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MEDITATION AND THE EEG |
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by Peter Fenwick In the 1950s, it was hoped that the EEG would be the ‘open sesame’ to an understanding of brain function, personality, and mental illness. It was thus not surprising that as EEG machines became smaller, and almost portable, those scientists interested in consciousness, meditation, and the esoteric should apply this new measurement method to genuine yogis fresh down from the Himalayas. Das and Gastaut (1955), Bagchi and Wenger (1957a, b), Anand et al. (1961), and Kasamatsu et al. (1957) were the first workers in this field. Das and Gastaut examined seven Indian meditation experts, and demonstrated a decrease in alpha amplitude and an increase in alpha frequency during meditation, and suggested that meditation was an ‘alerting’ procedure. They also detected generalized bursts of spindles of fast activity at the time that their practitioners claimed to be entering ‘samadhi’ (the ultimate goal of meditation). This early view of meditation as an ‘alerting’ phenomenon is now thought to be incorrect, and it is possible that the fast activity they detected was due to interference from the scalp muscles. However, the paper describing their results was not sufficiently detailed to indicate whether this was in fact so. Several subsequent papers, reviewed below, have mentioned the appearance of fast activity in the ultimate state of meditation. Bagchi and Wenger (1957b) lugged their EEG machines and portable generators up into the mountain caves of several ‘genuine’ Indian yogis, whose EEG they recorded during the meditation sessions, while banging cymbals behind them, flashing lights in their eyes, and plunging their feet into baths of cold water. They report that in some cases the alpha rhythm was unresponsive to this maltreatment. Thus, the second important concept arose, that of sensory withdrawal during the meditation period. These two ideas, fast rhythms during Samadhi and sensory withdrawal during meditation, are the foundation concepts of work on the EEG features of meditation. TM meditators were the subjects in several EEG studies of meditation conducted by Wallace (1970), Wallace et al. (1971), Banquet (1972, 1973; Banquet and Sailhan 1974), and Fenwick et al. (1977), all of whom came to essentially the same conclusion. At the beginning of meditation the alpha rhythm increases in amplitude and then generally slows in frequency by 1-3 Hz before spreading forwards into the frontal channels. In some meditators, bursts of theta activity are seen bitemporally, which may alternate with a recurrence of the alpha rhythm. In some studies only, bursts of beta spindling, mainly frontally, are seen in the deeper stages of meditation, and it has been suggested that these are concurrent with ‘transcendence’. All these changes are suggestive of lowered cortical arousal. Beta spindling is frequently seen in non-meditators who have become drowsy and passed into stage 1 sleep. Thus, the EEG changes point towards a behavioural state which is on the dimension of alertness to drowsiness. However, it must be remembered that EEG changes are non-specific, and it is not possible to argue from them to mental state. But physiologically this pattern of changes is not unique to meditation alone. Thus we cannot conclude on the basis of these findings that the meditation state is physiologically unique, but neither can we infer that the mental state of meditation is not distinct from low arousal states of drowsiness and light sleep. Much the most interesting question is whether the neurophysiological processes underlying meditation are in some respects similar to those of sleep. As has already been pointed out, it is never possible to argue back from an EEG picture to a mental state. There is a difference in mental state between meditation and sleep. However, if the same constellation of factors is found in both meditation and sleep, then it is possible that the same neurophysiological mechanisms are involved in both processes. As already described, the slowing and spreading forward of the alpha rhythm, intermittent bursts of theta activity, intermixed with alpha, and the occurrence of slow, rolling eyeball movements with the additional reported occurrence of myoclonic jerks during meditation (Banquet 1973; Fenwick 1974; Fenwick et al. 1977) all suggest that cortical arousal is decreased along the dimension of relaxed alertness to stage 1 sleep. Most of the phenomena indicate stage onset (not stage 1, but drowsiness) sleep. Stigsby et al. (1981), in an excellent study, carried out a computer analysis of EEGs during meditation, drowsiness, and sleep. They found that meditation was similar, in its spectral profile, to waking and drowsiness, but was quite different from stage 1 and 2 sleep, thus confirming the findings of Fenwick et al. (1977). There are other crucial differences between meditation and sleep. Firstly, an average subject not practising a technique of meditation cannot hold himself in the deepest stages of drowsiness, just before the onset of sleep, without rapidly falling through it into stage 1 sleep. Secondly, some features of the meditation experience are alien to those of deep drowsiness. For example both the subjective experience during meditation and the changes in rate and rhythm of breathing are atypical of stage 1 sleep or deep drowsiness. Thus one is forced to conclude that although the meditating state is in the direction of lowered cortical arousal it is different from that of deep drowsiness and stage 1 sleep in several significant respects. |