Fig. 4. An on-center receptive field. A. A spot of light directed into the center causes vigorous discharges. B. Illumination of the surround by a ring of light inhibits background discharge and causes responses when the light is turned off. C. Light covering the entire receptive field has a relatively small effect when turned on and off because antagonistic actions in the center and surround almost cancel each other. Illumination, indicated by the dark line, lasts 0.5 seconds. (Kuffler SW. The single-cell approach in the visual system and the study of receptive fields. Invest Ophthalmol Vis Sci 1973;12:794)