geometric hallucination
   Also known as geometrical hallucination, geometric visual hallucination, and optogeometric illusion. All four terms can be traced to the Greek noun geometria, which means land surveying. They are used to denote a * formed visual hallucination characterized by lines, planes, * spirals, and/or other geometrical forms or patterns. Some examples of geometric hallucinations are circles, ellipses, * herringbone patterns, spider webs, * lattices, * gratings, kaleidoscopes, mandalas, mosaics, symmetrical flowerlike patterns, and *fortifications. When they consist of irregular branching forms they are referred to as * dendropsia. When they consist of two-dimensional geometric forms such as tiles, squares, triangles, or hexagonal forms, they are referred to as tessellated hallucinations or *tessellopsia. Attempts to arrange geometric hallucinations in accordance with their phenomenological characteristics have yielded a classification of *form-constants devised by the German-American biological psychologist and philosopher Heinrich Klüver (1897-1979), and a classification of * dimensions of visual imagery devised by the American psychopharmacologists Ronald K. Siegel and Murray E. Jarvik. As to the pathophysiological correlates of geometric hallucinations, early hypotheses tended to focus on the involvement of the retina's choriocapillary circulation or its regularly arranged rods and cones, which were thought to be rendered visible by means of a process called 'transient retroreti-nal illumination'. Today, however, this entoptic model accounts for the mediation of no more than a fraction of the geometric hallucinations. As already noted by Klüver, geometric hallucinations tend to change places in keeping with the movements of the eyes, but to maintain their position relative to each other, suggesting that they are not mediated by the eye itself, but by the brain. As a consequence, the entoptic model of geometric hallucinations has now been largely abandoned in favour of a central model which attributes the mediation ofthe geometric patterns to neuronal discharges affecting the retinocorti-cal map (i.e. the patterns of connection between the retina and striate cortex), and/or neuronal circuits lying within striate cortex. Etiologically, geometric hallucinations are associated with a variety of neurological disorders (notably * aurae occurring in the context of paroxysmal neurological disorders such as migraine and epilepsy), *psychosis, and intoxication with substances such as cannabis, LSD, mescaline, alkaloids, antihis-tamines, nicotine, amphetamines, carbon dioxide, nitrous oxide, and numerous therapeutics and anaesthetics. They also occur regularly in the context of * hypnagogic and (to a lesser extent) *hypnopompic imagery. Moreover, some of the phenomena in the category geometric hallucination can be evoked experimentally by exerting bilateral pressure on the eye bulbs (resulting in so-called * pressure phosphenes), through the use of a flickering light (resulting in *photically induced hallucinations) or a weak electrical current applied to the eyeball or the visual cortex (as in cortical probing), and with the aid of transcranial magnetic stimulation (TMS) of the visual cortex. It has been suggested that geometric hallucinations share certain mechanisms for the activation of visual representation with those involved in the mediation of * mosaic vision. The term geometric hallucination derives from a classification of hallucinations governed by the theme of complexity. It is used in opposition to the terms * simple (or * elementary) hallucination and *complex hallucination.
   References
   Bressloff, P.C., Cowan, J.D., Golubitsky, M., Thomas, P.J., Wiener, M.C. (2001). Geometric visual hallucinations, Euclidian symmetry and the functional architecture of striate cortex. Philosophical Transactions of the Royal Society of London, 356, 299-330.
   Klüver, H. (1966). Mescal and Mechanisms of hallucinations. Chicago, IL: University of Chicago Press.
   Marshall, C.R. (1937). An inquiry into the cause of mescal visions. Journal of Neurology and Psychopathology, 17, 289-304.
   Siegel, R.K., Jarvik, M.E. (1975). Drug-induced hallucinations in animals and man.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.

Dictionary of Hallucinations. . 2010.

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