Examine Figure 5.17. You should see grayish diamonds at the crossroads among the black squares. Explain why.

The analogy may sound silly, but it illustrates something that happens in the retina. The receptors send messages to excite the closest bipolar cells (like giving them cookies) and also send messages to slightly inhibit them and the neighbors to their sides (like subtracting cookies). The net result will be to heighten the contrast between an illuminated area and its darker surround.
Actually, light striking the rods and cones decreases their spontaneous output. However, they have inhibitory synapses onto the bipolar cells, and therefore, light on the rods or cones decreases their inhibitory output. A decrease in inhibition means net excitation, so to avoid double negatives, we’ll think of the output as excitation of the bipolar cells. In the fovea, each cone attaches to just one bipolar cell. We’ll consider that simple case.