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Shirley Telles, Nagarathna S., Nagendra H.R., and Desiraju T.
Department of Neurophysiology, NIMHANS, Bangalore, India.
(Received September 15, 1993)

Abstract: Middle latency auditory evoked potentials were recorded in 18 male volunteers with ages between 25 and 45 years, 9 of whom had more than ten years of experience in OM meditation (senior subjects), whereas the other 9 had no meditation experience (naive subjects). Both groups were studied in two types of sessions. (1) Before, during, and after 20 minutes of mentally repeating "one" (control session), and (2) a similar session, though with 20 minutes of mentally chanting OM (meditation session). The senior subjects showed a statistically significant (paired t-test) increase in the peak amplitude of Na wave (the maximum negative peak between 14 and 18 ms) during meditation, while the same subjects showed a statistically significant reduction in the Na wave peak amplitude during control sessions. In contrast, the naive subjects had a significant decrease in the Na wave peak amplitude during meditation sessions and a nonsignificant trend of reduction during control sessions, as well. This difference between senior and naive subjects was significant (two way ANOVA). There were no significant changes in short latency wave V or Pa wave (the positive peak between the Na wave and 35 ms). The changes in the Na wave suggest that both meditation on a meaningful symbol, and mental repetition of a neutral word cause neural changes at the same level (possibly diencephalic). However, the change could be in opposite directions, and this difference could be correlated with differences in the duration of experience in meditation between senior and naive subjects.

Key words: Middle latency auditory evoked potentials; Meditation-Meaningful symbol- Neutral word.

   Meditation has been described as a training in awareness which, over long periods, produces definite changes in perception, attention and cognition (Brown, 1977). Objective evidence of such altered sensory perception exists, since yogis in deep meditation showed an absence of alpha blocking in the EEG even when exposed to diverse (visual, vibratory. thermal, and sensory) stimuli (Anand & Chhina, 1961). The effects of Transcendental Meditation (TM) on auditory evoked potentials (AEPs) have already been reported. The first study (Barwood, Empson, Lister & Tilley, 1978) showed that long latency AEPs (i.e., with latency more than 100 ms) did not change significantly during meditation. Later on, McEvoy, Frumkin and Harkins(1980), reported a change in short latency (brainstem) AEPs after meditation. Hence, if short latency AEPs change, whereas long latency AEPs remain unaltered during meditation, the effects of meditation on middle latency AEPs is interesting. This knowledge could help in understanding how neural processing at different stages alters with meditation. Hence the present study was carried out, recording middle latency AEPs during meditation on the syllable. OM, which has profound significance in Indian belief. Recordings were made at the end of a 10 day retreat during which subjects meditated for approximately 6 hours a day, and maintained silence for the remaining time.

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