The Neural Control of Vision

G. The Function of the On and Off Channels in Vision

action potentials discharged by an ON and an OFF retinal ganglion cellFigure 18 shows schematically the responses of an ON and OFF retinal ganglion cell. Two basic facts should be noted: 1. ON cells are excited by light increment (bright spot) and OFF cells to light decrement (dark spot). 2. The response is much weaker when large spots are used. This is because these cells have concentric center-surround organization with the surround being inhibitory. This arrangement makes such cells for the most part local difference detectors. Thus ganglion cells respond best to differences in illumination levels of their receptive fields that occurs between regions that do and do not include the center of their receptive fields.