Oliver Sacks’ Lucid Hallucinations

“We see with the eyes, but we see with the brain as well.

10% of visually-impaired people hallucinate, but no more than 1% acknowledge them because they fear that they will be seen as insane.

As you lose vision, as the visual parts of the brain are not getting any input, they become hyperactive and excitable and they start to fire spontaneously and you start to see things.

[although he did not reference Ramachandran, it reminded me of phantom limb pain in amputees]

Distorted human faces and cartoon images are the most common.

How the theater of the mind is generated by the machinery of the brain.

I’m blind in one eye, and not terribly good in the other. And I see the geometrical hallucinations… And when I see all these hexagons and complex things, which I also have, in visual migraine, I wonder whether everyone sees things like this, and whether things like cave art, or ornamental art may have been derived from them a bit.

So, what is going on? Fascinatingly, in the last few years, it’s been possible to do functional brain imagery, to do fMRI on people as they are hallucinating. And in fact, to find that different parts of the visual brain are activated as they are hallucinating. When people have these simple geometrical hallucinations, the primary visual cortex is activated. This is the part of the brain which perceives edges and patterns.

When images are formed, a higher part of the visual cortex is involved in the temporal lobe. And in particular, one area of the temporal lobe is called the fusiform gyrus. And it’s known that if people have damage in the fusiform gyrus, they maybe lose the ability to recognize faces. But if there is an abnormal activity in the fusiform gyrus, they may hallucinate faces. And this is exactly what you find in some of these people. There is an area in the anterior part of this gyrus where teeth and eyes are represented. And that part of the gyrus is activated when people get the deformed hallucinations.

There is another part of the brain which is especially activated when one sees cartoons. It’s activated when one recognizes cartoons, when one draws cartoons, and when one hallucinates them. It’s very interesting that that should be specific. There are other parts of the brain which are specifically involved with the recognition and hallucination of buildings and landscapes…

Now, at this level, in what’s called the inferotemporal cortex, there are only visual images, or figments or fragments. It’s only at higher levels that the other senses join in and there are connections with memory and emotion… Normally these are all part of the integrated stream of perception, or imagination. And one is not conscious of them.

It is only if one is visually impaired, or blind, that the process is interrupted. And instead of getting normal perception, you’re getting an anarchic, convulsive stimulation, or release, of all of these visual cells, in the inferotemporal cortex. So, suddenly you see a face. Suddenly you see a car. Suddenly this, and suddenly that. The mind does its best to organize, and to give some sort of coherence to this. But not terribly successfully.”

Here’s the Video of his talk at TED.