Thursday, June 12, 2008

Review of Scientific Literature on Yoga II

General Research on Meditations

Autonomic And Electroencephalographic Studies:

Transcendental meditation was described as a `fourth major state of consciousness’, based on the fact that 6 months to 3 years practice of TM was reported to cause some changes similar to those in sleep, i.e., a decrease in the heart rate and oxygen consumption, and an increase in the level or stability of the electrodermal response. However, there was also an increase in the EEG alpha wave amplitude and regularity, normally seen while awake (Wallace, 1970; Wallace, Benson and Wilson, 1971). A study on autonomic stability in TM practitioners revealed that meditators (compared to non-meditators) were more stable, with respect to rate of GSR habituation, multiple responses of GSR and the spontaneous fluctuation of GSR (Orme-Johnson, 1973). A controlled study by Banquet (1973) on 12 transcendental meditators with 2 years of experience showed increase in alpha amplitude with decreased frequency anteriorly, posterior theta, rhythmic beta waves during deep meditation and synchronization of anterior and posterior channels.

All the above-mentioned studies were on single sessions. In a later study on the EEG changes during TM, Tebecis (1975) showed considerable individual variation between the 2 separate sessions in the EEG patterns during meditation. Lang et al (1979), reported that the 24 hour urinary catecholamines was higher in advanced meditators compared to meditators with less experience. Neither increase in plasma nor adrenaline was found in advanced meditators after meditation preceded by exercise, whereas after another period of physical exercise, following meditation, neither plasma nor adrenaline decreased. Stigsby (1981) demonstrated the EEG pattern during TM different from sleep onset and sleep, but not different from wakefulness and drowsiness.

Contradictory autonomic changes were observed in Zen and Tantric meditations.

One set of studies reported changes suggestive of autonomic activation (Hirai, 1974: Corby et al., 1978), whereas another set of studies reported changes suggestive of autonomic relaxation evident through the reduction in oxygen consumption, decrease in respiratory rate and stable GSR (Sugi and Akutsu, 1968; Akishige, 1968; Elson et al., 1977).

Farrow and Hebert (1982) observed increase in the frequency and length of the breath suspension episodes in TM practitioners compared to controls. They also asked the subjects to indicate the experience of pure consciousness experience (complete quiescent mental state) by pressing an event marking button. The temporal distribution of the button presses was significantly associated to the episodes of breath suspensions, indicating that breath suspension is a physiological correlate of some episodes of experience of pure consciousness. This was substantiated by the results of another study (Fried, 1987) where the breathing pattern and the rate following relaxation with biofeedback-assisted guided imagery resembled the pattern observed in meditators, indicating the importance of breathing rate as an index of hypoarousal. It was observed that heart and breath rates were significantly different as an experienced meditator shifted at will from `single thought’ to `no thought’ state (Telles & Desiraju, 1992). In Brahmakumaris Raja Yoga meditators, there was a group significant increase in heart rate during meditation, while other parameters showed inter and intra individual differences. The heart rate varies with sympathetic and parasympathetic activity and hence no conclusion was made about the effects of this meditation on the autonomic nervous system (Telles & Desiraju, 1993). Senior Om meditators showed a decrease in heart rate along with increased peripheral vascular resistance, interpreted as a sign of mental alertness while being physiologically relaxed (Indian Journal of Physiology and Pharmacology, 1995, 39(4): 418-420).

Travis and Wallace (1997), demonstrated the appearance of skin conductance response, heart rate decceleration and the experience of transcendental consciousness at the onset of respiratory suspensions, with higher phasic autonomic activity at respiratory suspension than at breath holding. These easily measured markers could help focus research on the existence and characteristics of transcendental consciousness. In a separate group of meditators (n = 12, 20 days of meditation experience), there was a decrease in heart and breath rates (similar to the control session) and a decrease in skin resistance in meditation sessions alone (Telles, Nagarathna & Nagendra, 1998). These results also suggest meditation causes alertness with relaxation. A study on another relaxation technique and meditation, combined, called `cyclic meditation’, showed that this practice reduced the oxygen consumption significantly more than an equal period of supine rest (Telles, Reddy, & Nagendra, 2000).

There are two review articles for comprehensive understanding of the neurophysiological correlates of meditation practice (Shapiro, 1982 and Delmonte, 1984). The first one provides physiological and clinical comparisons of meditation with other self-control strategies, emphasizing the “uniqueness” of meditation. The other review by Delmonte highlights the state effects of meditation eventually generalized to become traits, viz, decreased electrocortical arousal, stronger orienting and recovery responses to stressors. He also describes the course of meditation practice i.e., it may begin with left hemisphere activity, which gives way to functioning more characteristically of the right hemisphere. However, in advanced meditation (no thought) both left and right hemisphere activity is suspended or inhibited. Finally, the review states that the inadequate evidence to support the notion of “unique state effects of meditation” is not adequate.

In summary, studies on TM reported mainly reduced sympathetic activity following meditation, though a single report did describe sympathetic activation in TM. Similarly, contradictory autonomic sympathetic changes were reported in Zen and Tantric meditators.

Event Related Evoked Potential Studies In Meditation:

Wandhofer et al. (1976), reported a study on auditory evoked potentials using loud tones and observed lower 12% baseline values of the latencies of the P1, N1 and P2 components in meditators compared to non meditators. However, in the meditators there was no change during meditation compared to the preceding baseline. A later study (Barwood et al., 1978) reported no consistent change in long latency AEP during TM. A study on short latency AEPs by McEvoy (1980) showed a slight modulation in the wave V latency, after meditation.

Studies on Chinese, Qi-Gong meditation showed increased amplitude of I to V components of BAEP during meditation (Guo-Long, Rong-qing, Guo-Zhang & Chi-ming, 1990). In contrast, decreased amplitudes of AEP-MLR and long latency AEP components occurred during meditation, believed to be due to inhibition of neural activity at thalamo-cortical, cortical levels during Qi Gong.

Banquet et al.(1979), compared the meditators with matched controls for reaction time (RT) during a series of visual stimuli. The meditators showed faster RT with less mistakes, and N100 and P200 of larger amplitude and shorter latency. The transient effects were opposite for the 2 groups, i.e., longer RT and larger P300 was observed following meditation while following rest there was no change in RT and decrease in P300. These results explain selective attention capacity and information processing strategies in meditation. Middle latency auditory evoked potentials were studied in senior Om meditators with 5-20 years of meditation experience. As described above, there were differences within and between subjects for the parameters studied. However there was a group significant decrease in the Nb wave latency of middle latency auditory evoked potentials during meditation, suggesting changes at the level of the association cortices (Telles & Desiraju, 1993). Om meditators with 15 days to 12 years of experience of meditation showed opposite direction changes in Na amplitude, an increased amplitude of the Na component of middle latency auditory evoked responses during meditation, suggesting increased activity at mesencephalic-diencephalic levels was seen in experienced meditators (Telles, Nagarathna, Nagendra & Desiraju, 1994).

In summary, Evoked potential studies on meditators, showed that brainstem neural centers, as well as those at thalamic, and primary sensory cortex were involved in meditation.

Studies of cerebral function during meditation using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI):

A positron emission tomography (PET) study (Herzog et al., 1990-91) on meditation showed intraindividual changes in regional cerebral metabolic rate of glucose (rCMRGlc), or regional glucose consumption, when meditation and non-meditation were compared. The ratio of frontal versus occipital rCMRGlc was significantly higher in meditation than in non-meditation, suggesting involvement of frontal cortical areas in meditation. There were also two more recent studies, one using P.E.T., the other using functional magnetic resonance imaging (fMRI).

Studies In Response To External Stimuli:

Two studies (Kasamatsu and Hirai, 1966: Hirai, 1974) on Zen meditators, demonstrated alpha suppression response, a sudden attenuation of alpha waves in response to a stimulus, which did not habituate to repeated click stimuli during Zen meditation whereas controls habituate after the fifth or sixth click. This reflects a “hypersensitivity” of attention during Zen meditation. In contrast, Anand, Chhina & Singh (1961), found that two yogis showed no alpha blocking to diverse stimuli while performing Raja Yoga meditation during which attention is supposedly focused inward (on a sound or word called a “mantra”) and withdrawn from the outside world. These two studies provide an indication that advanced meditators exhibited neurophysiological alterations indicative of their specific state of attention during meditation. There have been two reports of physiological reactions to stimuli during meditation (Wallace, 1970: Banquet, 1973). Both reports were minor parts of larger studies, and two reports directly contradict each other, one finding no response to stimuli and the other finding many responses and a failure to habituate. Becker and Shapiro (1981), replicated the two studies (Anand, et al., 1961: Kasamatsu and Hirai, 1966) on very experienced Zen, Yoga and TM meditators along with the two groups of controls. All five groups were presented with auditory clicks during meditation. EEG alpha suppression and skin conductance response showed clear habituation, which did not differ among groups. They also recorded N100, P200 and P300 components of AEP. Contrary to their expectation there were no difference between groups. They observed non-significant larger intial N100 responses to the clicks which lead to the speculation that enhanced N100 reflects selective attention during meditation. A later report (Heide, 1986), noted a difference in the heart-rate response but not in the electro dermal response evoked by 80 dB tones, when TM practitioners and non-meditators were compared.

In summary, the response of meditators meditating on either an external or internal object, to external stimuli is not conclusively worked out.

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* Naveen KV, PhD. National Institute of Naturopathy, Pune, sponsored this study.
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