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A visual receptive field is defined as the region of visual space in which a change in lightness or color will cause a change in the neuron’s firing rate. Almost all receptive fields have structure—different changes in different parts of the receptive field will have different effects on the neuron’s response. A key function of this receptive field structure is that neurons only respond to edges. When center and surround are balanced, the RGC (retinal ganglion cell) will not change its firing rate in response to uniform illumination.

Most neurons in the retina and thalamus have small receptive fields that have a very basic organization, which resembles two concentric circles. This concentric receptive field structure is usually known as center-surround organization. On-center retinal ganglion cells respond to light spots surrounded by dark backgrounds like a star in a dark sky. Off-center retinal ganglion cells respond to dark spots surrounded by light backgrounds like a fly in a bright sky.