Did you know that some people who are seeing ghosts are actually suffering from Charles Bonnet syndrome?
The disease is named after the Swiss naturalist Charles Bonnet, who described the condition in 1769.
He first documented it in his 89-year-old grandfather who was nearly blind from cataracts in both eyes but perceived men, women, birds, carriages, buildings, tapestries, physically-impossible circumstances and scaffolding patterns.
The condition can affect anybody at any age with diminishing eyesight. Even people with normal vision can develop it if they blindfold themselves for long enough.
Trick is, on opening our eyes, the nerve cells in the retina send a constant stream of impulses to the visual parts of the brain.
If the retina is damaged, the stream of impulses reduces, but - rather than lie dormant - other parts of the brain become hyperactive.
So when the brain isn’t receiving as many pictures as it is used to, it builds its own artificial images instead from the areas we use every day to process faces, objects, landscapes and colours.
What you hallucinate depends on which part of the brain these increases are located.