This apparent difference in light intensity is not actually there but is created by our visual receptors and is known as lateral inhibition. This process increases contrast and results in a sharpening of vision. In fact, computers sharpen images by almost the same process process. What this means is that the signals coming from the outside are actually altered before being sent to the brain so that what we see isn't necessarily there. The way this works is as follows.
Impulses originate in the eccentric cell when the cell is stimulated by light .
This signal is transmitted through the axon then to the optic nerve to the brain. The ability of an ommatidia to discharge impulses is related to the amount of light that neighboring ommatidia are receiving. Hartline found that if one ommatidia is receiving bright light and a neighbor is receiving dim light, the first ommatidia will inhibit the signal from it's neighbor. The result is that the dimmer signal gets even dimmer and the result is an increased difference between the two which the eye would perceive as an increase in contrast. References used in the Vision sections of this site.