of conscious human brain activities. Bull Math Biol 50:559-65 Yamamoto, Y., Hughson, R.L., Sutton, J.R., Houston, C.S., Cymerman, A., Fallen, E.L., Kamath, M.V. (1993) Operation Everest II: an indication of deterministic chaos in human heart rate variability at simulated extreme altitude. Biol Cybern 6