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.
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,Gurney, E., Myers, F.W.H. and Podmore, F. (1886). Phantasms of the Living, Vols. I and II. London: Trubner and Co.Sidgwick, Eleanor; Johnson, Alice; and others (1894). Report on the Census of Hallucinations, London: Proceedings of the Society for Psychical Research, Vol. X. 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.See Slade, P.D. and Bentall, R.P. (1988). Sensory Deception: a scientific analysis of hallucination. London: Croom Helm, for a review.
Types Of particular interest, for reasons to be discussed below, are those anomalous experiences which are characterised by extreme perceptual realism.
Apparitional experiences 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,See, for example, Green, C., and McCreery, C. (1975). Apparitions. London: Hamish Hamilton, pp. 192-196. and even objectsApparitions, pp. 197-199. 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.Apparitions, pp. 178-183.
Out-of-body experiences 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.Irwin, H.J. (1985). Flight of Mind: a psychological study of the out-of-body experience. Metuchen, New Jersey: The Scarecrow Press. Charles McCreeryMcCreery, C. (2008). Dreams and psychosis: a new look at an old hypothesis. Psychological Paper No. 2008-1. Oxford: Oxford Forum. Online PDF 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.Oswald, I. (1962). Sleeping and Waking: Physiology and Psychology. Amsterdam: Elsevier. On this model OBEs represent the intrusion of Stage 1 sleep processes into waking consciousness.
OBEs can be regarded as hallucinatory in the sense that they are perceptual or quasi-perceptual experiences in which, by definition, the ostensible viewpoint is not coincident with the physical body of the subject. Therefore, the normal sensory input, if any, that the subject is receiving during the experience cannot correspond exactly to the perceptual representation of the world in the subject's consciousness.
As with hallucinatory experiences in general, attempts to survey samples of the general population have suggested that such experiences are relatively common, incidence figures of between 15 and 25 percent being commonly reported.See Irwin, op.cit., for a review. The variation is presumably to be accounted for by the different types of populations sampled and the different criteria of 'out-of-body experience' used.
Dreams and lucid dreams
A dream has been defined by some (e.g. Encyclopædia Britannica) as a hallucinatory experience during sleep.
A lucid dream may be defined as one in which the dreamer is aware that they are asleep and dreaming. The term 'lucid dream' was first used by the Dutch physician Frederik van Eeden,van Eeden, F. (1913). A study of dreams. Proceedings of the Society for Psychical Research, 26, Part 47, pp. 431-461. who studied his own dreams of this type. The word 'lucid' refers to the fact that the subject has achieved insight into their condition, rather than the perceptual quality of the experience. Nevertheless, it is one of the features of lucid dreams that they can have an extremely high quality of perceptual realism, to the extent that the dreamer may spend time examining and admiring the perceptual environment and the way it appears to imitate that of waking life.See Green, C. (1968). Lucid Dreams. London: Hamish Hamilton, for examples.
Lucid dreams by definition occur during sleep, but they may be regarded as hallucinatory experiences in the same way as non-lucid dreams of a vivid perceptual nature may be regarded as hallucinatory, that is they are examples of 'an experience having the character of sense perception, but without relevant or adequate sensory stimulation [...]'.Drever, (1952). A Dictionary of Psychology. London: Penguin.
False awakenings A false awakening is one in which the subject believes they have woken up, whether from a lucid or a non-lucid dream, but is in fact still asleep.Cf. Green C. and McCreery C. (1994). Lucid Dreaming: the Paradox of Consciousness During Sleep. London: Routledge. Chapter 7. Sometimes the experience is so realistic perceptually (the sleeper seeming to wake in their own bedroom, for example) that insight is not achieved at once, or even until the dreamer really wakes up and realises that what has occurred was hallucinatory. Such experiences seem particularly liable to occur to those who deliberately cultivate lucid dreams. However, they may also occur spontaneously and be associated with the experience of sleep paralysis.
Laboratory-induced hallucinations Psychotic-like symptoms, such as hallucinations and unusual perceptual experience, involve gross alterations in the experience of reality. Normal perception is substantially constructive and what we perceive is strongly influenced by our prior experiences and expectancies. Healthy individuals prone to hallucinations, or scoring highly on psychometric measures of positive schizotypy, tend to show a bias toward reporting stimuli that did not occur under perceptually ambiguous experimental conditions.Bentall R.P, & Slade P.D. (1985). Reality testing and auditory hallucinations: a signal detection analysis. British Journal of Clinical Psychology, 24, 159 –169.Tsakanikos, E. & Reed, P. (2005). Seeing words that are not there: detection biases in psychometric schizotypy. British Journal of Clinical Psychology, 44, 295-299 During visual detection of fast-moving words, undergraduate students scoring highly on positive schizotypy had significantly high rates of false perceptions of words (i.e. reported seeing words that were not included in the experimental trials).Tsakanikos, E. & Reed, P. (2005). Do positive schizotypal symptoms predict false perceptual experiences in non-clinical population? Journal of Nervous and Mental Disease, 193, 809-812. Positive schizotypal symptoms in healthy adults seem to predict false perceptions in laboratory tasks and certain environmental parameters such as perceptual loadTsakanikos, E. (2006). Perceptual biases and positive schizotypy: the role of perceptual load. Personality and Individual Differences, 41, 951-958. and frequency of visual targetsReed, P., Wakefield, D., Harris, J., Parry, J., Cella, M. & Tsakanikos, E. (2008). Seeing non-existing events: effects of environmental conditions, schizotypal symptoms and sub-clinical characteristics. Journal of Behavior Therapy and Experimental Psychiatry, 39, 276-291. are critical in the generation of false perceptions. When detection of events becomes either effortless or cognitively demanding, generation of such biases can be prevented.Tsakanikos, E. (2006). Perceptual biases and positive schizotypy: the role of perceptual load. Personality and Individual Differences, 41, 951-958.
Subtypes Auditory hallucinations Auditory hallucinations, and in particular the hearing of a voice, are thought of as particularly characteristic of people with schizophrenia. However, normal subjects also report auditory hallucinations to a surprising extent. For example, Bentall and SladeBentall R. P. and Slade P. D. (1985). Reliability of a scale measuring disposition towards hallucination: a brief report. Personality and Individual Differences, 6, 527–529. found that as many as 15.4% of a population of 150 male students were prepared to endorse the statement "In the past I have had the experience of hearing a person's voice and then found that no one was there". They add:
Green and McCreeryGreen and McCreery, Apparitions, op.cit. p. 85. found that 14% of their 1800 self-selected subjects reported a purely auditory hallucination, and of these nearly half involved the hearing of articulate or inarticulate human speech sounds. An example of the former would be the case of an engineer facing a difficult professional decision, who, while sitting in a cinema, heard a voice saying, "loudly and distinctly": 'You can't do it, you know". He adds:
This case would be an example of what Posey and LoschPosey, T. B. and Losch, M. E. (1983). Auditory hallucinations of hearing voices in 375 normal subjects. Imagination, Cognition and Personality, 3, 99–113. call "hearing a comforting or advising voice that is not perceived as being one's own thoughts". They estimated that approximately 10% of their population of 375 American college students had this type of experience.
It has been suggested that auditory hallucinations are affected by culture, to the extent that when American subjects were examined they reported hearing stern authoritarian voices with violent or prohibitive suggestions, whereas voices heard in India and Africa tended to be playful and collaborative instead.
Hypnogogic and hypnopompic hallucinations occur in people without other symptoms and are considered non-pathological.
Sense of presence
This is a paradoxical experience in which the person has a strong feeling of the presence of another person, sometimes recognised, sometimes unrecognised, but without any apparently justifying sensory stimulus.
The nineteenth-century American psychologist and philosopher William James described the experience thus:
The following is an example of this type of experience:
Experiences of this kind appear to meet all but one of the normal criteria of hallucination. For example, Slade and Bentall proposed the following working definition of a hallucination: The experience quoted above certainly meets the second and third of these three criteria. One might add that the "presence" in such a case is experienced as located in a definite position in external physical space. In this respect it may be said to be more hallucinatory than, for example, some hypnagogic imagery, which may be experienced as external to the subject but located in a mental "space" of its own. Leaning, F. E. (1925). An introductory study of hypnagogic phenomena. Proceedings of the Society for Psychical Research, 35, 289–409.Mavromatis, A. (1987). Hypnagogia: the Unique State of Consciousness Between Wakefulness and Sleep. London: Routledge and Kegan Paul. Other explanations for this phenomenon were discussed by the psychologist Graham Reed who wrote that such experiences may involve illusion, misinterpretation or suggestion. He noted that the experiences are usually reported at moments of fatigue, stress, or during the night.Reed, Graham. (1988). The Psychology of Anomalous Experience. Prometheus Books. pp. 44–45.
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