Sidestepping what is defined as an "edge", quite a lot of work is done in the retina, including differential computation across cones - some "aggregator" cells will fire when it detect lines, movement, etc.
You can read on ganglion cells, bipolar cells and amacrine cells and see that a lot of preprocessing is done before even reaching the brain!
You can read on ganglion cells, bipolar cells and amacrine cells and see that a lot of preprocessing is done before even reaching the brain!