The Neural Control of Vision

C. The Retina

Five major cell types have been identified in the retina: The photoreceptors that form a single layer of cells against the inner wall of the eye, the horizontal cells, the bipolar cells, the amacrine cells, and the ganglion cells whose axons send the signals to the brain. There are two major classes of photoreceptors, the rods and the cones. Rods are for night vision and cones for day vision. Cones further subdivide into three color-selective types in old world primates and in humans, that have peak sensitivity in the short (blue), medium (green) and long (red) regions of the visible spectrum. The horizontal, bipolar, amacrine and ganglion cells also come in several subvarieties.

It is convenient to think of retinal organization as consisting of two major systems: the through system and the lateral system. Starting with the photoreceptors, the through system connects to the retinal ganglion cells via the bipolar cells. There are two lateral systems that are produced by virtue of the horizontal cells and the amacrine cells. These lateral networks give rise to the surround organization of receptive fields as seen in the retinal ganglion cells that will be discussed shortly. The interconnections among receptors, horizontal cells and bipolar cells take place in the outer plexiform layers (OPL); interconnections between bipolar cells, amacrine cells and retinal ganglion cells take place in the inner plexiform layers (IPL).

The rods and cones converge on retinal ganglion cells via different pathways. The cones connect with ganglion cells via cone bipolar cells. The rods connect with the ganglion cells via rod bipolars and amacrine cells.

The information from the retina is sent to the brain by the axons of the retinal ganglion cells. There are several different classes of retinal ganglion cells that differ not only in their response characteristics, but also are distinct anatomically. The two classes shown in Figure 2 are the ON and OFF which will be discussed below.