In recent years, a growing literature has addressed itself to the psychophysiological bases of altered states of consciousness (ASCs). An unprecedented interest in meditation, biofeedback, and other techniques for altering consciousness reflects in part the widespread notion that science has begun to understand the physiological bases of these states. Thus, based on research involving practitioners of Yoga, Zen, or Transcendental Meditation (TM), meditation has been considered a unique psychophysiological state, associated with a distinct configuration of autonomic and electrocortical changes. For example, it has been proposed on the basis of these data that the practice of Transcendental Meditation leads to the experience of a fourth major state of consciousness, distinct from waking, dreaming, and nondreaming sleep (Wallace, 1970).