Oliver Sacks has made a brilliant career of introducing readers to people who, because of abnormalities in their brains, perceive “reality” in ways that seem strange to “normal” people. In his book The Mind’s Eye, though, he becomes one of his own case studies: he developed melanoma in his right eye, and various surgeries and treatments left him with a large "scotoma": a degenerated portion of his right eye’s field of vision.

Ever the unflappable observer, Dr. Sacks shares with us some fascinating observations about the blind spot / "filling in" phenomenon discussed in the previous blog post:

I knew that the normal blind spot, which we all have, where the optic nerve enters the eye, is automatically filled in, so we are unaware of its existence. But the normal blind spot is tiny, whereas my own scotoma was huge, blotting out more than half of the entire visual field of the right eye. And yet, within a second or two of looking at a white surface, it could completely fill in, becoming white instead of black. The next day I tested this with a blue sky and found the same result. The scotoma became as blue as the sky, but this time I had no need to plot its margins with my finger, for when a flock of birds flew by, they suddenly disappeared into my scotoma, emerging on the other side a few seconds later — as if they had been cloaked in invisibility like a Klingon warship.

This filling in, I discovered, was strictly local, depending on a steady fixation of gaze. If there was a slight movement of the eye, the filling in dissipated, and the ugly black amoeba was back. Local, but persistent, for if I looked at a red surface for a few minutes and then at a white wall, I would see a large red amoeba (or Australia) on the wall, which would last about ten seconds before it turned white.

The blind spot, so called, does not just fill in color, it fills in patterns too, and I enjoyed experimenting with my own scotoma, testing its powers and limitations. It was easy to fill in a simple repetitive pattern — I started with the carpet in my office — though a pattern took a bit longer than a color, perhaps needing ten or fifteen seconds to duplicate. It would fill in from the edges, like ice crystallizing on a pond.…

I started to think of my visual cortex not just as a rigid duplicating device, but as an averaging device, capable of sampling what was presented to it and making a statistically plausible (if not photographically accurate) representation of it. [1]

Sacks’s injury pulled back the curtain on his brain’s inner workings in a way that most of us hope we never experience. The image of the scotoma filling in “from the edges, like ice crystallizing on a pond” — is extraordinary. You can almost feel the brain straining to create some kind of coherence in Sacks’s visual world. In the end, though, it’s complete and utter fiction.In the same book Sacks also tells of a man named Howard Engle, who, one normal-seeming morning, went to fetch his newspaper:

The July 31, 2001, Globe and Mail looked the way it always did in its make-up, pictures, assorted headlines and smaller captions. The only difference was that I could no longer read what they said. The letters, I could tell, were the familiar twenty-six I had grown up with. Only now, when I brought them into focus, they looked like Cyrillic one moment and Korean the next. Was this a Serbo-Croatian version of the Globe, made for export?…Was I the victim of a practical joke? I have friends who are capable of such things….I wondered what I might do to them that would improve on this piece of foolery. Then, I considered the alternative possibility. I checked the Globe’s inside pages to see if they looked as strange as the front page. I checked the want ads and the comics. I couldn’t read them either….

Panic should have hit me like the proverbial ton of bricks. But instead I was suffused with a reasonable, business-as-usual calm. “Since this isn’t somebody’s idea of a joke, then, it follows, I have suffered a stroke.” [2]

Indeed he had suffered a stroke, a minor one, in a part of the brain that handles vision. A few other parts of his world were a bit out of whack as well — he had some trouble recalling the names of everyday objects, and some difficulty recognizing colors and faces — but most everything else was normal. And maybe strangest of all, he had no trouble with writing: he could write as fluidly as ever. He couldn’t, though, read what he had written.

What makes the case of Howard Engel so fascinating is its subtlety. If a major brain injury causes big vision problems, that's not surprising: if you do massive damage to the brain, of course you won’t be able to see properly. In the case of Mr. Engel, though, “the world” is still so obviously “out there” — and yet it can’t be, for why else would this one little piece of “the world” be so…off? Why would almost everything look normal except for letters? Somehow the “off-ness” of this one small portion of vision makes us question how “real” everything else is.

The fact that vision is affected makes it even more unsettling. We rely more on vision than on any other sense for our feeling that the world is real. Yet Howard Engle's case forces us to ask: Can I trust what I see?

[1] Oliver Sacks, 2010. The Mind's Eye. New York: Vintage Books, pp. 174-176.

[2] Sacks, 2010, pp. 53-54.