Retinal photoreceptors (rods) are often arranged in groups that function together. These are referred to as center-surround receptive fields and the key to understanding them is to recognize that cells in the center of that field will innervate the bipolar cell differently than cells in the periphery. There are on and off-center bipolar cells such that some are activated by light hitting their central regions (on-center) and others are oppositely activated by light hitting the periphery (off-center). The adaptive advantage here is that contrast between adjacent regions is enhanced.
