I think everybody has the blood flow thing. But the degree to which they turn into "hallucinations" varies. By that I mean very elaborate geometric shapes wit

I think everybody has the blood flow thing. But the degree to which they turn into "hallucinations" varies. By that I mean very elaborate geometric shapes with a lot of motion and rotation. Sort of like a Persian rug in 3D and 4D (time), with a lot of scale. Fractals, etc. I suspect there is some common physiological reason why people find fractals "trippy" and say pictures of cars in traffic are not "trippy". Drugs tickle your brain in a certain ways, but as the OP suggests, they're not the only way your brain can be tickled like that. And some brains are more predisposed to such hallucinations than others.