A form constant is one of several geometric patterns which are recurringly observed during hallucinations and altered states of consciousness.
In 1926, Heinrich Kluver 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, Kluver 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.
Many of these shapes have an intriguing similarity to much of the imagery in Ernst Haeckel's Kunstformen der Natur.
Kluver's form constants have appeared in other drug-induced and naturally-occurring hallucinations, suggesting a similar physiological process underlying hallucinations with different triggers. Kluver's form constants also appear in near-death experiences and hallucinations of those with synesthesia.
Other triggers include psychological stress, or 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.
These shapes may appear on their own or with eyes shut in the form of phosphenes, especially when exerting pressure against the closed eyelid.
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.
The practice of the ancient art of divination may suggest a deliberate practice of cultivating form constant imagery and applying the brain's intuitive faculty and/or imagination to derive some meaning from transient visual phenomena.
Many religions represent geometric and/or repetitive forms as indicative of the divine, particularly in a starburst pattern.
Examples include mandalas, yantras (both of these specifically designed to evoke certain mental states), Islamic art and cathedral architecture.
Psychedelic art, inspired at least in part by psychedelic substances, frequently includes repetitive abstract forms and patterns such as tessellation, Moire patterns or patterns similar to those created by paper marbling, and, in later years, fractals. The op art genre of visual art created art using bold imagery very like that of form constants.