Just google geometry / check wikipedia. It's all about shapes likes squares, circles, hexagons, lines, tiles, repetitive form. Fractals are also technically geometric hallucinations I guess since they form mathematical formulae but they are so incredibly complex that it can seem almost random except for the fact that there are also repetitive or recursive patterns in it. There are fractal aspects about everyday life as well, like the fact that a mountain looks generally the same from very far away as zoomed in at small bits of rock and sand. The same often goes for coastlines. The reason is probably that as long as things are made from the same material or are influenced by the same forces it is universal and a lot similar laws apply to the macroscopics as to the microscopics. Of course there are also differences, some forces that seem random and chaotic add up to form new patterned combined forces.
Reasons thought of why geometric hallucinations exist are I think explained on this great old poster, no idea where to find it. Hopefully someone can provide a link to it. I think there are at least two phenomena to it that may be intricately linked to one another:
- the way neuronal systems are shaped in branched pathways seem to follow fractal and repetitive patterns that represent shapes and operations that can be analogous to mathematical algorithms.
- we seem to be programmed to detect and recognize patterns and shapes probably as part of an evolved trait that helped us interpret and memorize maps of our surrounding environments. The same thing seems to have happened to lifeform detection, and later on also to face expression detection and processing as part as social evolution.
Let alone patterns we learn to detect in our own lives that are part of some kind of goal or purpose in our life, like the meaning of a bullseye mark, clustering, borders and filling, and many many other things.
I think the way our brains are wired reflect these mechanisms and systems in certain ways even if heavily coded into the hardwire. Though scientists are already able to decode brain activity to distill visual information (I think from cats at least).
Because psychedelics act on all kinds of mechanisms in our brain that are involved in information processing it is not surprising that these phenomena I've mentioned up until now are all themes that play roles in visuals. Like seeing faces in objects where there aren't any, like being more sensitive to distortions in understanding lifeforms like when we are tripping in nature, fractals, etc.
These systems in the brain can also be so overloaded that we whiteout and sense things without using those algorithms to process sensory data into understandable bits. That's when we feel like boundaries dissolve and all the phenomena I mentioned disappear and melt together into one big sea.
Rational mysticism kicks ass.