Sunday, September 18, 2011

Your blind spot.

This is an easy and powerful demonstration of the fact that vision is only partly what the eye sees, and partly what the brain thinks. Vision is very significantly dependent upon how the brain processes information it receives from the eyes.

It is pretty common knowledge that the brain throws away unnecessary information it gets from the eyes, such as uninteresting peripheral imagery. Notice while reading this it seem like all you see are the words on this page, but now without taking your eyes off these words just “pay attention” to what is noticeable around the words and your screen. You can peripherally see lots more than just the words on this screen, but while attention is focused the brain discards this extraneous information from the field of perception.

What this experiment demonstrates is the less-well-known phenomenon that the brain supplies fictional vision when the eye fails to supply actual imagery. Very cool.

Thanks to my buddy Stuart Smith for pointing me to this.

The following article is copied from Serendip, reorganized into a single page for easier reading.

Most people (even many who work on the brain) assume that what you see is pretty much what your eye sees and reports to your brain. In fact, your brain adds very substantially to the report it gets from your eye, so that a lot of what you see is actually "made up" by the brain.

Some special features of the anatomy of the eyeball make it possible to demonstrate this to yourself. The front of the eye acts like a camera lens, differently directing light rays from each point in space so as to create on the back of the eye a picture of the world. The picture falls on a sheet of photoreceptors (red in the diagram), specialized brain cells (neurons) which are excited by light.

The sheet of photoreceptors is much like a sheet of film at the back of a camera. But it has a hole in it. At one location, called the optic nerve head, processes of neurons collect together and pass as a bundle through the photoreceptor sheet to form the optic nerve (the thick black line extending up and to the left in the diagram), which carries information from the eye to the rest of the brain. At this location, there are no photoreceptors, and hence the brain gets no information from the eye about this particular part of the picture of the world. Because of this, you should have a "blind spot" (actually two, one for each eye), a place pretty much in the middle of what you can see where you can't see.

Look around. Do you see a blind spot anywhere? Maybe the blind spot for one eye is at a different place than the blind spot for the other (this is actually true), so you don't notice it because each eye sees what the other doesn't. Close one eye and look around again. Now do you see a blind spot? Hmm. Maybe its just a little TINY blind spot, so small that you (and your brain) just ignore it. Nope, its actually a pretty BIG blind spot, as you'll see if you look at the diagram below and follow the instructions.


Close your left eye and stare at the cross mark in the diagram with your right eye. Off to the right you should be able to see the spot. Don't LOOK at it; just notice that it is there off to the right (if its not, move farther away from the computer screen; you should be able to see the dot if you're a couple of feet away). Now slowly move toward the computer screen. Keep looking at the cross mark while you move. At a particular distance (probably a foot or so), the spot will disappear (it will reappear again if you move even closer). The spot disappears because it falls on the optic nerve head, the hole in the photoreceptor sheet.

So, as you can see, you have a pretty big blind spot, at least as big as the spot in the diagram. What's particularly interesting though is that you don't SEE it. When the spot disappears you still don't SEE a hole. What you see instead is a continuous white field (remember not to LOOK at it; if you do you'll see the spot instead). What you see is something the brain is making up, since the eye isn't actually telling the brain anything at all about that particular part of the picture.

Alright, you say, that's kind of neat, but maybe the brain isn't "making it up." It just knows to put white where the blind spot is. Let's try another situation and see what happens.



Neat, what happens if you switch the yellow and the green?



Alright, so the brain can match surrounding colors, whatever they are. Can it do anything else?



Wild. It not only matches background colors, but completes the line across the blindspot too. I wonder what happens if the line enters the blindspot but doesn't come out the other side? (Try it yourself. Make the spot disappear and then bring a pencil in from the side so the tip is just into the spot). And how about if there are things that don't get interrupted by the blindspot at all? Can they affect what the brain makes up to put there?



V.S. Ramachandran has a nice article called "Blind Spots" in Scientific American (May, 1992, pp 86-91). A comparison of observations discussed there (the figure on the top right of page 88) with those you have just made leads to some interesting questions. Ramachandran also discusses interesting work of his own aimed at determining where in the visual pathways "filling in" occurs (cf. Ramachandran, V.S. and Gregory, R.L. (1991) Perceptual filling in of artificially induced scotomas in human vision. Nature 350: 699-702).

The blind spot also figures in some interesting discussion about how best to talk about what the brain does, the issue being whether the brain actually "fills things in" or instead simply ignores things about which it has no information. Daniel Dennett's lively and accessible Consciousness Explained (Little, Brown and Co., 1991) has a nice discussion favoring the latter (pp 344-366).

If you'd like to explore the issues here a little further (and have Mozilla Firefox or Internet Explorer as a browser), you can use a java applet to "map" your own blindspot, giving you a better sense of how big it actually is.

Presentation created by Paul Grobstein. Thanks to Heather Billik, BMC '96, for raising questions which led to the figure here, and to Lindsay Welch, BMC '98, for worrying about its relation to Ramachandran's paper.

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