The other day I discovered (or rediscovered - how could I have missed such a simple visual illusion?) that I have a three hands.
Here's the trick: place your open hands, palms toward you, over your face. Keep your little and ring fingers pressed together on your forehead, but leave your palms apart just enough to make room for your nose. If you're doing it right, you can see past your nose about 45 degrees into a small part of the room to your left with your right eye open, the opposite with your left eye open. (This is how magicians who "blindfold" a subject trick the audience.)
But now open both eyes, and slowly move both hands directly away from the center of your face. There between your hands is a third, misshapen hand. It is skinny enough to look sort of like a foot. But, damn it, it shouldn't be there!
Well, of course it should. It's a normal way for your binocular vision system to handle such a situation.
But viewing my odd, ghostly third hand, I though of Oliver Sacks's new book, "The Mind's Eye." In it, Sacks offers a series of essays about how nornally talented people have coped with strange kinds of vision disrputions. Sacks has been hooking us for years with such tales - as in his "The Man who Mistook his Wife for a Hat" - but this time he has a further surprise: Sacks himself suffers from a visual malady that makes him unable to recognize people he knows when they are out of their usual context.
Anyway, a reviewer of the book, Israel Rosenfield in the current Harper's magazine, thinks it opens a "deep and surprising" view of reading, perceiving, and understanding. He refers to a man with good vision - a writer and musician -who after a stroke could no longer read words or music, or even, at first, distinguish individual letters or multi-digit numbers. The man eventually learned to distinguish them (but not read his own writing) by sweeping his arm to trace out a letter. The reviewer called it reading by the act of writing ... or by movement.
Rosenfield notes that only animals - not plants - have brains. This is because animals move. Their world keeps changing, and they require a brain to keep track of this. He goes on to suggest that the stroke victim's very movement in tracing the shape of letters tuned into his brain's most basic reason for existence.
I imagine sitting before a group of young, wide-eyed children, showing them their very own third hands, and spinning a tale of its magic abilities. (Yes, I have such daydreams.) I'm not sure how this relates to Oliver Sacks, but it should.