How do we see what we see? Or maybe: how do we visualize what we see? According to Oliver Sacks,”we see with the eyes. But we see with the brain as well. And seeing with the brain is often called imagination. And we are familiar with the landscapes of our own imagination, our inscapes. We've lived with them all our lives.”
Steve Silberman, investigative reporter for Wired and other magazines, is now writing also for the Science Blog Network of the Public Library of Science (PLoS Blogs) and has just published a lengthy interview with Oliver Sacks, on “vision, his next book, and surviving cancer.” Read below an excerpt, and visit Steve Silberman’s blog at PLos.Silberman: In the book, you go on to describe some fascinating experiments you performed on your own vision, where you observed your brain “filling in” the blank spots in your visual field with patterns and hallucinations. The naive view of our visual system is that it works like a camera, passively receiving sense impressions and compiling them in the brain into a more or less accurate picture of the world. But the work of researchers like V.S. Ramachandran and many others has led us to understand that vision is a highly active and even speculative process, with the brain making guesses and predictions about what the eyes can’t see.
In the course of your illness, you discovered that your brain would generate elaborate patterns, even clouds or leaves, to hide the blank space in your vision caused by the tumor. You’ve always been interested in the brain’s generative visual activity — whether caused by illness or psychedelics — and wrote about it at length in your first book, Migraine. But what did you learn from these experiences about how the brain creates a seemingly seamless world out of fragmentary sense impressions?Sacks: In general terms, I learned that the brain is always busy. In particular, if a sensory input — whether it be vision or hearing or kinesthesia — is taken away, there will be some sort of compensation, and the cortical systems involved in those representations will become hyperactive. This first became clear to me when I spoke to various blind people. One man, for example, who had lost his sight when he was about 20, said that when he read Braille, he didn’t feel it in his fingers, he saw it. And there’s nice evidence that the occipital areas of the brain, and the inferotemporal areas — visual areas — are excited in that sort of situation.For myself, I was very struck by this “filling in” business. The first thing that struck me was when I was in hospital and I could pay more attention to these things — perhaps too much attention. But the scotoma in my vision, the blind area, was almost like a window looking into a landscape. I could see movement, and people, and buildings in it — things like those my brain concocts while I’m falling asleep or before a migraine. But this seemed to be going on continuously.And then there was an episode that very much startled me. Kate was in the room at the time too. I was washing my hands, and then for some reason I closed my left eye, and I continued to see the wash basin, the commode next to it, and the mirror very, very clearly — so clearly, in fact, that my first thought was that the dressing over the right eye must be transparent. But it was a huge, thick, opaque dressing. This was something quite different from an after-image. It was more like a strange persistence or perseveration of vision. The image wasn’t being erased in the usual way.But this sort of thing really only hit me after I had been lasered in June of ’07 and lost my central vision. Then the night I took off the bandage, I saw this great black amoeba — this thing shaped like Australia — but when I looked up at the ceiling, it immediately disappeared. It turned white and became camouflaged by taking on the color of its surroundings. I then found that I could fill it in with simple patterns, like the repeating geometric pattern on my carpet.Then I discovered another phenomenon which astounded me. Later that month, I went to Iceland for a friend’s wedding. Coming back on the plane, it was very hot, so I took off my shoes and socks. I liked playing with the scotoma, moving it around and putting things “into” it, so I used it to amputate my leg mid-shin. But then I started wiggling my toes, and gradually there was a strange, pinkish, protoplasmic extension around the stump of my leg. This formed itself into the shape of a foot with wiggling toes, and followed all my movements exactly. It didn’t look quite real — it had no skin texture or whatever — but it really was an astounding phenomenon, and made me feel that the visual area had become hypersensitive to other inputs, such as proprioceptive input or some sort of motor afferent.I also had — and still have — almost continuous hallucinations of a low order: geometric things, especially broken letters, some of them like English letters, some like Hebrew letters, some like Greek, some runes, and some a bit like numbers. They tend to have straight lines rather than curves, but they rarely form actual words. This is not something I said in the book, but if I smoke a little pot, they sometimes become words. And they tend to be in black and white — but when I smoke a little pot, they’re in color.
Oliver Sacks: what hallucination reveals about our minds.
