The syllable "OM" is known to have profound significance in Indian culture. Mental repetition of this syllable was shown to cause significant changes in middle latency auditory evoked potentials, which were suggestive of a facilitation of neural activity at either mesencephalic or diencephalic level (1). In contrast mentally repeating a neutral syllable "one", inhibited neural activity at the same level. This was observed in subjects who had been regularly meditating on "OM" every day for more than 10 years ("experienced meditators"). In "naive" subjects, with 15 to 20 days of experience in meditation, repeating both "OM" and "one" caused the same effect, viz an inhibition of neural activity at mesencephalic or diencephalic level.
It was also reported that highly experienced meditators (20 years experience on an average) show a significant reduction in heart rate and a non significant trend of reduction in oxygen consumption while mentally repeating "OM" compared to a period of non targeted thinking (2). These subjects simultaneously displayed an increase in cutaneous vasoconstriction, which was interpreted as an increase in mental alertness (3), in the presence of other signs of reduced arousal (reduced heart rate and oxygen consumption).
The present study was conducted to compare the autonomic effects of mentally repeating a meaningful syllable with mental repetition of a neutral one and with non targeted thinking, in subjects who were acquainted with both syllables, and had been meditating on the meaningful syllable for 20 days, comparable to the naive subjects described earlier (1). The duration of meditation was believed to influence the ability to comprehend the significance of the syllable and a comparison with results obtained in experienced meditators, could be made.