What do visual hallucinations tell us about the nature of consciousness

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20 February 2016

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The term ‘hallucination’ is difficult to define. There is a fine line between a ‘hallucination’ and an ‘illusion’. A hallucination differs from an illusion in that illusions are a product of misinterpretations of external stimuli whereas hallucinations need no such requirement making them an entirely internal process. A true hallucination can also be distinguished from a pseudo-hallucination in which the individual can recognise that what they’re seeing or hearing is not real. Hallucinations are also different from voluntary mental imagery, in that the thought has not uncontrollably forced itself onto our minds. Slade and Bentall (1988, cited in Blackmore, 2010) provided a working definition of a hallucination: “Any percept-like experience which (a) occurs in the absence of an appropriate stimulus, (b) has the full force or impact of the corresponding actual (real) perception, and (c) is not amenable to direct and voluntary control by the experiencer.” Hallucinations are frequently associated with schizophrenia. In result, those who hallucinate hesitate to come forward due to fear of being labeled crazy. The Society for Psychical Research’s ‘Census of Hallucinations’ is one of the first attempts to study hallucinations in the general population. 1684 people from 17,000 said they had previously suffered from a hallucination. It was found that visual hallucinations were more common than auditory hallucinations, and hallucinations where generally more common in females, with the most common type being a vision of a person (Blackmore, 2010).

One general model of hallucinations is the ‘neurophysiological dissociation’ theory proposed by Marrazzi (1962, as cited in Slade 1976). He found that LSD produced inhibition of the association areas without affecting the primary visual cortex. He proposed that hallucinogenic drugs have their effect by producing a functional dissociation between the receiving cortex and the association areas. This loss of control of the latter over the former is responsible for the hallucinatory experience. A similar theory to Marrazzi is the ‘perceptual release’ theory, which was first proposed by Hughlings Jackson who believed that hallucinations were a result of loss of control of one area of the brain over the rest. More generally, he argues that memories and internally generated images are naturally inhibited by information from the senses, and so such information is released when the sensory input is disrupted. Such a process tends to happen to people who indulge in sensory deprivation tanks, or to blind or deaf people.

West (1962, as cited in Slade 1976) further developed the perceptual release theory to account for a whole range of percept like experiences, including hallucinations. The central beliefs of West’s theory is first, percept-like experiences are based on neural traces, templates, or engrams which are the permanent record of memories in the brain, secondly these templates/engrams are woven into the basic material of fantasies, dreams and hallucinations, and lastly, this reorganised experience is prevented from emerging into consciousness by the presence of external sensory input. The release of the reorganised experience can occur when there is sufficient arousal to permit awareness, combined with impairment of a sensory input. The perceptual release theory differs from Marrazzi’s theory in two main ways. Firstly, perceptual release theory, in placing arousal level as a central concept, assumes that subcortical structures play an important role, whereas Marrazzi’s theory limits the area of dysfunction to structures within the cortex itself. Secondly, the perceptual release theory emphasises the disequilibrium between external sensory input and internal input from within, whereas Marrazzi does not (Slade, 1976).

The perceptual release theory places strong emphasis on external sensory input, which has been supported through research demonstrating that interruption of the senses induces hallucinatory experiences. The best way to induce hallucinations, or where they occur the most, is through sensory deprivation. Charles Bonnet Syndrome (CBS) is a common condition among people who have lost their sight in which they suffer visual hallucinations. Many studies have looked at the syndrome in order to provide a better understanding of visual hallucinations. One study exhibited a 64-year-old blind woman suffering from complex visual hallucinations (McNamara, Heros, Boller, 1982). It was found that neurosurgical removal of a large suprasellar meningioma helped remove her hallucinatory experiences. Such a case study provides evidence that hallucinations are related to both a sensory deprivation and an impairment in CNS functioning. Another study claimed that, through looking at patients suffering from CBS, the hallucinations were due to an interruption in the visual association areas of the cerebral cortex. The researchers suggested that hallucinatory experiences could terminate on improving visual function or addressing social isolation (Menon, Rahman, Menon, Dutton, 2003).

Deprivation of hearing can also induce hallucinations. Sometimes the deaf hear hallucinated sounds – they may even hear whole orchestras playing. Various case studies have demonstrated such a claim. Miller and Crosby (1979) found an 89-year-old woman experiencing the onset of musical hallucinations with longstanding progressive hearing loss. These hallucinations were well-formed perceptions of religious hymns and were nearly constant. Another study of a 52-year-old man proposed that the cause of musical hallucinations where due to progressive hearing loss and a result of sensory deprivation (Raghuram, Keshacan, Channabsavanna, 1980). Berrios (1990), through looking at a sample of 46 subjects experiencing musical hallucination, concluded that musical hallucinations were more common in females, and age and deafness seemed to play an important role in their development.

Studies looking at CBS or hearing loss are consistent with the claim that hallucinations can occur as a result of an interruption in sensory input. However, studies have also addressed possible consistencies between hallucinations. There is no limit to the variety of hallucinations, although some common features have been identified suggesting a consistency that reflects underlying sensory processes. Such common features include spirals, concentric patterns, wavy lines, and bright colours. These similarities were investigated through studying the effects of mescaline, a hallucinogenic drug (Kluver 1926, as cited in Blackmore, 2010). It was found that brightly coloured images occurred, with a tendency to take on four forms. The first was gratings and lattices, the second was tunnels, funnels and cones, the third was spirals, and the fourth was cobwebs. These forms seem to be evident in hallucinations caused by drugs, fever, and more. The reason for such consistency lies within the mapping between patterns of the retina and the columnar organisation of the primary visual cortex. Concentric circles from the retina are mapped into parallel lines in the visual cortex, with spirals, tunnels, lattices and cobwebs mapping onto lines in different directions. Inhibition is the cause of lines of activation in the visual cortex, and hallucinogenic drugs, lack of oxygen, certain diseases all affect inhibitory cells causing an excess of activity that can spread. In result, such consistent hallucinations occur similar to that of the four forms (Blackmore, 2010). However, there are also complex visual hallucinations in which the individual suffering from CBS/schizophrenia could see more concrete images such as monsters.

In conclusion, research into hallucinations has provided a better understanding of the nature of conscious perception. Hallucinations have provided an interesting test case for theories on the mechanisms of conscious perception. In result the conscious experience without a stimulus provides a very useful case for consciousness research. Studying hallucinations is therefore extremely beneficial, as it has helped provide a window into imagery, imagination, and mental representation.


Berrios, G.E. (1990) Musical hallucinations, A historical and clinical study. BJP, 156, 188-194

Blackmore, S. (2010). Consciousness. An Introduction (2nd edition). Hodder Arnold.

Hammeke, T.A. McQuillen, M.P. Cohen, B.A. (1983) Musical Hallucinations associated with acquired deafness. J Neurol Neurosurg Psychiatry, 46, 570-572

McNamara, M.E. Heros, R.C. Boller, F. (1982) Visual hallucinations in blindness: The Charles bonnet syndrome. International Journal of Neuroscience, 17, 13-15

Menon, G.J. Rahman, I. Menon, S.J. Dutton, G.N. (2003) Complex Visual Hallucinations in the Visually Impaired: The Charles Bonnet Syndrome. Survey of Ophthalmology, 48, 58-72

Miller, T.C. Crosby, T.W. (1979) Musical hallucinations in a deaf elderly patient. Annals of Neurology, 5, 301-302

Raghuram, R. Keshavan, M.D. Channabasavanna, S.M. (1980). Musical Hallucinations in a deaf middle-aged patient. Journal of Clinical Psychiatry, 41, 357.

Santhouse, A.M. Howard, R.J. Ffytche, D.H. (2000). Visual hallucinatory syndromes and the visual brain. Brain, 123, 2055-2064

Slade, P. (1976) Hallucinations. Psychological Medicine, 6, 7-13

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