Vision
an eye opener
The Greeks had this really crazy notion. It’s called the emission theory. According to which the light doesn’t come into your eyes. It comes out of your eyes!
Your eyes emit some kind of… ray. Which travels out and touches objects. And at the point of contact, vision occurs, as though the objects were lit up by a fire.
Then why can’t we see things at night, in the dark?
The Greeks had a simple explanation: there are two kinds of light: the internal source in your eye and the external source in the world (like
the Sun or an oil lamp). And, so said Plato, you need both in order to see.
The light from your eyes has to meet up with the daylight and blend with it. It takes two to tango.
If you close your eyes, you can’t emit any rays, so even though the daylight remains, you can’t see it. Duh.
But how come you can see something instantly even though it’s far away at the horizon? Because, dummy, you reached out to it with your ray and made the connection.
This is the way everyone understood vision for the next 2000 years.
To us it seems nuts. As though the world didn’t exist without us.
But it turns out that maybe the Greeks were right after all, at least metaphorically.
Though for bats, it’s quite literally true, except they are emitting beams of sound rather than light.
Like us, they construct an ongoing image of the world in their brain, made from two different sources: the sonic projection of what the bat expects, based on prior patterns of experience, and the errors it finds when the echo comes back. A continuous feedback loop of nearly instant course corrections until, about half the time, mouth and bug connect.
But none of this goes on in our conscious awareness. It never occurs to us that we are making a guess every time we look at something. We just think we are seeing what is actually there in front of us.
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Nor do we pay much attention to what we are seeing as long as it coincides with our expectations. We only really notice something if it’s not what we expected to see. And most of the time, we correct our view so quickly — we move the coffee pot just a little closer to the cup — that we don’t even notice we were just about to spill it.
The gaps between what we expected and what we found are filled in so fast that it all seems like one continuous strip on film.
But there are always gaps.
And movements to fill the gaps.
A photograph would show the world like this:
The retinal centers of each eye, the fovea, are like the headlights of your car, where only a small angle is in sharp focus, about twice the moon’s width.
So the world would appear on your retina like this:
Flipped upside-down and right-to-left. With a blind spot where the optic nerve connects to your brain. And everything blurred except for the center.
As we walk or move our heads, or as the object in front of us moves, we must constantly move the image back into this tiny focal area.
The eye movements are called saccades, a French word from the early 1700’s for a jerk — a violent check of a horse by giving a sudden pull on the reins.
It happens about 3-5 times a second, each lasting about 1/20th of a second, so your eyes are moving about 20% of the time you are awake (and even more when you are dreaming). Each image lasts about the same time a frame of a movie is on the screen, so in a sense, we are stitching everything together like a film — and projecting it onto a screen that we call “reality.”
Paradoxically, creating the illusion of a stable visual world requires the constant movement of our head and eyes, along with a constant stream
of changes in our brain. A series of guesses and compensations.
It’s not just our senses that must guess. We guess about everything. And we are wrong about everything. And only by getting it wrong can we ever get it right. An endless waltz.
