Monday, July 28, 2014

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What is EEG coherence?

When you close your eyes, the back of the brain almost immediately begins generating alpha2 EEG waves (10.5 - 12 cycles per second). The rear of the brain processes incoming visual information (almost 60% of all sensory input). Closing your eyes shuts off this information stream, and the rear of the brain goes into an “idling” or resting state, as indicated by the alpha2 and by a reduction of cerebral metabolic rate. (Pictured are Susan Vegors, Dr. Fred Travis, and Dr. David Orme-Johnson in the MUM EEG lab, about 1988.)

When you begin the practice of the Transcendental Meditation technique, another highly coherent kind of alpha, called alpha1 (7.5-10 cycles per second) begins in the frontal cortex and spreads rapidly to encompass the entire brain 3-5. Cerebral metabolic rate increases in the front of the brain 6, which along with the increased alpha1, indicates “restful alertness”, “inner wakefulness” and “inner directed attention”, a distinctly different state from ordinary rest 3-5,7. The frontal cortex is known as the CEO or executive control center of the brain. This highly coherent alpha1 EEG emanating from the CEO of the brain to the entire brain is unique to the practice of the Transcendental Meditation technique (see chart).

However, although the EEG effects observed during and outside of TM practice have been largely seen in the alpha band in frontal areas, broad band EEG coherence spanning theta, beta, alpha, and to some extent gamma have also been observed, especially during periods of transcendental consciousness 8-11.  

Reviews of alpha coherence and synchrony have implicated alpha in organizing different cortical processes distributed throughout the brain that are important for a wide range of perceptual, motor and cognitive tasks 1,2,12-16.

 “It is widely accepted that information is stored in neural networks and human behavior arises from extremely complex communication between neurons in these networks and also between separate networks or assemblies…. If we consider that the spatial resolution of scalp EEG is in the range of some centimeters it comes evident that with this method it is not possible to investigate network properties on a micro-level, such as the synchrony of a few connected single cells. Instead, with EEG we can only make inferences about synchronicity on a large scale… If two cortical areas show very similar brain activity, and this brain activity is related to each other in a fixed manner, one can deduce from it that the two brain areas are somehow functionally related, as they do the same thing at the same time. … Phase coherence is then a measure to quantify the similarity between two signals from the brain…. One can thus conclude that these two brain sites are somehow functionally related to each other and are involved in task relevant processes.” 1(p. 1003).

1. Sauseng P, Klimesch W. What does phase information of oscillatory brain activity tell us about cognitive processes? Neurosci Biobehav Rev 2008;32(5):1001-1013.

2. Palva S, Palva JM. New vistas for α-frequency band oscillations. Trends Neurosci 2007;30(4):150-158.

3. Wallace RK. The Physiological Effects of Trancendental Meditation: A Proposed Fourth Major State of Consciousness. In: Orme-Johnson DW, Farrow JT, eds. Scientific Research on the Transcendental Meditation Program Collected Papers. Vol 1. 2 ed. Livingston New York: MERU; 1970:43-78.

4. Wallace RK. The Physiology of Meditation. Scientific American 1972;226:84-90.

5. Wallace RK, Benson H, Wilson AF. A wakeful hypometabolic physiologic state. Am J Physiol 1971;221:795-799.

6. Ludwig M. Brain activation and cortical thickness in experienced meditators. San Diego, CA: Psychology Department, The California School of Professional Psychology; 2011.

7. Wallace RK. Physiological effects of Transcendental Meditation. Science 1970;167:1751–1754.

8. Badawi K, Wallace RK, Orme-Johnson D, Rouzere AM. Electrophysiologic characteristics of respiratory suspension periods occurring during the practice of the Transcendental Meditation Program. Psychosom Med May-Jun 1984;46(3):267-276.

9. Levine PH, Hebert R, Haynes CT, Strobel U. EEG coherence during the Transcendental Meditation Technique. In: Orme-Johnson D, Farrow, J., ed. Scientific Research on the Transcendental Meditation Program :Collected Papers. Vol 1. Livingston Mannor NY: Maharishi European Research University Press; 1977:187-207.

10. Travis FT, Arenander A. Cross-sectional and longitudinal study of effects of Transcendental Meditation practice on interhemispheric frontal asymmetry and frontal coherence. Int J Neurosci 2006;116(12):1519-1538.

11. Farrow JT, Hebert JR. Breath suspension during the Transcendental Meditation technique. Psychosom Med 1982;44(2):133-153.

12. Hummel F, Gerloff C. Larger interregional synchrony is associated with greater behavioral success in a complex sensory integration task in humans. Cereb Cortex 2005;15:670–678.

13. Hummel FC, Gerloff CG. Interregional long-range and short-range synchrony: a basis for complex sensorimotor processing. Progress in Brain Research 2006;159:223-236.

14. Sauseng P, Klimesch W, Doppelmayr M, et al. EEG alpha synchronization and functional coupling during top-down processing in a working memory task. Hum Brain Mapp 2005;26(148–155).

15. Sauseng P, Klimesch W, Gruber WR, Birbaumer N. Oscillatory phase synchronization: a brain mechanism of memory matching and attention. Neuroimage 2008;40:308–317.

16. Sauseng P, Klimesch W, Schabus M, Doppelmayr M. Fronto-parietal coherence in theta and upper alpha reflect central executive functions of working memory. Int J Psychophysiol 2005;57:97–103.

 

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