There's a delicate line between normal brain functionality and breakdown, between reality and hallucination. One place where the border wavers is in the visual phenomena that accompany certain migraine headaches. Half an hour or more before pain begins, sometimes, a blind spot appears off-center in the field of view. It's not an afterimage, and it's not associated with either eye's retina --- it's much farther back in the brain itself. Over a period of many minutes, the blind spot grows slowly. Around its edge there's a flickering zig-zag crenelated kind of aura, the so-called "fortification" illusion.
Weird! (I've only observed it a couple of times, decades ago; fortunately I haven't had many migraines over the years. If you see such an aura and wish to preempt a headache, you may want to try taking an analgesic, caffeine, and/or lots of water. Good luck!) Perhaps the illusion reveals a spreading biochemical reaction in the visual cortex, a diffusion of blood vessel constriction, or something else. In any case, it's a striking example of the physical nature of perception: the material substrate that underlies and forms the basis of thought. That's something easy to forget about when discussing "mind" in the abstract world of philosophy.
(see http://www.migraine-aura.org/EN/index.html for work by Dr. Klaus Podoll and colleagues at the Migraine Aura Foundation concerning migraine aura & migraine art; see also OcularMigraines (3 Jan 2004), ... )