Have you ever seen your blind spot?
There’s a fantastically interesting demonstration over on Serendip, a science hub supported by Bryn Mawr College. Check it out.
The gist of it is that vertebrate eyes, due to the way they evolved, are naturally set up in a way that leaves a hole in the vision. The retina, which senses light, is behind the nerve fibers that transfer the signals to the brain. In order to get those nerve fibers out of the eye, they bundle up near the back and go back into the skull– but since the retina is normally behind the fibers, there’s nothing to sense light in that spot. (Check out the image to the right in case that description’s not clear.)
The interesting thing, however, is that this doesn’t result in a gaping hole in your vision. Make no mistake, there’s absolutely no way, given the anatomy, that that portion of your eye detects light. Then why is your vision whole?
Enter the brain. Your brain, as the Serendip demonstrations show, is remarkably adept at filling in holes. In the demonstration, you adjust your vision while focusing on a single point, until a large black dot elsewhere on the page disappears. If you do this on a white background, then what you see is an unblemished field of white. But more remarkably, if there is a line running through the blind spot, your brain fills in that line, even though it’s getting no information for that spot. If the blind spot occurs in a field filled with a regular pattern, then your brain fills in the pattern appropriately.
I’m constantly amazed by this for a couple of reasons. First, it’s downright incredible. That our brains are sophisticated enough to patch up the image we’re seeing based on what’s around our blind spot– that’s just cool. And we don’t even notice it. Before I experienced the Serendip demonstration, I had no idea that my brain was doing all this work behind the scenes. Clearly, it’s doing its job, and it’s doing it well.
But the second thing that hits me about this is what it implies about our knowledge. The blind spot demonstration shows the power of our brains, which is incredible, but it’s also a reminder about the limitations of what we can know through perception. My brain is working hard and doing a great job at patching up that image and drawing the line right through the blind spot when that dot disappears, but it doesn’t change the fact that the dot disappears. True, if I stayed in that particular arrangement forever, never moving my eyes, I’d never realize that what I was seeing was not the whole picture. The whole thing seems accurate and truthful– but it’s not. The brain may continue the patterns, but it fails to perceive the big honking black dot in front of it. What we see is not always how it is. Continue reading
