Edge Processing in the Retina

Why is it important to understand receptive fields? In any sensory system, the receptive field is what initially organizes sensory input and sets up the higher-order processing which will occur later in the sensory pathway.

For example, the center-surround organization of bipolar cell receptive fields sets the retina up for its first job: to detect edges. Edges are where an area of high photon flux is near an area of low photon flux. A pen-and-ink drawing of an elephant is activating edge detectors and telling us what the shape of the elephant is in a very profound way. Bipolar cells are edge detectors.

Bipolar cells tend to be a bit dramatic and over-emphasize edges. This can be seen in the optical illusion known as Mach bands, where a gradual change in the number of photons over distance is encoded as a darker stripe and lighter stripe even though no such stripes exist. If you take a piece of paper with a hole in it and cover the bands here, you will see there are no dark and light bands adjacent to the edges between different shades of gray, even though you perceive them when you look at the entire image.