Lateral Inhibition

This is a rough side cut of your eye. The stuff we care about is labeled.

The basic mechanics of the eye are thus: Light goes in the front, does a bunch of stuff we don't care about for the purposes of this explanation, and hits the retina wall. The retina wall processes the light (and modifies the data) and sends it along the optic nerve to the brain where you can process what you're looking at.

The modification of the data in the retina is what we want to talk about. Specifically what's called Laterial Inhibition. So let's draw a picture to see what happens in the retina.

This is a simplified view of the retina layer. Light hits the receptors, the receptors pass on info to the cells above them, who pass on into to the cells above them, that have a long string that joins up into the "optic nerve."

The Horizontal and Amarcine cells do not actually relay information. They monitor the two junctions on the communication line. If there is very little information, they release chemicals that destroy the information before it gets passed on. If there is a lot of information, they release duplicates of that information. Again: Very little information? Squash it. Lots of information? Duplicate it.

This graph doesn't explain compression. There are about 10 light receptors to every middle cell. And there are about 10 top cells for every middle cell. So our brains only receive 1/127th of the information our physical eye sees. That's why we have the Amarcine and Horizontal cells -- to squelch the too-little-to-matter information, and to amplify when changes are there.

So what does this mean to us in useful terms? Here, have a picture:

If you look closely, you'll notice that the left and the right side of each band is a different value. See it? Actually, they're all solid values, but our eye sees them as gradients. What the Horizontal and Amarcine cells are enhancing is contrast relationships. The signal says, "This is brighter than this," and the Amarcine cell says, "Yeah, and more!" and we end up seeing one side darker than it is, and one side lighter than it is, making it appear as a gradient.