Within-subject and between-subject variability in responses to a mind-body intervention in electrophysiological brain activity.

INTRODUCTION: Mind-body interventions (MBIs) increasingly used in health care include alterations in the ordinary state of consciousness that might be essential for their effects on psychological and physical patterns encoded in the unconscious. Little is known about the effects of repeated practice or the time of the repetitions. METHODS: EEG-derived Bispectral-Index (BIS) is clinically used to monitor anesthetic depth, a deviation from the normal waking consciousness. We used BIS-monitoring in six participants who repeated a MBI ten times within 3-4 days at different times of day. RESULTS: BIS values decreased significantly from an awake baseline of 97.5 to a mean of 83.2, showing a magnitude of reduction similar to that observed during the standardized hypnosis induction of a suggestibility test. Marked drops in BIS occurred during the breaks before and after the four tasks of the MBI and within certain tasks. There was a steady decline in BIS during the 30-min MBI, and some deepening with increasing exercise. Changes in BIS were significantly more pronounced when the MBI was performed at midday compared to mornings, particularly in individuals with an eveningness chronotype. DISCUSSION: The reproducibly observed decrease of BIS during the MBI we take as surrogate for involved trance states in support of the hypothesis that this facilitates access to the unconscious and modulation of physiological and psychological programs encoded there. The within-subject variability in the corresponding EEG-derived BIS with repetitions of the MBI is unexpectedly high, although still lower than between-subject variability. Some improvement was observed with repeated practice, although not in a linear relationship. The dependence of these effects on time of day warrants further investigation.