Gamma rhythm

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Gamma waves

A gamma wave is a pattern of neural oscillation in humans with a frequency between 25 to 100 Hz, though 40 Hz is typical.

According to a popular theory, gamma waves may be implicated in creating the unity of conscious perception (the binding problem). 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 electroencephalography 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.

Linked to unity of consciousness?

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 and Christof Koch 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 scepticism over the idea that 40 Hz oscillations is sufficient condition for visual awareness.

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.

Gamma waves are observed as neural synchrony from visual cues in both conscious and subliminal stimuli. This research also sheds light on how neural synchrony may explain stochastic resonance 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 Tibetan 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.

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 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 been suggested 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.

Moreover, recent studies using magnetoencephalography (MEG), which does not suffer the potential artifacts associated with EEG, have identified gamma activity associated with sensory processing, mainly in the visual cortex.

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 wave – (0.1–4 Hz)
• Theta wave – (4–7 Hz)
• Alpha wave – (8–12 Hz)
• Mu wave – (8–13 Hz)
• Beta wave – (12–30 Hz)

Gamma and Negative Emotions

Gamma waves also seem to be envolved in the processing of negative stimuli. Not only are they more prominent in parts of the brain that relate to emotions such as the amygdala, but according to Oya et al. (2002), when subjects were presented with negative stimuli, there was an increase in gamma power compared to when presented with neutral or positive stimuli. Moreover, Oathes et al. (2008) that those with General Anxiety Disorder (GAD) had higher delta waves after a anxiety inducing procedure.

See also

• Electroencephalography
• Epilepsy
• Sensory Integration Dysfunction
• SMR==External links==

References

• Bruce Bower (2004). Synchronized thinking. Brain activity linked to schizophrenia, skillful medication. Science News 166 (20): 310.
• Brain Waves ("40Hz") Research. Brain Waves ("40Hz") Research. URL accessed on June 16, 2005.
• Are neocortical gamma waves related to consciousness?. National Library of Medicine PubMed. URL accessed on June 16, 2005.
• Oya, H., Kawasaki, H., Howard, M.A., Adolphs, R., 2002. Electrophysiological
  responses in the human amygdala discriminate emotion categories of complex
  visual stimuli. Journal of Neuroscience 22, 9502–9512.
• Oathes, D. J., Ray, W. J., Yamasaki, A. S., Borkovec, T. D., Castonguay, L. G., Newman, M. G., & Nitschke, J. (2008). Worry, generalized anxiety disorder, and emotion: Evidence from the EEG gamma band. Biological Psychology, 79(2), 165-170. doi:10.1016/j.biopsycho.2008.04.005

Further reading

• Meditation Gives Brain a Charge, Study Finds. WashingtonPost.com. URL accessed on June 16, 2005.

External links

• EpilepsyHealth.com - 'A Sampling from Chapter 3' Biofeedback, Neurofeedback and Epilepsy, Sally Fletcher (2005)
• Nature.com - 'Thinking in Synchrony', Henry Gee (February, 1999)