What do visual hallucinations tell us about the nature of consciousness?
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...
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