A gamma wave or gamma Rhythm is a pattern of

neural oscillation Neural oscillations, or brainwaves, are rhythmic or repetitive patterns of neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms within individual neurons or by ...

in humans with a frequency between 25 and 140 Hz, the 40- Hz point being of particular interest. Gamma rhythms are correlated with large scale brain network activity and

cognitive Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought, ...

phenomena such as

working memory Working memory is a cognitive system with a limited capacity that can hold information temporarily. It is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term memory, ...

attention Attention is the behavioral and cognitive process of selectively concentrating on a discrete aspect of information, whether considered subjective or objective, while ignoring other perceivable information. William James (1890) wrote that "Atte ...

, and perceptual grouping, and can be increased in amplitude via

meditation Meditation is a practice in which an individual uses a technique – such as mindfulness, or focusing the mind on a particular object, thought, or activity – to train attention and awareness, and achieve a mentally clear and emotionally cal ...

neurostimulation Neurostimulation is the purposeful modulation of the nervous system's activity using invasive (e.g. microelectrodes) or non-invasive means (e.g. transcranial magnetic stimulation or transcranial electric stimulation, tES, such as tDCS or transcr ...

. Altered gamma activity has been observed in many mood and

disorders such as

Alzheimer's disease Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

, and

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...

Discovery

Gamma waves can be detected by

electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...

magnetoencephalography Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs (su ...

. One of the earliest reports of gamma wave activity was recorded from the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...

of awake monkeys. Subsequently, significant research activity has concentrated on gamma activity in visual cortex. Gamma activity has also been detected and studied across

premotor The premotor cortex is an area of the motor cortex lying within the frontal lobe of the brain just anterior to the primary motor cortex. It occupies part of Brodmann's area 6. It has been studied mainly in primates, including monkeys and humans. ...

, parietal, temporal, and

frontal Front may refer to: Arts, entertainment, and media Films * ''The Front'' (1943 film), a 1943 Soviet drama film * ''The Front'', 1976 film Music *The Front (band), an American rock band signed to Columbia Records and active in the 1980s and ea ...

cortical regions Gamma waves constitute a common class of oscillatory activity in neurons belonging to the

cortico-basal ganglia-thalamo-cortical loop The cortico-basal ganglia-thalamo-cortical loop (CBGTC loop) is a system of neural circuits in the brain. The loop involves connections between the cortex, the basal ganglia, the thalamus, and back to the cortex. It is of particular relevance t ...

. Typically, this activity is understood to reflect

feedforward Feedforward is the provision of context of what one wants to communicate prior to that communication. In purposeful activity, feedforward creates an expectation which the actor anticipates. When expected experience occurs, this provides confirmato ...

connections between distinct brain regions, in contrast to

alpha wave Alpha waves, or the alpha rhythm, are neural oscillations in the frequency range of 8–12 Hz likely originating from the synchronous and coherent (in phase or constructive) electrical activity of thalamic pacemaker cells in humans. Historica ...

feedback across the same regions. 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. Gamma wave activity is most prominent during alert, attentive wakefulness. 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 The scalp is the anatomical area bordered by the human face at the front, and by the neck at the sides and back. Structure The scalp is usually described as having five layers, which can conveniently be remembered as a mnemonic: * S: The ski ...

EEG Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...

, because the frequency band of gamma waves overlaps with the

electromyographic Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an medical instrument, instrument called an electromyograph to produce a record called an electromyog ...

frequency band. Thus, gamma signal recordings could be contaminated by muscle activity. Studies utilizing local muscle paralysis techniques have confirmed that EEG recordings do contain EMG signal, and these signals can be traced to local motor dynamics such as

saccade A saccade ( , French for ''jerk'') is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishi ...

rate or other motor actions involving the head. Advances in signal processing and separation, such as the application of

independent component analysis In signal processing, independent component analysis (ICA) is a computational method for separating a multivariate signal into additive subcomponents. This is done by assuming that at most one subcomponent is Gaussian and that the subcomponents ar ...

or other techniques based on

spatial filtering A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of light or other electromagnetic radiation, typically coherent laser light. Spatial filtering is commonly used to "clean up" the o ...

, have been proposed to reduce the presence of EMG artifacts. In at least some EEG textbooks, we 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. Clinical EEG may not do these things.

Function

Conscious perception

Gamma waves may participate in the formation of coherent, unified

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

, also known as the problem of combination in the

binding problem The consciousness and binding problem is the problem of how objects, background and abstract or emotional features are combined into a single experience. The binding problem refers to the overall encoding of our brain circuits for the combination o ...

, due to their apparent synchronization of neural firing rates across distinct brain regions. Robert Pollack
The Missing Moment
1999 40-Hz gamma waves were first suggested to participate in visual consciousness in 1988, .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 Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was an English molecular biologist, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in deciphering the helical struc ...

and

Christof Koch Christof Koch ( ; born November 13, 1956) is a German-American neurophysiologist and computational neuroscientist best known for his work on the neural basis of consciousness. He is the president and chief scientist of the Allen Institute for B ...

in 1990 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 40-Hz oscillations are a sufficient condition for visual awareness. A number of experiments conducted by

Rodolfo Llinás Rodolfo Llinás Riascos (born 16 December 1934) is a Colombian-born American neuroscientist. He is currently the Thomas and Suzanne Murphy Professor of Neuroscience and Chairman Emeritus of the Department of Physiology & Neuroscience at the NYU ...

supports a hypothesis that the basis for consciousness in awake states and dreaming is 40-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 40-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 40-Hz oscillation seen in wakefulness and in dreaming is proposed to be a correlate of cognition, resultant from coherent 40-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: :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: 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. 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

. Gamma waves are observed as neural synchrony from visual cues in both conscious and

subliminal Subliminal may refer to: * Subliminal stimuli, sensory stimuli below an individual's threshold for conscious perception * Subliminal channel, in cryptography, a covert channel that can be used over an insecure channel * Subliminal (rapper) (born ...

stimuli. This research also sheds light on how neural synchrony may explain

stochastic resonance Stochastic resonance (SR) is a phenomenon in which a signal that is normally too weak to be detected by a sensor, can be boosted by adding white noise to the signal, which contains a wide spectrum of frequencies. The frequencies in the white no ...

in the nervous system.

Clinical relevance

Mood disorders

Altered gamma wave activity is associated with

mood disorder A mood disorder, also known as an affective disorder, is any of a group of conditions of mental and behavioral disorder where a disturbance in the person's mood is the main underlying feature. The classification is in the ''Diagnostic and Stat ...

s such as

major depression Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Introdu ...

bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...

and may be a potential

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...

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 In neuroscience, the default mode network (DMN), also known as the default network, default state network, or anatomically the medial frontoparietal network (M-FPN), is a large-scale brain network primarily composed of the dorsal medial prefro ...

, which is normally suppressed during tasks requiring significant attention. Rodent models of depression-like behaviors also exhibit deficient gamma rhythms.

Schizophrenia

Decreased gamma-wave activity is observed in

. 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. The neurobiological basis of gamma dysfunction in schizophrenia is thought to lie with

GABAergic In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter GABA. For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic neuron produces GABA. A ...

interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...

s involved in known brain wave rhythm-generating networks.

Antipsychotic Antipsychotics, also known as neuroleptics, are a class of Psychiatric medication, psychotropic medication primarily used to manage psychosis (including delusions, hallucinations, paranoia or disordered thought), principally in schizophrenia but ...

treatment, which diminishes some behavioral symptoms of schizophrenia, does not restore gamma synchrony to normal levels.

Epilepsy

Gamma oscillations are observed in the majority of

seizures An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or neural oscillation, synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much o ...

and may contribute to their onset in

. Visual stimuli such as large, high-contrast gratings that are known to trigger seizures in

photosensitive epilepsy Photosensitive epilepsy (PSE) is a form of epilepsy in which seizures are triggered by visual stimuli that form patterns in time or space, such as flashing lights; bold, regular patterns; or regular moving patterns. PSE affects approximately o ...

also drive gamma oscillations in visual cortex. During a focal seizure event, maximal gamma rhythm synchrony of

s is always observed in the seizure onset zone, and synchrony propagates from the onset zone over the whole epileptogenic zone.

Alzheimer's disease

Enhanced gamma band power and lagged gamma responses have been observed in patients with

(AD). Interestingly, the tg APP-PS1 mouse model of AD exhibits decreased gamma oscillation power in the lateral

entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in the ...

, which transmits various sensory inputs to the

hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...

and thus participates in memory processes analogous to those affected by human AD. Decreased hippocampal slow gamma power has also been observed in the 3xTg mouse model of AD. Gamma stimulation may have therapeutic potential for AD and other

neurodegenerative A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

diseases.

Optogenetic Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by expression of light-sensitive ion channels, pumps or enzymes specifically in the target cells. On the level of individ ...

stimulation of fast-spiking

s in the gamma wave frequency range was first demonstrated in mice in 2009. 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, reduces

amyloid beta Amyloid beta (Aβ or Abeta) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid precursor protein (APP), which is ...

load and activates

microglia Microglia are a type of neuroglia (glial cell) located throughout the brain and spinal cord. Microglia account for about 7% of cells found within the brain. As the resident macrophage cells, they act as the first and main form of active immune de ...

in the well-established 5XFAD mouse model of AD. 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. However, the precise molecular and cellular mechanisms by which gamma band stimulation ameliorates AD pathology is unknown.

Fragile X syndrome

Hypersensitivity and memory deficits in

Fragile X syndrome Fragile X syndrome (FXS) is a genetic disorder characterized by mild-to-moderate intellectual disability. The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled. Physical features may ...

may be linked to gamma rhythm abnormalities in

sensory cortex The sensory cortex can refer informally to the primary somatosensory cortex, or it can be used as a term for the primary and secondary cortices of the different senses (two cortices each, on left and right hemisphere): the visual cortex on the occi ...

and

. For example, decreased synchrony of gamma oscillations has been observed in

auditory cortex The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...

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.

Meditation

High-amplitude gamma wave synchrony can be self-induced via

. Long-term practitioners of meditation such as

Tibetan Tibetan may mean: * of, from, or related to Tibet * Tibetan people, an ethnic group * Tibetan language: ** Classical Tibetan, the classical language used also as a contemporary written standard ** Standard Tibetan, the most widely used spoken dial ...

Buddhist Buddhism ( , ), also known as Buddha Dharma and Dharmavinaya (), is an Indian religion or philosophical tradition based on teachings attributed to the Buddha. It originated in northern India as a -movement in the 5th century BCE, and ...

monks exhibit both increased gamma-band activity at baseline as well as significant increases in gamma synchrony during meditation, as determined by scalp EEG. fMRI on the same monks revealed greater activation of right

insular cortex The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian bra ...

and

caudate nucleus The caudate nucleus is one of the structures that make up the corpus striatum, which is a component of the basal ganglia in the human brain. While the caudate nucleus has long been associated with motor processes due to its role in Parkinson's di ...

during meditation. The neurobiological mechanisms of gamma synchrony induction are thus highly

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

. 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 Society for Neuroscience (SfN) is a professional society, headquartered in Washington, DC, for basic scientists and physicians around the world whose research is focused on the study of the brain and nervous system. It is especially well kn ...

, the current

Dalai Lama Dalai Lama (, ; ) is a title given by the Tibetan people to the foremost spiritual leader of the Gelug or "Yellow Hat" school of Tibetan Buddhism, the newest and most dominant of the four major schools of Tibetan Buddhism. The 14th and current Dal ...

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."

External links

EpilepsyHealth.com
- 'A Sampling from Chapter 3' ''Biofeedback, Neurofeedback and Epilepsy'', Sally Fletcher (2005)
Gamma: Insight and Consciousness… Or just Microsaccades?
- A summary of recent research. 2009-06-26.

References

{{DEFAULTSORT:Gamma Wave Electroencephalography Neuropsychology de:Elektroenzephalografie#Gamma-Wellen