In archaeology, the term entoptic phenomena relates to visual experiences derived from within the eye or brain (as opposed to externally, as in normal vision). In this respect they differ slightly from the medical definition, which defines entoptic phenomena as only applying to sources within the eye, not the brain.
Entoptic hallucination, geometric hallucinations, or phosphenes, are one of the most commonly reported visual effects of psychedelics. Phosphenes are a sensation of light caused by excitation of the retina by mechanical or electrical means.
Klüver gave them the name form constants. In addition to psychedelics, form constants may for example also be triggered by epilepsy, sensory deprivation, migraine headaches and fever. Klüver categorised four types of patterns: lattices, cobwebs, tunnels and spirals. Another term for such geometrical patterns is entoptic images.