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 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, 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 calm ...
or
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 trans ...
.
Altered gamma activity has been observed in many
mood and
cognitive disorders such as
Alzheimer's disease,
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 electrica ...
,
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 wit ...
.
Discovery
Gamma waves can be detected by
electroencephalography or
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 (s ...
. 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 e ...
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 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 instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...
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 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 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 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 ...
.
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 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 St ...
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 ...
or 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, p ...
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.
Schizophrenia
Decreased gamma-wave activity is observed in 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 wit ...
. 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 interneurons involved in known brain wave rhythm-generating networks. Antipsychotic
Antipsychotics, also known as neuroleptics, are a class of psychotropic medication primarily used to manage psychosis (including delusions, hallucinations, paranoia or disordered thought), principally in schizophrenia but also in a range of ...
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 synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with l ...
and may contribute to their onset in 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 electrica ...
. 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 interneurons 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 Alzheimer's disease (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, ...
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 stimulation of fast-spiking interneurons 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 load and activates microglia 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 may be linked to gamma rhythm abnormalities in sensory cortex and 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, ...
. 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 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 calm ...
. 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 b ...
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 d ...
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 D ...
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."
See also
Brain waves
* Delta wave
Delta waves are high amplitude neural oscillations with a frequency between 0.5 and 4 hertz. Delta waves, like other brain waves, can be recorded with electroencephalography (EEG) and are usually associated with the deep stage 3 of NREM sleep, ...
– (0.1 – 3 Hz)
* Theta wave
Theta waves generate the theta rhythm, a neural oscillation in the brain that underlies various aspects of cognition and behavior, including learning, memory, and spatial navigation in many animals. It can be recorded using various electrophysi ...
– (4 – 7 Hz)
* Mu wave
The sensorimotor mu rhythm, also known as mu wave, comb or wicket rhythms or arciform rhythms, are synchronized patterns of electrical activity involving large numbers of neurons, probably of the pyramidal type, in the part of the brain that c ...
– (7.5 – 12.5 Hz)
* SMR wave – (12.5 – 15.5 Hz)
* 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 ...
– (7 (or 8) – 12 Hz)
* Beta wave
Beta waves, or beta rhythm, are a neural oscillation (brainwave) in the brain with a frequency range of between 12.5 and 30 Hz (12.5 to 30 cycles per second). Beta waves can be split into three sections: Low Beta Waves (12.5–16 Hz, " ...
– (12 – 30 Hz)
* Gamma wave – (32 – 100 Hz)
*High frequency oscillations
High-frequency oscillations (HFO) are brain waves of the frequency faster than ~80 Hz, generated by neuronal cell population. High-frequency oscillations can be recorded during an electroencephalagram (EEG), local field potential (LFP) or elec ...
– (over ~80 Hz)
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