NPR has an brief but interesting piece on a blind man who has visual hallucinations.
Stewart, the person in question, lost his sight due to hereditary sight-loss, but has developed Charles Bonnet syndrome, a curious condition where playful visual hallucinations are common.
Two things about this condition are striking: firstly, the hallucinations are typically complex and intricate but the damage is typically only to the retina, the cortex remains intact.
Secondly, unlike many other conditions where hallucinations are common, the person typically retains complete insight. They know they are hallucinating and typically don’t mistake hallucinations for the real world.
While the person interviewed in this radio segment is blind, Charles Bonnet syndrome can occur in people with partial sight, who may have only lost vision in one part of their visual field (often due to macular degeneration). In these cases, even when the hallucinations can ‘blend in’ with true vision, the person usually knows the difference.
One of the most remarkable things about the interview is that the Stewart’s hallucinations can be triggered by quite idiosyncratic things (such as foods and thoughts) and that he takes such joy in the experience.
If you want to read more about the syndrome, the Fortean Times published a great article on it back in 2004.