illusory visual spread
   Also known as illusory visual perseveration. Both terms were introduced in or shortly before 1949 by the British neurologist Macdonald Critchley (1900-1997) to denote a type of "visual perse-veration characterized by the visual extension, expansion, or prolongation of a stimulus-object, "in other words, a kind of spatial persevera-tion of objects seen". In illusory visual spread, objects within the visual field are often perceived as multiple copies, and neighbouring objects may appear to take on the colours and texture of these objects. Critchley illustrates the latter phenomenon as follows: "The pattern of a striped or chequered garment would seem to extend over the face of the wearer. The pattern of cretonne curtains would often seem to extend along the adjacent wall." As to the pathophysiology of illusory visual spread, it has been suggested that the visual parietal regions may be involved in its mediation. In an etiological sense the phenomenon is associated primarily with " aurae occurring in the context of paroxysmal neurological disorders such as migraine and epilepsy, and with the use of " hallucinogens such as LSD and mescaline. Illusory visual spread is classified by Critchley as a type of visual perseveration, which is in turn classified as a "reduplicative phenomenon or a type of " metamorphopsia.
   References
   Critchley, M. (1949). Metamorphopsia of central origin. Transactions of the Ophthalmologic Society of the UK, 69, 111-121.
   Critchley, M. (1953). The parietal lobes. London: Edward Arnold & Co.
   ffytche, D.H., Howard, R.J. (1999). The perceptual consequences of visual loss: 'positive' pathologies of vision. Brain, 122, 1247-1260.
   Santhouse, A., Howard, R., ffytche, D. (2000). Visual hallucinatory syndromes and the anatomy of the visual brain. Brain, 123, 2055-2064.

Dictionary of Hallucinations. . 2010.

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