TheRetina

Therods are mostsensitive to light and dark changes, shape and movement and only containone type of light-sensitive pigment. Rods are not good for color vision. However, in a dim room, we use mainly our rods, but we are "color blind".Rods are more numerous than cones in the periphery of the retina. Nexttime you want to see a dim star at night, try to looking at it with yourperipheral vision...use your ROD VISION to see the dim star. There areabout 120 million rods in the human retina.

The cones are not as sensitive to light as the rods. However,cones are most sensitive to one of three different colors (green, red orblue). Signals from the cones are sent to the brain which then translatesthese messages into the perception of color. However, cones only work inbright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited fordetecting fine details. There are about 6 million cones in the humanretina. Some people cannot tellsome colors from others - these peopleare "color blind". Someone who is color blind does not have a particulartype of cone in the retina or one type of cone may be weak. In thegeneral population, about 8% of all males are color blind and about0.5% of all females are color blind.

The fovea,shown here on the left, is the region of the retina that provides for themost clear vision. In the fovea, there or NO rods...only cones. Thecones are also packed closer together here in the fovea than in the restof the retina. Also, blood vessels and nerve fibers go around the foveaso light has a direct path to the photoreceptors.

One part of the retina does NOT containany photoreceptors. This is our "blind spot". Therefore anyimage that falls on this regionwill NOT be seen. It is in this region that the optic nerves cometogether and exit the eye on their way to the brain.

(Image adapted from Biodidac)

To demonstrate the blind spot, look atthe image below or draw it ona piece of paper: Forthose of you without a graphicalbrowser, the image is a small dot on the left side separated by about 6-8inches from a small + on the right side. Close your left eye.Hold the image (or place your head from the computer monitor) about 20inches away. With your right eye, look at the dot. Slowly bring theimage (or move your head) closer while looking at the dot. At a certaindistance, the + will disappear from sight...this is when the + falls onthe blind spot of your retina. Reverse the process. Close your right eyeand look at the + with your left eye. Move the image slowly closer to youand the dot should disappear. Here is another image that will help you find your blind spot. Forthis image, close your right eye. With your left eye, look at the redcircle. Slowly move your head closer to the image. At a certaindistance, the blue line will not look broken!!