DISCUSSION The present study has shown that mental repetition of a syllable either meaningful (MOM) or neutral (COM), causes a decrease in heart and breath rate. Non targeted thinking (NT) for the same period of time does not have a similar effect. This result is similar to reports about the physiological changes during Transcendental Meditation (TM) (8, 9). During the practice of TM subjects silently repeat a particular mantram (see below) throughout each 20 min session, returning focus to it whenever attention wanders (10). A unique mantram is assigned to each individual, with the idea that a particular sound resonates optimally with a specific nervous system. The mantram is not "meaningful" in the sense used in the present study. It was noted that the change which was unique to the repetition of the meaningful syllable ("OM"), during MOM sessions was a significant decrease in the skin resistance level (SRL). This change is opposite to that observed in the studies on TM by (8.9), in which an increase in skin resistance was reported. It is now well recognized that different subdivisions of the sympathetic nervous system need not always be activated simultaneously. The responses to a given stimulus vary, depending on the individual and on the stimulus (11). Hence a subject may have a reduction in HR (reduced cardiac sympathetic tone) at the same time as a decrease in skin resistance level (SRL), which is believed to be due to increased sudomotor sympathetic tone. The interpretation of change in skin resistance is open to continuing debate. The sweat glands are believed to be the major contributors to changes in the spontaneous electrodermal activity (EDA) as is observed by recording the SRL (12). Human sweat glands are generally known to receive predominantly sympathetic cholinergic innervation (13). Hence though the EDA is regulated by the sympathetic nervous system, the EDA and cardiac sympathetic control systems are recognized to be organized differently (14). The HR is believed to be influenced by somatic activity and movement control mechanisms, whereas the EDA appears to be determined primarily by motivational or attentional arousal. Hence mental repetition of both "OM" and "one" may reduce somatic activity (HR reduction), while the mental repetition of "OM" appears to cause an increase in attentional arousal (reduced SRL), as well. In addition to the conventional concept of an increase in skin resistance signifying an increase in sympathetic tone, the sensitivity of the skin resistance (response) to significant stimuli is well known (the "lie detection test"). It was reported (15) that the generation of frequent and large electrodermal skin responses were the only indication that in prosopagnosic patients, who have lost their ability to recognize faces, the process of recognition is still taking place. It was also noted that in these patients the results of cognizance were not available to consciousness. While these findings cannot be directly applied to the present results, they may suggest that even though mental repetition of both "OM" and "one" cause comparable changes in HR and RR, the neural processes involved in recognizing the significance of the syllable "OM" are active, as reflected by the decrease in SRL. Previous studies have demonstrated that subjects with a short experience of meditation (comparable to the subjects of the present study), show similar inhibition in sensory neural processing at mesencephalic and diencephalic levels, while repeating both "OM" and "one" (1). In the same study, in experienced meditators neural processing at these levels was significantly facilitated while repeating "OM". These observations on sensory neural processing, were based on recordings of middle latency auditory evoked potentials. In both short duration and experienced meditators there were significant changes (in opposite directions) in the amplitude of the Na wave. This is a negative wave occurring between 14-18 msec after the stimulus, with a neural generator at mesencephalic /diencephalic level (16). An increase in peak amplitude observed in experienced meditators, suggests facilition of sensory neural processing (17). In a separate study (2) on the autonomic changes in experienced "OM" meditators, there was no change in the skin resistance level while repeating "OM". This is in contrast to the short duration meditators of the present study. Hence to sum up the results of the present and previous studies, (i) a decrease in HR, RR (conventional indicators of relaxation) follow the mental repetition of any syllable, (ii) subjects who have brief (20 days) experience of medication on a meaningful syllable ("OM"), show a significant decrease in SRL during repetition of that syllable, but not while repeating a neutral syllable or during non targeted thinking. Subjects with longer meditation experience do not show this change, (iii) experienced meditators show significant facilitation of sensory neural processing at mesencephalic /diencephalic levels while repeating the syllable on which they meditate. Naive meditators do not show this change. These results suggest that the process of understanding the significance of a meaningful syllable may be continuous, and is significantly influenced by the duration of meditation. The decrease in SRL was observed in naive meditators but not in experienced meditators, hence this change may represent an intermittent stage during which changes in sensory evoked potentials (EPs) are not yet obtained. The present results also support the idea that the SRL may indicate cognitive involvement in the object of meditation in addition to being a conventional indicator of sympathetic tone. |