditation such as Tibetan Buddhist monks exhibit both increased gamma-band activity at baseline as well as significant increases in gamma synchrony during meditation, as determined by scalp EEG.<ref name="pmid15534199"/> fMRI on the same monks revealed greater activation of right insular cortex and caudate nucleus during meditation.<ref name="dalailama">{{cite web|url=https://www.dalailama.com/news/2007/how-thinking-can-change-the-brain |title=How Thinking Can Change the Brain|publisher=The Office of His Holiness the Dalai Lama|date=2007-01-29|author=Sharon Begley|access-date=2019-12-16}}</ref> The neurobiological mechanisms of gamma synchrony induction are thus highly plastic.<ref>{{cite news| url=https://www.washingtonpost.com/wp-dyn/articles/A43006-2005Jan2.html | newspaper=The Washington Post | title=Meditation Gives Brain a Charge, Study Finds | first=Marc | last=Kaufman | date=January 3, 2005 | access-date=May 3, 2010}}</ref> This evidence may support the hypothesis that one's sense of consciousness, stress management ability, and focus, often said to be enhanced after meditation, are all underpinned by gamma activity. At the 2005 annual meeting of the Society for Neuroscience, the current Dalai Lama commented that if neuroscience could propose a way to induce the psychological and biological benefits of meditation without intensive practice, he "would be an enthusiastic volunteer."<ref>{{cite web|author=Reiner PB |date= 2009-05-26| publisher=Scientific American| title = Meditation On Demand | url =http://www.scientificamerican.com/article.cfm?id=meditation-on-demand|access-date=2019-12-16}}</ref>
=== Death === Elevated gamma activity has also been observed in the brain around the time of death. In a 2013 study, electroencephalography (EEG) recordings from rats undergoing experimental cardiac arrest revealed a transient surge of synchronized gamma oscillations within roughly the first 30 seconds after the heart stopped, preceding the isoelectric ("flat") EEG. This activity was global across the cortex and highly coherent, and it showed cross-frequency coupling and directed connectivity at levels exceeding the normal waking state.<ref name="Borjigin2013">{{cite journal |last1=Borjigin |first1=Jimo |last2=Lee |first2=UnCheol |last3=Liu |first3=Tiecheng |last4=Pal |first4=Dinesh |last5=Huff |first5=Sean |last6=Klarr |first6=Daniel |last7=Sloboda |first7=Jennifer |last8=Hernandez |first8=Jason |last9=Wang |first9=Michael M. |last10=Mashour |first10=George A. |title=Surge of neurophysiological coherence and connectivity in the dying brain |journal=Proceedings of the National Academy of Sciences |date=27 August 2013 |volume=110 |issue=35 |pages=14432–14437 |doi=10.1073/pnas.1308285110 |doi-access=free |pmid=23940340 |pmc=3761619 |bibcode=2013PNAS..11014432B }}</ref>
A 2023 study extended these findings to humans by analyzing EEG and electrocardiogram signals from four comatose patients before and after the withdrawal of ventilatory support. Two of the four patients exhibited a rapid and marked surge of gamma power following the onset of global hypoxia—increasing by a factor ranging from roughly 2- to 391-fold relative to baseline—together with increased cross-frequency coupling of gamma waves with slower oscillations and increased interhemispheric functional and directed connectivity in the gamma band. The activity was concentrated in the temporo-parieto-occipital junction, a posterior region sometimes described as a "hot zone" thought to be important for conscious processing.<ref name="Xu2023">{{cite journal |last1=Xu |first1=Gang |last2=Mihaylova |first2=Temenuzhka |last3=Li |first3=Duan |last4=Tian |first4=Fangyun |last5=Farrehi |first5=Peter M. |last6=Parent |first6=Jack M. |last7=Mashour |first7=George A. |last8=Wang |first8=Michael M. |last9=Borjigin |first9=Jimo |title=Surge of neurophysiological coupling and connectivity of gamma oscillations in the dying human brain |journal=Proceedings of the National Academy of Sciences |date=9 May 2023 |volume=120 |issue=19 |article-number=e2216268120 |doi=10.1073/pnas.2216268120 |doi-access=free |pmid=37126719 |pmc=10175832 |bibcode=2023PNAS..12016268X}}</ref>
Because gamma oscillations are associated with conscious awareness, the authors proposed that these surges may represent a neural correlate of covert consciousness, potentially relevant to the vivid near-death experiences reported by some survivors of cardiac arrest. They cautioned against strong conclusions: the sample was very small, the two patients who showed the surge both had a documented history of seizures (though none occurred in the hours before death), and because the patients did not survive, the observed activity could not be correlated with any reported subjective experience.<ref name="Xu2023" /> Earlier commentators had similarly argued that comparable activity in animals, while intriguing, was unlikely on its own to account for near-death experiences.<ref> {{cite journal |last1=Greyson |first1=Bruce |last2=Kelly |first2=Edward F. |last3=Dunseath |first3=W. J. Ross |title=Surge of neurophysiological activity in the dying brain |journal=Proceedings of the National Academy of Sciences |date=2013 |volume=110 |issue=47 |pages=E4405 |doi=10.1073/pnas.1316937110 |doi-access=free |pmid=24198339 |pmc=3839773 |bibcode=2013PNAS..110E4405G }}</ref>
==See also==
===Brain waves=== * Delta wave – (0.1 – 4 Hz) * Theta wave – (4 – 7 Hz) * Mu wave – (7.5 – 12.5 Hz) * SMR wave – (12.5 – 15.5 Hz) * Alpha wave – (7 (or 8) – 12 Hz) * Beta wave – (12 – 30 Hz) * Gamma wave – (32 – 100 Hz) * High-frequency oscillations – (over ~80 Hz)
==External links== * [http://www.epilepsyhealth.com/biofeedback.html EpilepsyHealth.com] – 'A Sampling from Chapter 3' ''Biofeedback, Neurofeedback and Epilepsy'', Sally Fletcher (2005) * [http://scienceblogs.com/developingintelligence/2009/06/26/gamma-insight-and-consciousnes/ Gamma: Insight and Consciousness… Or just Microsaccades?] – A summary of recent research. 2009-06-26.
==References== {{Reflist|30em}}
{{EEG}} {{SleepSeries2}} {{DEFAULTSORT:Gamma Wave}} Category:Electroencephalography Category:Neuropsychology