2. What is a hypnotic "trance" ?

2.5. Trance as distinct from sleep or stupor

I think we can fairly conclude from the research on hypnosis done so far that 'trance' may in fact have useful meaning for describing the subjective experience of subjects in hypnotic situations, but is not explained, or even described, by any one simple theory yet proposed, either neurological or psychological. All of the current theories seem to leave aspects unexplained.

Clearly, selective cerebral inhibition and activation of some kind is involved at various stages of a hypnotic induction, but not yet in any way we can uniquely distinguish from other forms of waking response to changing stimuli in other situations. And certainly hypnotic response does not rely upon the generalized inhibition found in the action of depressant drugs or in the normal sleep state. It is a much more highly specific effect, if indeed it truly is distinct in some way, as subjective data appear to suggest.

The most common neurological theories of hypnosis over the years as a form of partial sleep have mostly been based on (1) the superficial resemblance of a classically induced subject to a near-sleeping person, (2) on the ease with which a deeply hypnotized subject will fall off to sleep on suggestion or if hypnosis is not explicitly ended, and (3) because various drugs that induce sleep-like or stuporous states can produce some of the same characteristics as hypnotic trance.

It has been very consistently determined that trance itself has nothing at all to do with sleep, and is much more easily distinguished from a sleeping state physiologically than from a waking state. Measurements attempted included a number of famous early experimental studies in the 1930's, on such variables as EEG measurements, cerebral circulation, heart rate, respiration, basal metabolism, and various behavioral parameters. Representative of these experiments comparing hypnosis and sleep was: M.J. Bass, "Differentiation of the hypnotic trance from normal sleep," Journal of Experimental Psychology, 1931, 14:382-399.

Though the mentation in hypnosis often resembles dreaming, it appears much closer to daydreaming in character than to normal night time dreaming.

Clark Hull, in his 1936 classic Hypnosis and Suggestibility describes a number of experimental setups for distinguishing the mental characteristics of sleep from those of hypnotic trance.

One thing suggested by this is that if sleep can be viewed as largely a generalized cortical inhibition, and trance is not in any determinable way identified with sleep, that trance is not a form of sleep or a stupor. This is also easily determined by observing the range of activities possible in hypnotized subjects (compared to waking subjects and those under the influence of depressant drugs).