Science Questions

Iím losing my sight, so why do I still have hallucinations?

Sun, 19th Apr 2009

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Question

Theresa, Suffolk asked:

Iím losing my sight, so why do I still have hallucinations?

Answer

Chris -   The answer is, Theresa, that you can actually get signals coming from the visual system not just from the photoreceptors, the things that convert light waves into brainwaves; you can also get signals coming from all of the things that create the visual world for you.  That means the other bits of your retina (that are not harmed by whateverís causing you to go blind), and you can also get them coming from the brain itself, in the same way as you can get tinnitus in your ears.  We think that might be a bit like phantom limb syndrome, for instance. 

There are a whole raft of reasons why people get hallucinations, and of course the normal hallucination that we all get, every time we go to bed at night, is that we dream!  This is where the bits of the brain that are involved in doing certain jobs during the day, when you go to sleep they become active again, and they re-play some of what youíve been doing during the day.  That includes the visual parts of the brain that create the visual experience, so even though your eyes may no longer work, you may not be able to physically see, you still have visual memories, and those can still be played out and you can still experience them.  One of my friends who went blind at a young age said he loved going to sleep because it reminded him what colours were like.  He could experience colours again in his mindís eye, so when people said something was read, blue or green he could understand and appreciate those colours again.

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