Subjects
There were two groups of nine subjects each. They were all males, with ages ranging from 25 to 45 years. The first group (average age 32.1 ± 4.2 years) consisted of "Senior subjects" who had more then 10 years experience in meditation (average 12.4 ± 1.8 years). The second group (average age 36.2 ± 3.8 years) cosisted of "naive" subjects who were strongly motivated to practice mediation, but had no experience prior to the present, intensive 10 day retreat. The study was explained to the subjects. and their signed informed consent was taken, according to the ethical guidelines of the Indian Council of Medical Research (New Delhi, India)

Sessions
All the subjects were studied in 2 types of sessions, each lasting 32 min. (a) The "control" or nonmeditation session consisted of 20 min. of sitting with eyes closed, mentally repeating a neutral word i.e. "one". This period was preceded and followed by two 6 min. periods of sitting relaxed, with eyes closed. The next session was recorded after a gap of about 15 minutes during which the electrodes were disconnected, and the subjects were allowed to get up. This next session (b), was a meditation or "test" session, which was identical in design to the control session described above, except the 20 min. spent repeating "one," were spent mentally chanting the syllable OM.

Meditation
IIndian teachings consider the syllable OM to be the force behind all thought. Either uttering or thinking about OM is supposed to cause a quiet mental state. The meditation practiced in the present study involves mentally chanting the syllable "Om".

Recording of Evoked Potential
Middle latency auditory evoked potentials (AEP-MLRs) were recorded in the 70ms poststimulus time period, from Cz, referenced to the left earlobe, with the ground electrode on the forehead. The preamplifier band width was set at 1-1500 Hz. 1500 responses were averaged for each period. Sweeps containing artifactually large signals were rejected through software specification. The rejection level was expressed as a percentage of the full scale range of analogto- digital convertor. This level was set at 100%. The number of sweeps was also known from the print-out (NICOLET, USA, Clinical averaging package). Click stimuli of 40 µs duration and alternating polarity were delivered at 5 Hz binaurally, through acoustically shielded earphones (Amplivox, UK). The intensity was kept at one and half times the threshold of hearing. This value was (on an average) 45 dB above the threshold of hearing, which was approximately 24 dB with stimulus parameters kept as described above. This was sufficient to evoke the potentials consistently, at the same time leaving the subjects undisturbed.

TABLE 1

Peak Latencies and Peak Amplitudes of AEP - MLRs Recorded on Senior Subjects in
Meditation (M) and Non-Meditation (NM) Sessions
 

a N = 9
b p < .05, t-test for paired data, during to pre.
c p < .05, t-test for paired data, during M to during NM
d p < .001, t-test for paired data, during to pre

AEP-MLRs Components
Peak amplitudes of short latency wave V, and middle latency Na and Pa waves were measured from the baseline existing at the beginning of sweep. Peak latency was measured from the time of click delivery. The middle latency auditory evoked response components (AEP-MLRs) were described as follows: wave V was the maximum positive peak between 5 and 8 ms, the Na wave was the maximum negative peak between 14 and 18 ms, and Pa wave was described as the maximum positive peak between the Na wave and 35 ms. These descriptions are similar to those in other studies on AEP-MLRs (Erwin & Buchwald, 1986).

Data Analysis
The data were analyzed in two ways, viz a) using a two factor ANOVA (Factor A= senior subjects versus naive subjects and Factor B = Premeditation versus during meditation). The two factor ANOVA was repeated using the values obtained in control sessions as follows: Factor A= Senior subjects versus naive subjects and Factor B = Precontrol versus during control period. The t-test for paired data (two tailed) was used to check for statistical significance in the following comparisons: (1) premeditation with meditation, (2) Precontrol with control period, (3) meditation with control period.These two tests were carried out on the peak latencies and peak amplitudes of short latency wave V and middle latency Na and Pa waves.