People afflicted with Charles Bonnet Syndrome see beings from another world. Many scientists would call these beings hallucinations. Others call this syndrome a portal to a parallel reality.
People with Charles Bonnet Syndrome (or "Bonnet-people") are otherwise mentally sound. The beings appear when the Bonnet-people's vision deteriorates as a result of eye diseases such as age-related macular degeneration -- or when patients have had both eyes removed. Charles Bonnet Syndrome is more common in older people with a high level of education.
Bonnet-people report that they see apparitions resembling distorted faces, costumed figures, ghosts, and little people.
A few years ago I experienced an unexplained, if harmless, "retinal occlusion" -- essentially a spontaneous bruising of the retina. As the vision in the afflicted eye worsened due to the burst capillaries, I began seeing intricate patterns, especially in low ambient light. I've since identified the patterns as "entopic" images, thought by some to have served as the basis for early cave art.)
Although I never saw any "beings," I could sometimes make out suggestive detail of both biological and mechanical objects, and even interact with them through what, at the time, seemed to be a sort of telepathy. (I never thought what I was experiencing was anything but a subjective phenomenon, but that didn't make it any less interesting.)
Here's an account of one such Bonnet-like encounter:
In front of my face, at reading distance, there appeared to be multiple rows of compressed text, each word encapsulated in an ellipse. Each row moved rapidly from the right to the left -- too fast for me to make out any sort of narrative, but acutely responsive, so that I could visually choose a specific word-balloon and have it persist for a moment before vanishing -- instantly replaced by a stream of words with similar connotations. It was like looking into the mind of a language database or some futuristic heads-up display word processor. It also had the feel of a timed quiz or test of some sort; I can see something like it eventually becoming a high-bandwidth Web application.
I can't help but be vaguely reminded of Philip K. Dick's experience with the "phosphene activity" that inspired the eye-fooling body-suits of "A Scanner Darkly."