how do non drug induced Visual Hallucinations work ? what happens in the brain that makes the eyes see something that isn’t there?
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You don’t experience the sensation of vision with your eyes, you experience it with your brain. You can experience visual stimuli in the absence of eyes, but you can not experience visual stimuli with intact eyes and without a visual area in the brain.
Visual hallucinations aren’t as much a case of seeing something that isn’t there, but seeing what your visual area generates in the absence of input from the eyes. This can be due to any number of reasons involving mental illness, epilepsy, near-death experiences, trance experiences, substances (especially psychedelics) and it is also known to occur in blind people.
Think of it like this: The brain is constantly generating experiences and imagery. The information from your eyes doesn’t generate an image of the outside world, it restricts the amount of freedom your brain has to generate visual experiences. Without certain restrictions, it has more freedom to generate different experiences/images.
For a wonderful overview of this idea: **Defining and measuring hallucinations and their consequences – what is really the difference between a veridical perception and a hallucination? Categories of hallucinatory experiences (Pages: 23-45) –
Jan Dirk Blom** in: Collerton, D. (ed), 2015. *The Neuroscience of Visual Hallucinations.* Chichester: Wiley-Blackwell.
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