Agamma wave orgamma rhythm is a pattern ofneural oscillation in humans with a frequency between 30 and 100 Hz, the 40 Hz point being of particular interest.[1] Gamma waves with frequencies between 30 and 70hertz may be classified aslow gamma, and those between 70 and 150 hertz ashigh gamma. Gamma rhythms are correlated withlarge-scale brain network activity andcognitive phenomena such asworking memory,attention, andperceptual grouping, and can be increased in amplitude viameditation[2] orneurostimulation.[1][3] Altered gamma activity has been observed in manymood andcognitive disorders such asAlzheimer's disease,[4]epilepsy,[5] andschizophrenia.[6]
Gamma waves can be detected byelectroencephalography ormagnetoencephalography. One of the earliest reports of gamma wave activity was recorded from thevisual cortex of awake monkeys.[7] Subsequently, significant research activity has concentrated on gamma activity in visual cortex.[8][9][10][11]
Gamma activity has also been detected and studied acrosspremotor,parietal,temporal, andfrontal cortical regions.[12] Gamma waves constitute a common class of oscillatory activity in neurons belonging to thecortico-basal ganglia-thalamo-cortical loop.[13] Typically, this activity is understood to reflectfeedforward connections between distinct brain regions, in contrast toalpha wave feedback across the same regions.[14] 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.[15] Gamma wave activity is most prominent during alert, attentive wakefulness.[13] However, the mechanisms and substrates by which gamma activity may help to generate different states of consciousness remain unknown.
Some researchers contest the validity or meaningfulness of gamma wave activity detected byscalpEEG, because the frequency band of gamma waves overlaps with theelectromyographic (EMG) frequency band. Thus, gamma signal recordings could be contaminated by muscle activity.[16] Studies utilizing muscle paralysis techniques have confirmed that scalp EEG recordings do contain significant EMG signal,[17][18] and these signals can be traced to local motor dynamics such assaccade rate[19] or other motor actions involving the head. Advances in signal processing and separation, such as the application ofindependent component analysis or other techniques based onspatial filtering, have been proposed to reduce the presence of EMG artifacts.[16]
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.
Gamma waves may participate in the formation of coherent, unifiedperception, also known as the problem of combination in thebinding problem, due to their apparent synchronization of neural firing rates across distinct brain regions.[20][21][22]40 Hz gamma waves were first suggested to participate in visual consciousness in 1988,[23] 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 andChristof Koch in 1990[24] 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 that40 Hz oscillations are a sufficient condition for visual awareness.[25]
A number of experiments conducted byRodolfo Llinás supports a hypothesis that the basis for consciousness in awake states and dreaming is40 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 nonspecific40 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, the40 Hz oscillation seen in wakefulness and in dreaming is proposed to be a correlate of cognition, resultant from coherent40 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 byAndreas K. Engelet al. in the journalConsciousness and Cognition (1999) that argues for temporal synchrony as the basis for consciousness, defines the gamma wave hypothesis thus:[26]
The suggested mechanism is that gamma waves relate to neural consciousness via the mechanism for conscious attention:
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 theprecept [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.[21]
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.[27] This brings adistributed 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 thebinding problem.[20]
Gamma waves are observed asneural synchrony from visual cues in both conscious andsubliminal stimuli.[28][29][30][31] This research also sheds light on how neural synchrony may explainstochastic resonance in the nervous system.[32]
Altered gamma wave activity is associated withmood disorders such asmajor depression orbipolar disorder and may be a potentialbiomarker 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 thedefault mode network, which is normally suppressed during tasks requiring significant attention. Rodent models of depression-like behaviors also exhibit deficient gamma rhythms.[33]
Decreased gamma-wave activity is observed inschizophrenia. Specifically, the amplitude of gamma oscillations is reduced, as is the synchrony of different brain regions involved in tasks such asvisual oddball andGestalt perception. People with schizophrenia perform worse on these behavioral tasks, which relate to perception and continuous recognition memory.[34] The neurobiological basis of gamma dysfunction in schizophrenia is thought to lie withGABAergicinterneurons involved in known brain wave rhythm-generating networks.[35]Antipsychotic treatment, which diminishes some behavioral symptoms of schizophrenia, does not restore gamma synchrony to normal levels.[34]
Gamma oscillations are observed in the majority ofseizures[5] and may contribute to their onset inepilepsy. Visual stimuli such as large, high-contrast gratings that are known to trigger seizures inphotosensitive epilepsy also drive gamma oscillations in visual cortex.[36] During a focal seizure event, maximal gamma rhythm synchrony ofinterneurons is always observed in the seizure onset zone, and synchrony propagates from the onset zone over the whole epileptogenic zone.[37]
Enhanced gamma band power and lagged gamma responses have been observed in patients withAlzheimer's disease (AD).[4][38] Interestingly, thetg APP-PS1 mouse model of AD exhibits decreased gamma oscillation power in the lateralentorhinal cortex, which transmits various sensory inputs to thehippocampus and thus participates in memory processes analogous to those affected by human AD.[39] Decreased hippocampal slow gamma power has also been observed in the 3xTg mouse model of AD.[40]
Gamma stimulation may have therapeutic potential for AD and otherneurodegenerative diseases.Optogenetic stimulation of fast-spikinginterneurons in the gamma-wave frequency range was first demonstrated in mice in 2009.[41] 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,[3] reducesamyloid beta load and activatesmicroglia in the well-established 5XFAD mouse model of AD.[42] 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.[1] However, the precise molecular and cellular mechanisms by which gamma band stimulation ameliorates AD pathology is unknown.
Hypersensitivity and memory deficits due toFragile X syndrome may be linked to gamma rhythm abnormalities in thesensory cortex andhippocampus. For example, decreased synchrony of gamma oscillations has been observed in theauditory 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.[40]
High-amplitude gamma wave synchrony can be self-induced viameditation. Long-term practitioners of meditation such asTibetanBuddhist monks exhibit both increased gamma-band activity at baseline as well as significant increases in gamma synchrony during meditation, as determined by scalp EEG.[2] fMRI on the same monks revealed greater activation of rightinsular cortex andcaudate nucleus during meditation.[43] The neurobiological mechanisms of gamma synchrony induction are thus highlyplastic.[44] 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 theSociety for Neuroscience, the currentDalai 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."[45]
Elevated gamma activity has also been observed in moments precedingdeath.[46]
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