The Fortean Times has an online article about the unusual experiences that can occur in a condition called macular degeneration, where light sensing cells in the part of the eye called the macular cease to work. As well as blindness in the central part of vision, hallucinations can occur.
“Hallucinations? What do you mean?” I asked, totally nonplussed. He outlined several forms of hallucination that were plaguing him. The first one to manifest was what Don described as looking like “a ball of string or basketwork, a globular shape with an aperture on one side”. He would see this image as if projected onto walls or other surfaces. He could sometimes make out a small face inside the aperture, and on the occasions when this became particularly evident the basket-like effect would adjust around it like a bizarre headdress.
This hallucinatory state is known as Charles Bonnet syndrome, after the 18th century philosopher who noticed the condition in his father.