The Null Device

Charles Bonnet Syndrome

Macular degeneration is an incurable disorder of the eye, affecting mainly elderly people and causing partial blindness. The curious thing about it is that, in some cases, sufferers see vivid hallucinations. (via MindHacks)
When Don was visiting the graveyard where his wife is buried, he sat for a while on a bench. He suddenly saw one end of the church on the far side of the cemetery become illuminated. Then there appeared great crowds of figures of both sexes and in all manner of dresses moving in a stately way towards the church this time they were not advancing towards him. They entered the large area of illumination and vanished.
A further visual effect which Don considered to be rather spectacular was the disappearance of people in front of him, especially presenters on stage in lecture situations. First the persons head would vanish and then the torso, yet Don would be able to see the background behind where the now invisible figure was standing with perfect, uninterrupted clarity.

It is speculated that the hallucinations, which are known as Charles Bonnet Syndrome, are caused by the brain attempting to fill the gaps in its input, interpolating the noise coming from malfunctioning eyes with past experience. Which, of course, happens all the time, except in normal circumstances, the input is more or less trustworthy. It is also believed that many more people suffer from the condition than would admit to it, lest others consider them to be going insane.

If such a condition is possible, it raises the question of how much of our everyday experience do we really get from our senses, and how much do our brains infer. I've seen the claim that the human eye's output is far too poor to give the vivid images we perceive, and much of our perception is the result of post-processing in the brain. Could the bulk of our perceived reality be hallucination, which, in normal conditions, happens to work usefully?

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