visual perseveration
   Also known as perseveration. Both terms are indebted to the Latin verb perseverare,which means to maintain or to keep on stating. The term visual perseveration was introduced in or shortly before 1949 by the British neurologist Macdonald Critchley (1900-1997) to denote the illusory reoccurrence of visual percepts after the stimulus-object has moved out of focus. As Critchley explains, "This experience is not ordinarily a persistent one, but is intermittent. Only very rarely can it be demonstrated at will. There are two main varieties, namely: (1) visual perseveration in time, or paliopsia; and (2) illusory visual spread, a visual extension, expansion, and elongation; in other words, a kind of spatial perseveration of objects seen." A third variety of visual perseveration is the *trailing phenomenon. As to the pathophysiology of visual perseveration, it has been suggested that the visual parietal regions may be involved. Visual perseveration is commonly classified as a * reduplicative phenomenon or as a type of *metamorphopsia (which is itself classified as a * sensory distortion). Conceptually, it is related to reduplicative phenomena occurring in any of the other sensory modalities. Thus perseveration in the auditory modality is referred to as auditory perseveration or *palinacusis, and perseveration in the tactile or somatosensory modality as perseverative somaesthetic sensation or * palinaesthesia. Irrespective of the sensory modality involved, perseveration tends to be associated in an etiological sense 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. The term perseveration is also used in psychiatry to denote a formal thought disorder characterized by the aimless repetition of words, sentences, or themes.
   References
   Critchley, M. (1949). Metamorphopsia of central origin. Transactions ofthe 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.
   Klee, A., Willanger, R. (1966). Disturbances of visual perception in migraine. Acta Neurolog-ica Scandinavica, 42, 400—114.
   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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