A form constant is one of several geometric patterns which are recurringly observed during hypnagogia, hallucinations and altered states of consciousness.

In 1926, Heinrich Klüver systematically studied the effects of mescaline (peyote) on the subjective experiences of its users. In addition to producing hallucinations characterized by bright, "highly saturated" colors and vivid imagery, Klüver noticed that mescaline produced recurring geometric patterns in different users. He called these patterns "form constants" and categorized four types: lattices (including honeycombs, checkerboards, and triangles), cobwebs, tunnels, and spirals.^[1]

In 1988 David Lewis-Williams and T.A. Dowson incorporated the form constant into his Three Stages of Trance model, the geometric shapes comprising the visuals observed in the model's first stage.

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 experiences and sensory experiences of those with synesthesia. Other triggers include psychological stress, threshold consciousness (hypnagogia), insulin hypoglycemia, the delirium of fever, epilepsy, psychotic episodes, advanced syphilis, sensory deprivation, photostimulation, electrical stimulation, crystal gazing, migraine headaches, dizziness and a variety of drug-induced intoxications.^[1] These shapes may appear on their own or with eyes shut in the form of phosphenes, especially when exerting pressure against the closed eyelid.^[2]

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 the primary visual cortex. Concentric circles in the retina are mapped into parallel lines in the 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.^[1]

Author Michael Moorcock once observed in print that the shapes he had seen during his migraine headaches resembled exactly the form of fractals. The diversity of conditions that provoke such patterns suggests that form constants reflect some fundamental property of visual perception.

Form constants have a relationship to some forms of abstract art, especially the visual music tradition, as William Wees noted in his book Light Moving in Time about research done by German psychologist Heinrich Klüver on the form constants resulting from mescaline intoxication. The visual and synaesthetic hallucinations this drug produced resembles, as Wees noted, a listing of visual forms employed in visual music:

• Text from "Hallucinogens and Creativity" page by Susan Opar
• "Spontaneous pattern formation in large scale brain activity: what visual migraines and hallucinations tell us about the brain"; online video of lecture by Jack Cowan
• "Spontaneous pattern formation in the primary visual cortex"
• free version of "What Geometric Visual Hallucinations Tell Us about the Visual Cortex"