Aetiological theories are of three kinds:
1. overstimulation affecting different levels of information processing;
2. failure of inhibition of mental functions;
3. distortion of the processing of sensory information at the interpretive level.
The work of Penfield and Perot(15) has suggested that overstimulation may be a pathogenetic mechanism. They stimulated the temporal regions of 500 patients, of whom 8 per cent reported scenic hallucinations, some in several modalities. Stimulation of the visual occipital cortex led to simple hallucinations like flashes, circles, stars, or lines. This phenomenon, known as Formkonstanz,(16) has been observed in drug-induced experimental psychosis, which is the most obvious overstimulation paradigm. It is interesting that schizophrenic patients can usually distinguish drug-induced hallucinations from those arising from their disorder. Using neural network theories, Hoffmann(1Z> simulated hallucinations by using Hopfield networks; overloading the storage capacity of the network generated what can be considered as the equivalent of hallucinations.
The disinhibition theory originated with Hughlings Jackson, who considered that productive symptoms were caused by the disinhibition of controlling neural activities, while negative symptoms resulted from damage to the systems which generate the productive symptoms. A modern approach to disinhibition theory is sensory deprivation research using dark and sound-proofed environments, but this has yielded inconsistent results. Hallucinations, narrowly defined, seldom occur after deprivation, which may be of greater relevance to the vivid, usually visual, imaginative experiences by certain people described by Galton (18) in 1880, and later by Jaensch,(!9) as 'eidetic types'. Disinhibition may also underlie the 'hypnagogic hallucinations' which can occur in healthy persons shortly before they fall asleep.
The role in the production of hallucinations of the postsensory interpretation and evaluation of stimuli is uncertain. In these terms hallucinations are a sort of deception, but this is not a sufficient description of their nature. Recent neurophysiological hypotheses and findings from neuroimaging studies have suggested that there is an 'inner censorship' (29 which deals with the ambiguities of perceptions by setting hierarchies of contingencies.
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