The Neural Control of Vision
C. The Retina

The receptive fields of retinal ganglion cells, as shown in Figure 4, are circular and are comprised of two regions, a central, excitatory one, and a concentric surround region that is typically inhibitory. As a result of this arrangement, a small spot of light confined to the center area produces a vigorous response in ganglion cells whereas a larger spot that impinges on both the center and the surround regions produces a much reduced response as will be shown later.