Our retinas are flat and 2D, surfaces not solids.
But human beings perceive the world vividly in 3D. How can it be? Where does the extra D come from?
An excellent online article by David Hubel lists a whole range of mechanisms that let us perceive depth. This experiment is about just one: stereopsis.
Human eyes are spaced about 60 mm apart, so slightly different images fall on our left and right retinas. By taking advantage of this data (about 50, 000 times per waking hour by my calculation) our brains judge the distance of nearby objects – and more than that; we can perceive objects as 3D entities. We see a ball is a ball, not just a flat disc.
Stereopsis needs two eyes. If you lost one you would not be ‘stereo you’ any more. How would it feel?
At this point you will probably just shut one eye and look around. That’s what I did, and shrugged and thought, ‘What’s the big deal?’
Actually stereo sight versus non-stereo sight is a big difference. This experiment is intended to demonstrate just how big.
Two spoons, one little, like a teaspoon, and one bigger, like a desert spoon.
Close one eye (and keep it closed until told to open it again).
Take one spoon in your left hand and the other spoon in your right, and hold them side by side in front of you, an inch or two apart. Your computer screen is a good background.
Now move the small spoon towards you until it appears exactly the same size as the big spoon.
At this point you should be able to tell that the small spoon is a bit closer. This ability to discriminate is probably because to see the two spoons you have to focus at different distances, providing the brain with some information about distance. But if you are like me the impression will be only slight and the scene before you will seem almost flat.
Now open the eye you’ve been keeping closed. Ka-boom! Suddenly you can see two spoons in 3D and the little one is much closer than you realized.
– Though there any many ways we can judge the distance of objects, probably no other provides that unique subjective experience (“It’s solid-looking”) – what a philosopher would probably call the qualia. The fact of the two eyes seeing slightly different things and that this is the main mechanism behind our seeing a solid world was only realized for the first time in the 19th century.
– I have a friend who has lost an eye (and sadly can’t do this experiment therefore). He describes how, without stereopsis, anything that flies suddenly into his field of view might be a bird, some way off, or a fly quite close by. For a moment he can’t tell. Is it a bird? Is it a fly? (Superman is ruled out.)
Once he focuses, he knows which it is – but, just for a second, it is an AFO – an Ambigous Flying Object.
– It took me a while to come up with the two spoons as a good experiment. I tried all sorts of ideas for viewing objects with one eye shut, but then I was inspired by a Mitchell and Webb comedy sketch that exploits the ambiguous nature of spoon sizes.