just a quick knee jerk example here, it seems the visual cortex has an intrinsic noise that is normally filtered out but is shunted through when psychedelics are in the mix.

https://en.wikipedia.org/wiki/Form_constant

>" Many observers see geometric visual hallucinations after taking hallucinogens such as LSD, cannabis, mescaline or psilocybin; on viewing bright ickering lights; on waking up or falling asleep; in “near-death” experiences; and in many other syndromes. Kl ¨ uver organized the images into four groups called form constants: (I) tunnels and funnels, (II) spirals, (III) lattices, including honeycombs and triangles, and (IV) cobwebs. "<

http://psychedelic-information-theory.com/geometric-hallucin...