Hallucinations in the sane

Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation.

It is now widely recognized that hallucinatory experiences are not merely the prerogative of those suffering from mental illness, or normal people in abnormal states, but that they occur spontaneously in a significant proportion of the normal population, when in good health and not undergoing particular stress or other abnormal circumstance.

The evidence for this statement has been accumulating for more than a century. Studies of benign hallucinatory experiences go back to 1886 and the early work of the Society for Psychical Research, which suggested approximately 10% of the population had experienced at least one hallucinatory episode in the course of their life. More recent studies have validated these findings; the precise incidence found varies with the nature of the episode and the criteria of ‘hallucination’ adopted, but the basic finding is now well-supported.

Of particular interest, for reasons to be discussed below, are those anomalous experiences which are characterised by extreme perceptual realism.

A common type of anomalous experience is the apparitional experience, which may be defined as one in which a subject seems to perceive some person or thing that is not physically present. Self-selected samples tend to report a predominance of human figures, but apparitions of animals, and even objects are also reported. Notably, the majority of the human figures reported in such samples are not recognised by the subject, and of those who are, not all are of deceased persons; apparitions of living persons have also been reported.

Out-of-body experiences (OBEs) have become to some extent conflated in the public mind with the concept of the near-death experience. However, the evidence suggests that the majority of out-of-body experiences do not occur near death, but in conditions of either very high or very low arousal. McCreery has suggested that this latter paradox may be explained by reference to the fact that sleep may be approached, not only by the conventional route of low arousal and deafferentation, but also by the less familiar route of extreme stress and hyper-arousal. On this model OBEs represent the intrusion of Stage 1 sleep processes into waking consciousness.

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