Part of the show Naked Science Question and Answer
Jill in Cambridge asked:
I've been blind for forty years and when I dream, I can still see. I can see things I haven't seen like my grandchildren and places I haven't been to. How is this possible?
Humans are incredibly visual creatures. We devote over a third of our brain power just to being able to see. That makes us quite similar to dogs, although dogs of course live in less of a visual world and more of a smelly world. A dog's nose is 3000 times more sensitive to smell than a human. When you go to sleep and have a dream, the regions of the brain you use during the day to do various tasks light up if you have a brain scan while dreaming. If you look at people who are dreaming and look at the visual areas of the brain you'll see that they're becoming very active. When you wake people up when they're showing those signs and ask them what was happening, they'll say that they were in a field or walking around and experiencing something. They can give you a pretty graphic description. When you're blind, often what happens is that when you've seen once, the bits of the brain that used to do the seeing have laid down a pretty powerful memory of what's out there in the world around you. You know what colours there are you know what objects look like. When you go to sleep, although there's no direct input from the eyes now, still can generate those images and they're every bit as real as they were when you really were seeing. I have a lot of blind friends that they actually love going to sleep because it reminds them of what seeing is like, and it also reminds them of what colours are like.