Seven normal healthy male volunteers in the age range of 29 to 55 yrs (the average being 42.3+ 9.8 yrs), who were experienced in meditation (range 5-20 yrs), were studied. Using identical stimulus conditions, records were also obtained on 7 age-matched "naive" male subjects with ages ranging from 30 to 55 yrs (average 45.6+9.5 yrs) who did not meditate
     Each session was of 32 min duration, 20 min spent in meditation, and the two 6 min periods before and after meditation were spent sitting relaxed, with eyes closed. These sessions were repeated thrice in each subject. The meditators were also studied during 3 control sessions, which were similar in design except that the 20 min period which was meant for meditation was spent "just sitting" with eyes closed. The 7 "naive" subjects were studied in 3 repeat sessions which followed the same sequence as the "control" sessions. The effect of focusing on a meaningless word/syllable could not be checked, and therefore the results cannot help in understanding the changes which occur specially when focusing on a meaningful symbol.
     The meditation practice is claimed to attain a single thought state, in deep relaxation. The single thought is the syllable "OM" which is chanted aloud during the initial training in meditation, which subsequently leads to an effortless mental chanting of the syllable.
     Middle latency auditory evoked potentials (AEP-MLRs) were recorded from Cz-Al with the preamplifier bandwidth set at 1-1500 Hz, in the 70 msec post-stimulus time period. A total of 1500 responses were averaged for each record (NICOLET, USA, Clinical Averaging Package, 1980 version). Click stimuli of 40 msec duration were delivered at 5 Hz binaurally through acoustically shielded earphones (Amplivox, UK). The intensity was set at 1½ times the threshold of hearing noted with the above stimulus parameters. This was 45 dB HL on an average, low enough to allow the meditators to be undisturbed and yet of sufficient intensity to allow recording of the potentials consistently. Peak latency of Na, Pa, and Nb waves were measured from the baseline values at the beginning of the sweep. Latency was measured from the time of click delivery.
     The AEP-MLRs components were as follows the Na wave was the largest negative peak between 14 and 18 msec, Pa wave as the largest positive peak between the Na wave and 35 msce, and the largest negative peak between 35 and 65 msec was the Nb wave. These descriptions are similar to those in other studies on AEP-MLRs3.
     Values of peak amplitudes and peak latencies were averaged for each of the 3 periods during the repeat sessions of a subject. Statistical analysis of changes between (i) Meditation or control periods and the preceding eyes closed period; (ii) between meditation and control periods was done by using the paired t-test. For the "naive" subjects the values of the control period were compared with those of the preceding period.