It seems as if what is being discussed here is a fairly universal feature of human consciousness known as the 'form constant'.
There's a pretty fucking fabulous lecture
here on how these images are generated by various sort of 'noise' spilling over into the visual cortex from the rest of the brain during various 'non-normal' states of consciousness.
A paper which summarizes the information upon which the lecture is based can be found in .pdf
here.