Peter Naish "Hypnotic and Psychotic Hallucinations: The Temporal Connection"

There are intriguing parallels between the hallucinations experienced in conditions such as Schizophrenia and those that can be induced in hypnosis.  Indeed, Szechtman et al (1998) considered them sufficiently similar to justify employing hypnotic auditory hallucinations as a research analogue for the ‘hearing voices’ of schizophrenia.  A hint that similar underlying processes may be involved in the two conditions is to be found in the Schizotypy/Susceptibility correlation.  Schizotypy is a measure of a healthy person’s tendency to experience some of the effects that would be commonplace for a schizophrenic patient.  Many people will claim experiences such as occasionally feeling that there is something different about themselves when looking in a mirror, or thinking they heard someone speak when they were alone.  These are schizotypal experiences.  Regarding hypnosis, not everyone is equally responsive: the range of susceptibilities in the general population is normally distributed (the same bell-shaped curve that appears with intelligence for example).  It turns out that people who score high on a measure of schizotypy also tend to score high on measures of hypnotic responsiveness.

Until recently the perceptual distortions achieved by people who are highly susceptible to hypnosis had to be taken ‘on trust’; sceptics were inclined to the view that, at best, they were no different from imagining.  Recently, however, evidence gathered from brain-scanning suggests that hypnotised people really do ‘see’ things that are not there, and that the pattern of neural activity bears more resemblance to actual seeing than to imagining.

Even with the advent of scanning, it has not been easy to work with schizophrenic patients.  One problem is the difficulty of arranging a hallucination ‘to order’, hence the use of a hypnosis analogue by Szechtman et al.  Nevertheless, recently information about neural activity during hallucinations has been emerging.  The data gleaned are consistent with the belief that, while schizophrenic experiences are clearly self-generated, the sufferer remains ignorant of their source so ascribes them to an external agency.  The situation is very much the same for hypnotised people, when given a traditional suggestion such as “Your arm is getting lighter and is going to float up.”  Typically, the person will announce that “The arm is moving by itself.”

Observations such as these raise numerous questions - certainly more than they answer.  Thus, although brain scans may reveal similar patterns of activity during real and hallucinatory experiences, simply to know the locations of the activity does not reveal the mechanism of hallucination.  Moreover, while we might come to accept that a diseased brain can deliver a faulty representation of reality, it is less obvious how a hypnotised person is able at one and the same time to generate a misperception ‘to order’ then experience it as if being externally derived.

The research to be described initially sidesteps some of these issues by examining a more tractable phenomenon: time distortion.  It is well known that people emerging from a session of hypnosis tend grossly to underestimate the duration of the experience.  Examining the phenomenon in more detail reveals that it is probably the result of an internal ‘clock’ running more slowly.  Furthermore, the studies have shown that people who are able to generate more convincing hallucinations also experience more time distortion.

Taken on its own, the impact of hypnosis upon time perception would seem of relevance only to researchers in hypnosis.  However, it turns out that other hallucination-generating conditions, not only schizophrenia but Parkinson’s disease too, are associated with an inability to make accurate time judgements.  The phenomenon has been particularly well researched in Parkinson’s patients and the results of analogous studies in hypnosis turn out to be remarkably similar.

Because the underlying cause of Parkinson’s disease is quite well understood it is possible to identify the brain circuitry involved – both in time-keeping and in developing and controlling a sense of conscious awareness.  The processes implicated are closely linked to what we know about the mechanisms of schizophrenia and hypnosis, and by ‘triangulating’ in this way a plausible over-arching picture begins to emerge.  This is a picture which makes it possible to hypothesise about the nature of veridical and hallucinatory experiences, of imagination and agency, and to some extent of consciousness itself.

Szechtman H, Woody E, Bowers KS and Nahmias C (1998) Where the imaginal appears real: A positron emission tomography study of auditory hallucinations.  Proceedings of the National Academy of Sciences  95 1956-1960.