Gamma wave
A gamma wave or gamma rhythm is a pattern of neural oscillation in humans with a frequency between 25 and 140 Hz, the 40 Hz point being of particular interest.[1] 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[2] or neurostimulation.[1][3] Altered gamma activity has been observed in many mood and cognitive disorders such as Alzheimer's disease,[4] epilepsy,[5] and schizophrenia.[6]
Not to be confused with gamma rays.Clinical relevance[edit]
Mood disorders[edit]
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.[33]
Schizophrenia[edit]
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.[34] The neurobiological basis of gamma dysfunction in schizophrenia is thought to lie with GABAergic interneurons 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]
Epilepsy[edit]
Gamma oscillations are observed in the majority of seizures[5] 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.[36] 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.[37]
Alzheimer's disease[edit]
Enhanced gamma band power and lagged gamma responses have been observed in patients with Alzheimer's disease (AD).[4][38] 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.[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 other neurodegenerative diseases. Optogenetic stimulation of fast-spiking interneurons 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] reduces amyloid beta load and activates microglia 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.
Fragile X syndrome[edit]
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.[40]
Other functions[edit]
Meditation[edit]
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.[2] fMRI on the same monks revealed greater activation of right insular cortex and caudate nucleus during meditation.[43] The neurobiological mechanisms of gamma synchrony induction are thus highly plastic.[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 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."[45]