Travis, F.T. & Wallace R.K. (1997) Autonomic Patterns during Respiratory Suspensions: Possible markers of Transcendental Consciousness. Psychophysiology, 34: 39-46.

(Also see, Travis, F.T. & Pearson, C. (2000). Distinct Phenomenological and Physiological Correlates of 'Consciousness Itself.' International Journal of Neuroscience,100, 77-89.)

Abstract
Two experiments investigated physiological correlates of transcendental consciousness during Transcendental Meditation sessions. In the first, experimenter-initiated bells, based on observed physiological patterns, marked three phases during a Transcendental Meditation session in 16 subjects. Inter-rater reliability between subject and experimenter classification of experiences at each bell was quite good. During phases including transcendental consciousness experiences, skin conductance responses and heart rate deceleration occurred at the onset of respiratory suspensions or reductions in breath volume. In the second experiment, this autonomic pattern was compared to that during forced breath holding. Phasic autonomic activity was significantly higher at respiratory suspension onset than at breath holding onset. These easily measured markers could help focus research on the existence and characteristics of transcendental consciousness.