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

Figure 20 provides a schematic of how the ON and OFF ganglion cells arise. By virtue of the beautiful work carried out initially by Dowling and Werblin, it has been established that the photoreceptors of the vertebrate retina all hyperpolarize to light, yield only graded potentials, and utilize the neurotransmitter glutamate. The ON and OFF systems originate at the level of the bipolar cells. Two major classes of bipolar cells are the ON and the OFF. The receptors make sign conserving synapses with the OFF bipolar cells and sign inverting synapses with ON bipolar cells that have a unique neurotransmitter receptor site (mGluR6). It has been discovered by Watkins and Evans and by Slaughter and Miller that when the glutamate analog 2-amino-4-phosphonobutyrate (APB) is applied to the retina the responses of ON bipolar cells and ON retinal ganglion cells to light are blocked. This remarkable discovery made it possible to determine how the ON and OFF systems connect in the retina, in the central nervous system, and why they have evolved.