“Hallucinations relate to the higher cognitive functions of the brain. I was reading about someone at Columbia who asked volunteers to differentiate between houses and faces. Signals in the frontal cortex became active whenever subjects expected to see a face, irrespective of what the actual stimulus was. They would look at a house and "see" a face.

It's called, you know Predictive coding. The brain has an expectation of what it will see, then compares this with information from the eyes. When this goes tits up, hallucinations occur. Our eyes don't present to our brains exact photographs of the things we see. They are more like sketches and impressions chattering along the optic nerve for the brain to interpret. That's what optical illusions are about. The brain's software is perfectly capable of simulating a vision in this way”