[gnaritas]

Actually they do, and beyond that, the retina contains neurons that do the basic processing before travelling down the optic nerve to the brain. The retina is also basically brain tissue and is part of the central nervous system.

[pygy_]

Indeed, among other things, they detect edges, movement, and preprocess color from RGB to something conceptually closer to YUV (with color encoded along two contrast axes: red-green and blue-yellow).

https://en.wikipedia.org/wiki/YUV