I found this article incredibly fascinating. I just had to post it here for posterity.
From Quanta Magazine:
There is still much more to understand about hallucinations. Jean-Paul Sartre experimented with mescaline in Paris in 1935 and found it distorted his visual perception for weeks. Houses appeared to have “leering faces, all eyes and jaws,” clock faces looked like owls, and he saw crabs following him around all the time. These are much higher-level hallucinations than Klüver’s simple form constants. “The early stages of visual hallucination are very simple — these geometric patterns,” Ermentrout said. But when higher cognitive functions kick in, such as memory, he said, “you start to see more complex hallucinations and you try and make sense of them. I believe that all you’re seeing is the spontaneous emergence of [stored memories] as the higher brain areas become more excited.”
Heinrich Klüver classified the shapes he saw while under the influence of hallucinogenic drugs into four categories, known as “form constants.”
Diagram showing how lines in the visual field (circular regions on the left) map to lines in the striate cortex, part of the visual cortex involved in directly processing visual information.
You can download this article as a pdf.