A little while ago I watched a series called The Brain with David Eagleman and one thing that really stuck with me was how the eyes and brain interact. The eye

A little while ago I watched a series called The Brain with David Eagleman and one thing that really stuck with me was how the eyes and brain interact. The eyes are not connected straight to the main brain, they feed into a 'frame buffer' which creates an image. The brain then looks at what is in the 'frame buffer' and can also add/feedback into it. This is partially why psychosis can seem so real. One part of our brain feeds into the 'frame buffer', the other part of the brain interpreting what you see can't tell the difference between input from the eyes and feedback from the brain. Those spiders you are seeing are as real to you as if you really do see them with your eyes. This also goes some way to explaining how the 'gorilla walking through the basketball game' illusion works and other visual tricks such as not seeing the blackout when your eyes move - you really didn't see that cyclist etc. (I've explained that in my simple language from recollection as it's how I easily understand it.)