Electroencephalography and Clinical Neurophysiology, 1978, 4 5 : 6 7 1 - - 6 7 3 © Elsevier/North-Holland Scientific Publishers, Ltd. Laboratory AUDITORY
T.J. BARWOOD, J.A.C. EMPSON 2, S.G. LISTER and A.J. TILLEY
Department o f Psychology, The University, Hull (England) (Accepted for publication: July 4, 1978)
Objective studies of the psychophysiology of meditating subjects have, to some extent, supported the notion that there is a psychophysiologically distinct state which is normally only achieved during meditation. Anand et al. (1961) reported a decrease in EEG responsiveness to stimuli (as measured by alpha blocking) in subjects practising Zen meditation. Banquet (1973) confirmed this observation, using subjects practising transcendental meditation (TM), and also found a slowing of the alpha rhythm, with an increase in amplitude, and, in some subjects, the development of trains of theta waves. It has, however, recently been suggested that that the EEG of meditation is in fact indistinguishable from that of drowsiness, or light sleep (e.g. Younger et al. 1975; Fenwick et al. 1977). Attentional factors have been shown to affect the late components of the auditory evoked potential (Davis 1964). The primary purpose of this experiment was to test the hypothesis that the reported changes in awareness during meditation should be accompanied by changes in the late components of the auditory evoked potential (AEP), and also to assess any difference between potentials obtained during meditation and during sleep. Secondly, the experiment allowed an investigation of any changes in AEP after a period of meditation, compared with before.
Su bjec ts The 8 subjects comprised the local TM instructor and 7 acolytes, chosen by him for their experience and knowledge of meditative technique. They had all
I We wish to thank Patrick J. Shaw and the other subjects for their forbearance. 2 Requests for reprints should be sent to Dr Jake Empson, Department of Psychology, The University, Hull, HU6 7RX, England.
been regularly meditating (every day) for between 18 months and 6 years.
Procedures Subjects, wearing headphones, adopted their normal sitting posture for meditating. Average responses were recorded to 3 blocks of 50 stimuli, before meditation (baseline), during meditation, and afterwards. The stimuli were 1 sec tones, 1500 Hz and approximately 30 dB, presented once every 15 sec to one ear only. Subjects were given 2 min to settle down at the beginning of each of the 3 blocks of stimuli. (During a pilot experiment stimuli were presented to both ears, giving an apparent source of sound in the centre of the head. The subject found it impossible to meditate, and would not continue with the experiment until one of the headphones was disconnected.) The EEG was averaged over 500 msec following the onset of the tone, at a rate of 1 point every 2 msec. The sleep recordings took place on the same evening, with the subjects lying in bed, wearing their headphones. When sleep onset was clearly evident (either sleep spindles or K-complexes had appeared) the tape was started, and averaging started after a lapse of 2 min. Monopolar EEG was recorded from the vertex, referenced to the left mastoid. EOGs were also routinely recorded, both to check on EOG contamination of the EEG during the waking records, and to aid in the assessment of sleep depth. The apparatus used was an Elema-Schonander 8-channel encephalograph and a PDP 8 computer, programmed with the Advanced Averager series of programs supplied by the Digital Equipment Corporation.
Data analysis The paper records were visually examined to check that there were no EOG contaminations of EEG immediately following each stimulus. Also, the number of eye movements, defined as 70 pV deflections in
T. BARWOOD ET AL.
672 TABLE I
Peak latency (msec).
Eye movements (no. of 70 pV deflections in EOG).
N1 P1 N2
113.5 203.0 388.8
101.7 205.7 406.0
106.2 209.7 386.5
119.5 199.0 393.0
levels of eye movements and after meditating. Fewer eye movements were produced during sleep than during meditation (t = 3.92, P < 0.01) (see Table III). the EOG, were counted for each of the 4 blocks of stimuli. The AEPs were analysed in terms of the amplitude and latency of the N1, P1 and N2 peaks.
For all 3 peaks, the largest average amplitude was obtained during the sleep condition. There were, however, no statistically significant differences between baseline amplitude and any of the subsequent recordings for the N1 and P1 peaks (see Table II). N2 amplitude was not significantly greater during meditation than during baseline or afterwards. N2 amplitude during sleep was not significantly different from baseline level, or from that during meditation, although it was significantly greater than the amplitude after meditation (t -- 2.43, P < 0.05).
There was no difference between the AEPs elicited during meditation and baseline periods, so our primary hypothesis, that there should be some change consistent with the subjective accounts of nonattention to stimuli, may with confidence be rejected. There is also no firm evidence to support the hypothesis that the AEP changes after meditation. Unlike subjects of Younger et al. (1975) and Fenwick et al. (1977), ours did not appear to give any signs of going to sleep while meditating. The number of eye movements in meditation was intermediate between the number observed during wakefulness and sleep. The EOG excursions during sleep were of the slow, rolling type, while those during meditation were more similar to the wakeful movements, produced by saccades and blinks. Presumably our subjects were prevented from falling asleep during meditation (in the way that those of Fenwick et al. did) by the repeated stimulation of the tones. Taken together, our EEG and EOG measures are not inconsistent with the idea that TM is normally similar to drowsiness or light sleep. However, our subjects all reported having meditated fairly satisfactorily, so it could be argued that light sleep is not essential to the experience of meditation, although it may be a frequent consequence of it.
(3) Eye movements
(1) Latency of evoked potential peaks The N1 and P1 peaks came at about 100 and 200 msec each, as conventionally described for auditory evoked responses (e.g. Regan 1972). The N2 peaks consistently appeared at about 400 msec, rather later than the norm to clicks. There were no systematic differences between the 4 conditions (see Table I).
(2) Amplitude of evoked potential peaks
Fewer eye movements were produced during meditation than baseline (t = 2.51, P < 0.05), but there was no significant difference between baseline
Auditory evoked potentials to tone stimuli were recorded from 8 practised meditators before, during, and after meditation, and also during light sleep. No consistent changes were noted between baseline and meditating AEPs, or between meditating and sleep AEPs.
Peak amplitude (pV).
N1 P1 N2
--4.46 4.13 --3.25
--3.69 3.97 --3.21
--3.41 6.27 --1.37
---9.15 14.86 --8.35
Potentiels ~voquds auditifs et m~ditation transcendentale Des potentiels ~voqu~s auditifs ~ des sons purs ont ~t~ enregistr~s chez 8 sujets pratiquant depuis long-
AEPs AND TM temps la MT, avant, pendant et apr~s une p~riode de m~ditation et 4galement pendant un sommeil l~ger. Aucune difference marquante n'a 4t~ not4e entre les potentiels recueillis au repos et pendant la m~ditation, ou entre ceux recueillis en cours de m~ditation et ceux obtenus pendant le sommeil.
References Anand, B., Chhina, G. and Singh, B. Some aspects of electroencephalographic studies in Yogis. Electroenceph, clin. Neurophysiol., 1961, 13 : 452--456. Banquet, J.P. Spectral analysis of the EEG in meditation. Electroenceph. clin. Neurophysiol., 1973, 35: 143--151.
673 Davis, H. Enhancement of evoked potential in humans related to a task requiring a decision. Science, 1964, 145: 182--183. Fenwick, P.B.C., Donaldson, S., Gillis, L., Bushman, J., Fenton, G.W., Perry, I., Tilsley, C. and Serafinowicz, H. Metabolic and EEG changes during TM: an explanation. Biol. Psychol., 1977, 5: 101-118. Regan, D. Evoked Potentials in Psychology, Sensory Physiology and Clinical Medicine. Chapman and Hall, London, 1972. Younger, J., Adriance, W. and Berger, R.J. Sleep during TM. Percept. Mot. Skills, 1975, 40: 953-954.