Gamma Wave

Matthieu Ricard

Neural oscillation is rhythmic or repetitive neural activity in the central nervous system. For example Delta waves (0-4 Hz on an EEG) is the lowest frequency neural oscillation and is associated with deep sleep. Gamma waves are the highest frequency pattern at 25-100 Hz (though 40 Hz is typical), and according to a popular theory, they may be related to subjective awareness.

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.

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 (how the brain merges all its many inputs into a unified perception). Neural oscillations have been suggested as the mechanism of binding. 40 Hz oscillations in particular may be causally implicated in visual awareness as well as in visual binding (merging visual data into the overarching consciousness).

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.

Gamma waves are observed as neural synchrony from visual cues in both conscious and subliminal stimuli. This also sheds light on how neural synchrony may explain stochastic resonance in the nervous system (the fact that adding random noise can paradoxically improve the output of some systems). They are also implicated in REM sleep, which involves visualizations, and also during anesthesia.

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. 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. 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.

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 (electronics firing in muscles cells)– to relations to other neural function, such as minute eye movements. However, proponents 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.

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