How does an MRI scan "see" a hallucination?

26 September 2010

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Question

If a hallucination is seeing what isn’t there, how would a brain scan then see the same thing which isn’t there?

Answer

We put this to Professor Paul Fletcher and Professor Jack Gallant...

Paul - I don't know if that's a great metaphysical question. I mean, the brain scanner is looking at how the brain behaves when it is seeing something that isn't there, so it's not so much interested or able to see the content of that although as we've just heard from Jack actually, the possibility of seeing what the brain thinks is there is possibly something for the future.

Chris - Any comment on that Jack?

Jack - Yeah. I think there's growing evidence that when you have a visual hallucination, what's actually happening is the visual areas of your brain are being activated, essentially top down from inside out and the visual experiences you have in the visual hallucination are -since the brain subsystems were being operated in those cases are visual, then you both experience visual events and you could decode visual events because you're decoding from the same parts of the brain that are encoding visual information normally.

Chris - And hence, what you've got is this system where you think it's real because it's the same bit of the brain that would say, "Yup, I'm experiencing something" but it's just being internally generated.

Jack - Right.

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