Signal processing in the retina

Retinal cells do not work on their own. They function like a small brain out in front: they are organized into circuits that are already looking for specific things, like how much light there is, where the edge between light and dark falls, and what is moving.

There is a main path heading inward, in three steps. First the photoreceptor, the cone or rod that takes in the light and releases a chemical messenger called glutamate. That message is picked up by the bipolar cell, the middle step. And the bipolar cell hands it to the ganglion cell, which has the output cable to the brain.

The curious part is that bipolar cells do not all respond the same way. The ones called ON depolarize when light reaches them, meaning they switch on; the OFF ones do the opposite, they hyperpolarize. So the same point of light generates two messages of opposite sign.

And there is another path, off to the side. Horizontal cells stretch out laterally and quiet the bipolar cells around them. That damping of the neighbors is what makes contrast stand out: a point looks sharper if what is next to it is switched off. The distribution is not even either: in the macula each cone has its own ganglion cell, while several rods funnel into a single one.