PHY 242 Net Research Project for 11/08/99

VISION: THE EYES HAVE IT

Without a doubt, the most astounding sense possessed by animals is the sense of sight. The front end of the visual system is the eye. Although the details of how the eye accomplishes its task are immense, the general function is easy to understand. The eye can be divided into the following:

1. A focusing system, consisting of the cornea and the lens. The cornea and lens form a real image on the retina.
2. The iris, which controls the amount of light entering the eye by adjusting the size of the opening (the pupil).
3. The retina, which transforms the real image into electrical impulses that are sent to the brain along the optic nerve.

These components are shown below in the diagram borrowed from LaserSite, one of many commercial sites dedicated to medical eye treatment.

The Outer Eye

The cornea is essentially a fixed focal length convergent lens. The second lens in this system is simply called the lens. It is a tough but flexible collection of transparent fibers. Radial zonule fibers keep the lens pulled tight into the proper shape for distant viewing. When you wish to view a nearby object, a ciliary muscle that wraps around the lens contracts to counter the zonule fibers. The iris is opened and closed by two sets of muscles that control the iris in a fashion similar to that of the lens; the radial set opens the iris while the circular muscle closes the iris. The Neurology Dept at University of Washington has a very nice site which outlines how the eye muscles control the pupil and change the shape of the lens during accommodation. It also addresses the functioning of the retina.

The Retina

Essentially, the retina is renewable film. The focused image stimulates photoreceptor cells in the retina. There are two types of receptor cells. The rod cells respond to all wavelengths of light and are sensitive to low light levels. The cone cells (which require higher light intensity) come in three varieties, sensitive to three regions of the spectrum. Roughly, the regions are centered on the colors blue, green and red. But there is considerable overlap in their response. The greatest concentration of cone cells is at the fovea, located at the center of the retina. The fovea provides the greatest image resolution. The rods are located outside the fovea and provide vision for low light levels such as at dusk and night. (For those really interested in the technical details of the retina, check out the University of Utah Medical Center WebVision page.)

Color Perception

Ideas about how we perceive color have been around for many centuries. It was Newton who showed that "white" light was composed of many colors by refracting light through a prism. Hemholtz, Young, and Maxwell all contibuted to color theories in the late 1800's. In the simplist theories, all colors are composed of three primary colors. One choice for primary colors is red, green and blue (RGB). Although many colors can be achieved by adding primary colors in varying proportions, perception plays an important role. (Check out a color wheel) For example, yellow is often considered to be a pure or primary color. Indeed, a monochromatic (single frequency) source with wavelength 570 nm will appear yellow to most observers. But yellow can also be achieved by the combination of two monchromatic sources of red (650 nm) and green (525 nm). Green can be achieved by combining yellow and blue. Humans can distinguish about 100 different hues or colors. But humans will often disagree on color. This is because color perception is a function of the photoreceptor cell response and the complex visual processing that occurs in the brain. J. Scrugg has a page on Color Theory , in which he discusses several aspects of color. Be sure to check out the Color link and the Addition link which discusses the RGB color scheme used for monitors.

(Psychiatrist Oliver Sacks has written a fascinating travelogue called The Island of the Colorblind, in which he discusses hereditary colorblindess on the Micronesian islands of Pingelap and Pohnpei.)

Some Tricks to Play With Your Eyes

• Look very closely at a friends eyes as they look intently at an object. You'll notice that the eyes are constantly jittering. This movement is purposeful. When light energy is absorbed by a photoreceptor cell, a chain of chemical processes is initiated that results in a signal along the optic nerve. It takes time, about .1 seconds, for the system to recover. (Incidently, it is the related "persistence of visual images" that makes video images such as those on TV appear to be continuous, when you are actually viewing a sequence of still images.) Hence, the brain constantly moves the eyeball in order to move the image around on the retina, allowing the cells to recover. The eye muscles that control movement are controlled by the brain and the images appear stable, despite the constant agitated motion. If you close one eye and gently push the on the open eye you will notice the view moves! Your brain did not tell the eye muscles to do this and the result is that the image appears to move. (Careful, don't poke yourself in the eye!)

• There is a blind spot on the retina (near the fovea) where the axions from the photoreceptor/ganglion cells connect to the optic nerve. There are no photoreceptors there. You don't normally notice it, since one eye always "covers" for the other. But you can easily locate it. And the effects are rather interesting. The brain has a habit of filling in missing information. Check out this blindspot page that provides several interesting diagrams to try.

• At low light levels, you must use the rod cells in your retina. But there aren't any rod cells in the central area of the fovea. Hence, with low light levels you must actually look slightly off to the side of the object you are trying to see. You do this automatically and it's a little difficult to catch yourself doing it. Look up at a bright star or planet (the moon is too large) and notice how it disappears if you try to look directly at it. It takes some time to do this, so be patient.

• One more interesting trick. The brain does a lot of processing in fusing the information from the eyes into an image. Cut a small slit (about .5 cm wide) in a piece of paper. Close one eye and look through the slit with the other eye. You should be able to see only a small portion of the scene in front of you. While holding the paper fixed with your head, rapidly swing your head back and forth. Notice that you can "see" the full picture quite well. Yet there is never more than a slice of the total scene on your retina at any one time. How's that for complex imaging!

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