Oliver Sacks: What hallucination reveals about our minds
Oliver Sacks, one of the foremost leaders in the popularisation of the intricate workings of the brain. In this TED talk he speaks of how hallucinations give us an insight into the weird and wonderful way the brain responds to the deterioration of visual and audio stimulus, with the main focus being on a condition called Charles Bonnet syndrome, a condition where visually impaired people can experience particularly strong and vivid hallucinations. He also touches on how brain trauma (such as a tumor or cyst) can cause hallucinations.
Personally, having a loved one who has suffered from a brain tumor, and who experience terribly from hallucinations which went undiagnosed for some time, I can see how these kinds of things cause people to question their own sanity.