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

We need to add one complexity to this picture. Curiously, in the primate retina, there are only ON rod bipolar cells. This means that if they are blocked as it can be done with APB, the rod system is eliminated entirely. This is exactly what happens when APB is administered to the eye. The result is that the animal becomes night blind.

These findings suggest that the reason the ON and OFF channels have emerged is to be able to send signals to the central nervous system rapidly with excitatory signals for stimuli made visible either by virtue of light increment or light decrement. This enables the organism to rapidly process both kinds of information. If this were not the case organisms could not respond quickly enough to stimuli seen by virtue of light increment. This processing capacity comes about by virtue of the ON bipolar cells that have the mGluR6 neurotransmitter receptors that invert the photoreceptor signal and hence excite the ON ganglion cells when stimuli are seen by virtue of light increment.