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==History==
In 1926, [[Heinrich Klüver]] systematically studied the effects of [[mescaline]] ([[peyote]]) on the [[subjective experience]]s of its users. In addition to producing hallucinations characterized by bright, "highly [[Saturation (color theory)|saturated]]" colors and vivid imagery, Klüver noticed that mescaline produced recurring [[geometric pattern]]s in different users. He called these patterns "form constants" and categorized four types: [[lattice (group)|lattices]] (including [[Hexagonal lattice|honeycomb]]s, [[checkerboard]]s, and [[triangle]]s), [[Spider web|cobweb]]s, tunnels, and [[spiral]]s.<ref name="Bressloff2002">{{cite journal | last = Bressloff | first = Paul C. |author2=Cowan, Jack D. |author3=Golubitsky, Martin |author4=Thomas, Peter J. |author5=Weiner, Matthew C. |date=March 2002 | title = What Geometric Visual Hallucinations Tell Us About the Visual Cortex | journal = Neural Computation
In 1988 [[David Lewis-Williams]] and T.A. Dowson incorporated the form constant into their Three Stages of Trance model, the geometric shapes comprising the visuals observed in the model's first stage.
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==Precipitants==
Klüver's form constants have appeared in other drug-induced and naturally occurring hallucinations, suggesting a similar physiological process underlying hallucinations with different triggers. Klüver's form constants also appear in [[near-death experience]]s and sensory experiences of those with [[synesthesia]]. Other triggers include [[psychological stress]], [[hypnagogia|threshold consciousness (hypnagogia)]], [[insulin hypoglycemia]], the delirium of [[fever]], [[epilepsy]], [[psychosis|psychotic episodes]], advanced [[syphilis]], [[sensory deprivation]], [[photostimulation]], [[electrical stimulation]], [[crystal gazing]], migraine headaches, [[dizziness]] and a variety of [[drug]]-induced [[Substance intoxication|intoxication]]s.<ref name="Bressloff2002" /> These shapes may appear on their own or with eyes shut in the form of [[phosphene]]s, especially when exerting pressure against the closed eyelid.<ref>{{cite journal|last1=Tyler|first1=CW|title=Some new entoptic phenomena.|journal=Vision Research|date=1978|volume=18|issue=12|pages=1633–9|doi=10.1016/0042-6989(78)90255-9|pmid=726316}}</ref>
It is believed that the reason why these form constants appear has to do with the way the visual system is organized, and in particular in the mapping between patterns on the retina and the columnar organization of primary visual cortex. Concentric circles in the retina are mapped into parallel lines in visual cortex. Spirals, tunnels, lattices and cobwebs map into lines in different directions. This means that if activation spreads in straight lines within the visual cortex, the experience is equivalent to looking at actual form constants.<ref name="Bressloff2002" />
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