While the mechanics of eye is a well-understood and fascinating subject, how the brain processes and interprets the information that is passed from the retina along the optic nerve is less understood and even more fascinating. (For a primer on the mechanics of the eye, visit THE EYE.)
The brain must interpret the activity information it receives from each receptor cell (a rod or cone cell) on the retina, not unlike the information from each pixel of the CCD "film" in a digital camera. (CCD stands for Charged Coupled Device. The CCD is composed of million of pixels on which the image is focused.) But unlike the single number that represents each pixel of the CCD, the information from the rods and cones is constantly changing and is processed in complicated ways along the optic nerve. Mammals cannot survive with single snapshots of the world in front of them. The constantly changing scene in front of us must be interpreted in a way that allows us to do such things as track motion and recognize objects.
That is not as easy! Consider as examples:
- Each cell has to "rest" after being stimulated, hence the eye must constantly move the image about on the retina. (An experiment to show this is described below.)
- Recognizing the face of a friend, or the gate of his or her walk, requires a rapid interpretation of an ever-changing scene. The speed and accuracy with which the brain can do this is still far beyond any optical recognition systems devised by engineers.
- Athletes can run and catch a ball at the same time. Although the head is bouncing with each step, the brains uses information from the eye muscles and the inner ear to cancel out that motion and allow the athlete to smoothly track the flight of the ball.
Some Tricks
Below are series of experiments that will provide a glimpse of the complex processing of the visual system by "playing a few tricks on the eyes".
- JITTERY EYES- Look very closely at a friend's 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 lower portion of the eyelid of 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!)
- BLIND SPOT - 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.
- LOOKING ASKEW - 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.
- FUSING THE SCENE - 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!
- 3-D AND THE MAGIC EYE - How do we see in three dimensions? The answer lies in the fact that we have two eyes - stereo vision. Each eye sees the scene in front of us at a slightly different angle. Our brain processes those slightly different scenes and provides us with our sense of depth. You can convince yourself with a simple experiment. Cover one eye and have a friend hold a pencil a few feet in front of you. Bring your hand in from the side (not from the front) and try to touch the pencil. You'll probably miss! Now look with both eyes and try it ... no problem!
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Stereo vision was understood hundreds of years ago and spawned the first generation of stereographs in the 19th century. Check out the larger version of the Muir and Roosevelt stereograph at the right. The two scenes look the same at first, but there were taken with a "double-camera" with two lenses and two films. If you relax your eyes and let the left eye foucs only on the left picture and the right eye only on the righ picture, your brain will fuse the two photographs into a single 3-D reconstruction. Placing a card up to your nose along the dividing line of the two pictures may help. | 
provided by the Sierra Club |
More recently, this same concept has spawned the world of the Magic Eye sterogram. What makes the sterogram different, is that the picture is hidden in a depth field map. At first glance, the stereogram appears to be just a repeated pattern - like some wallpaper. Normally, your two eyes are "aimed" at the same point. But you can easily see the image if you relax and allow your eyes to focus as if your were looking at a distant object. Now your eyes pick up two different points on the picture and the hidden 3-D image pops right out. The image is a 3-D relief, as if someone had tightly wrapped the "wallpaper" around the object.
Don't get frustrated if this does not happen immediately. Concentrating or trying too hard is usually counterproductive. Just relax and let your gaze naturally drift beond the image ... almost the same as when you dirft of into a reverie during a physics lecture. Try the two images below:
There are may sites where you can access Magic Eye images, such as http://www-ai.ijs.si/sirds/sirds.html. If you would like more information on how the Magic Eye sterograms work, check out http://www.techmind.org/stereo/sintro.html.
- A FEW MORE TRICKS - Below are few more images that are constructed in such a way as to exposed some of the subtleties of the brain's processing.
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