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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Seeing involves more than just the eyes. It involves the brain too. And not just one part of the brain but a series of different parts of the brain. Each part of this system does a lot of processing and interpreting of the image to turn it in to something the brain can recognise.
Vision involves both ‘bottom-up’ and ‘top-down’ processing. Bottom-up processing is where light enters the eyes and your visual system does it’s best to turn this in to information that the brain can use. Top-down processing is where your brain tells your visual system what it expects to see. This means that after all this work the visual system does to produce an image, you are more likely to see what you expect and less likely to see what you don’t expect to see.
We can also say we can train our visual system to see certain things based on our experiences. So, for example, a hunter can become really good at recognising the shapes of different animals through this top-down processing. Someone who plays with Lego can become really good at recognising the different shaped blocks.
By extension, some visual hallucinations seem to be the result of what people expect to see, based on their previous experiences, through this top-down processing. This means that our ordinary thoughts, hopes and fears from our minds can appear in front of us as things that we see. In this way, hallucinations as a result of top-down processing will always be meaningful to the person experiencing them.
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