Asked by: Christian Sant Cassia, Malta
Hallucinations are actually more common in visually impaired people. As many as 15 per cent of visually impaired or blind people may experience Charles Bonnet Syndrome (CBS), which takes its name from the 18th-century Swiss naturalist who first described the phenomenon experienced by his grandfather.
In a healthy eye, the optic nerve is accustomed to dealing with as much data as a high-speed broadband connection. When vision is reduced or lost due to damage to the eye (from cataracts or macular degeneration, for example) the neurons in the visual cortex are suddenly left with nothing to do. The visual cortex is already adapted to automatically fill in gaps in our visual field, to compensate for the blind spot.
It seems that, in CBS sufferers, this mechanism just takes wilder and wilder guesses. Victims report dancing monkeys, floating faces and other strangeness.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.