Gamma wave
Encyclopedia
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, though 40 Hz is prototypical.
According to a popular theory, gamma waves may be implicated in creating the unity of conscious perception (the binding problem
Binding problem
The binding problem is one of a number of terms at the interface between neuroscience and philosophy which suffer from being used in several different ways, often in a context that does not explicitly indicate which way the term is being used. Of the many possible usages, two common versions may be...
). However, there is no agreement on the theory; as a researcher suggests:
- Whether or not gamma wave activity is related to subjective awareness is a very difficult question which cannot be answered with certainty at the present time.
History
Gamma waves were initially ignored before the development of digital electroencephalographyElectroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
as analog electroencephalography is restricted to recording and measuring rhythms that are usually less than 25 Hz. One of the earliest reports on them was in 1964 using recordings of the electrical activity of electrodes implanted in the visual cortex of awake monkeys.
History of idea
The idea that distinct regions in the brain were being stimulated simultaneously was suggested by the finding in 1988 that 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 Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
and Christof Koch
Christof Koch
Christof Koch is an American neuroscientist working on the neural basis of consciousness. He is the Lois and Victor Troendle Professor of Cognitive and Behavioral Biology at California Institute of Technology, where he has been since 1986...
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.
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.
Role in attentive focus
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 the precept, and thereby bringing the precept 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.
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 precept 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
Binding problem
The binding problem is one of a number of terms at the interface between neuroscience and philosophy which suffer from being used in several different ways, often in a context that does not explicitly indicate which way the term is being used. Of the many possible usages, two common versions may be...
.
Gamma waves are observed as neural synchrony
Neural oscillations
Neural oscillation is rhythmic or repetitive neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms localized within individual neurons or by interactions between neurons...
from visual cues in both conscious and subliminal
Unconscious mind
The unconscious mind is a term coined by the 18th century German romantic philosopher Friedrich Schelling and later introduced into English by the poet and essayist Samuel Taylor Coleridge...
stimuli. This research also sheds light on how neural synchrony may explain stochastic resonance
Stochastic resonance
Stochastic resonance is a phenomenon that occurs in a threshold measurement system when an appropriate measure of information transfer is maximized in the presence of a non-zero level of stochastic input noise thereby lowering the response...
in the nervous system. They are also implicated in REM sleep, which involves visualizations, and also during anesthesia.
Contemporary research
A 2009 study published in Nature successfully induced gamma waves in mice brains. Researchers performed this study using optogenetics (the method of combining genetic engineering with light to manipulate the activity of individual nerve cells). The protein channelrhodopsin-2 (ChR2), which sensitizes cells to light, was genetically engineered into these mice, specifically to be expressed in a target-group of interneurons. These fast-spiking (FS) interneurons, known for high electrical activity, were then activated with an optical fiber and laser—the second step in optogenetics. In this way, the cell activity of these interneurons was manipulated in the frequency range of 8–200 Hz. The study produced empirical evidence of gamma wave induction in the approximate interval of 25–100 Hz. The gamma waves were most apparent at a frequency of 40 Hz; this indicates that the gamma waves evoked by FS manipulation are a resonating brain circuit property. This is the first study in which it's been shown that a brain state can be induced through the activation of a specific group of cells.Relation to meditation
Experiments on TibetanTibetan people
The Tibetan people are an ethnic group that is native to Tibet, which is mostly in the People's Republic of China. They number 5.4 million and are the 10th largest ethnic group in the country. Significant Tibetan minorities also live in India, Nepal, and Bhutan...
Buddhist monks have shown a correlation between transcendental mental states and gamma waves.
A suggested explanation is based on the fact that the gamma is intrinsically localized. Neuroscientist Sean O'Nuallain suggests that this very existence of synchronized gamma indicates that something akin to a singularity - or, to be more prosaic, a conscious experience - is occurring. This work adduces experimental and simulated data to show that what meditation masters have in common is the ability to put the brain into a state in which it is maximally sensitive and consumes power at a lower (or even zero) rate, briefly. The "Zero power hypothesis" suggests that the lower power states may correspond to a "selfless" state and the more typical non-zero state, in which gamma is not so prominent, corresponds to a state of empirical self.
As hinted above, gamma waves have been observed in Tibetan Buddhist monks. A 2004 study took eight long-term Tibetan Buddhist practitioners of meditation and, using electrodes, monitored the patterns of electrical activity produced by their brains as they meditated. The researchers compared the brain activity of the monks to a group of novice meditators (the study had these subjects meditate an hour a day for one week prior to empirical observation). In a normal meditative state, both groups were shown to have similar brain activity. However, when the monks were told to generate an objective feeling of compassion during meditation, their brain activity began to fire in a rhythmic, coherent manner, suggesting neuronal structures were firing in harmony. This was observed at a frequency of 25–40 Hz, the rhythm of gamma waves. These gamma-band oscillations in the monk’s brain signals were the largest seen in humans (apart from those in states such as seizures). Conversely, these gamma-band oscillations were scant in novice meditators. Though, a number of rhythmic signals did appear to strengthen in beginner meditators with further experience in the exercise, implying that the aptitude for one to produce gamma-band rhythm is trainable.
Such evidence and research in gamma-band oscillations may explain the heightened sense of consciousness, bliss, and intellectual acuity subsequent to meditation. Notably, meditation is known to have a number of health benefits: stress reduction, mood elevation, and increased life expectancy of the mind and its cognitive functions. The current Dalai Lama
14th Dalai Lama
The 14th Dalai Lama is the 14th and current Dalai Lama. Dalai Lamas are the most influential figures in the Gelugpa lineage of Tibetan Buddhism, although the 14th has consolidated control over the other lineages in recent years...
meditates for four hours each morning, and he says that it is hard work. He elaborates that if neuroscience can construct a way in which he can reap the psychological and biological rewards of meditation without going through the practice each morning, he would be apt to adopt the innovation. The aforementioned study in which gamma states were induced in mice may be a step in this direction.
Opposing evidence
Many neuroscientists are not convinced of the gamma wave argument. Arguments against it range from the possibility of mismeasurement - it has beensuggested that EEG-measured gamma waves could be in many cases an artifact of electromyographic activity -
to relations to other neural function, such as minute eye movements
However, proponents like O'Nuallain and Andreas Engel argue that gamma evidence persists even with careful signal separation.
Bearing this theory in mind, a number of questions remain unexplained regarding details of exactly how the temporal synchrony results in a conscious awareness or how a new percept "calls for" the synchrony, etc.
Other brain waves
- Delta waveDelta waveA delta wave is a high amplitude brain wave with a frequency of oscillation between 0–4 hertz. Delta waves, like other brain waves, are recorded with an electroencephalogram and are usually associated with the deepest stages of sleep , also known as slow-wave sleep , and aid in characterizing the...
– (0.1–4 Hz) - Theta wave – (4–7 Hz)
- Alpha waveAlpha waveAlpha waves are neural oscillations in the frequency range of 8–12 Hz arising from synchronous and coherent electrical activity of thalamic pacemaker cells in humans...
– (8–12 Hz) - Mu waveMu waveMu waves, also known as the comb or wicket rhythm, are electromagnetic oscillations in the frequency range of 8–13 Hz and appear in bursts of at 9 – 11 Hz. Mu wave patterns arise from synchronous and coherent electrical activity of large groups of neurons in the human brain...
– (8–13 Hz) - Beta waveBeta waveBeta wave, or beta rhythm, is the term used to designate the frequency range of human brain activity between 12 and 30 Hz . Beta waves are split into three sections: High Beta Waves ; Beta Waves ; and Low Beta Waves...
– (12–30 Hz)
External links
- EpilepsyHealth.com - 'A Sampling from Chapter 3' Biofeedback, Neurofeedback and Epilepsy, Sally Fletcher (2005)
- Gamma: Insight, Consciousness, or Microsaccades? - A summary of recent research (6/26/2009)
- How Thinking Can Change the Brain, dalailama.com. 2007-01-29.