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The Retina

The drawing below is a diagram of horizontal cross section of the eye.

The normal (or corrected) light-focusing system of the eye (the cornea and lens) forms an image on the retina in the back of the eye. The retina is a complex sheet of cells that contains the light-sensitive receptors: about 100,000,000-125,000,000 rods and 6,000,000 cones. Because it detects light, the retina functions something lie the film in a camera. But film passively records light. The retina actively changes what it receives. One active process the retina does is to adapt. The retina adjusts its sensitivity separately to the amount of light reaching different parts. This is why the eye can see things in brightly lit places and in shadows at the same time. A photograph can show clearly a brightly lit area or a shaded area, but not in the same picture.

The fovea in the middle of the retina receives light from the middle of the visual field, where you look directly. This is the part of the visual field you can see detail and color by far best, and this is where most of the 6 million cones are concentrated. The 100+ million rods are located away from the fovea, so they carry out peripheral vision ("side" vision, away from what you look at directly). Your peripheral vision is very sensitive to light and motion, but cannot detect detail or color well. The diagram below shows the density of cones and rods in the retina from the nasal side (side by the nose) through the fovea to the temporal side (side by the temples in front of the ears)

The retina breaks up the visual image into over 100 million pieces, because each rod and cone picks up its own bit of the image. This is sort of like this video display, which forms its image from about a million small dots. The brain puts these pieces back together to produce perception of objects and events.

The retina has two other kinds of neurons: horizontal cells and amacrine cells. These cells transmit information among neighboring rods and cones and their connections to the brain. They make neighboring bipolar and ganglion cells interact, mostly through lateral inhibition

Lateral inhibition is the result of inhibition that neighboring brain pathways have on each other. In the retina, neighboring pathways from rods and cones to ganglion cells show lateral inhibition. It is very important because it sharpen edges and contours. Pictures from space have edges and contours enhanced by the same kind of process done by computer.