Humans use numerous visual cues to perceive distance. One of the most accurate of those
cues is binocular disparity. Binocular disparity takes advantage of the fact that you have
two eyes. Because the eyes are separated by about 2 inches, each eye has a slightly
different view of the world. Binocular vision results when the brain combines these
disparate images to determine true stereoscopic depth. Basically, the farther an object is
away from you, the more similar the view in your two eyes. And the closer the object is to
you, the more disparate the two images.
Take a look at the four colored dots in the middle of the figure at the top of the page
through the blue/red glasses. You should now only see two dots, one above the other. One
of those dots should appear slightly closer than the other dot. The reason you see only
one dot when you are really looking at four different colored dots is because the eye
looking throuth the blue filter sees only the red dot and the eye looking through the red
filter sees only the blue dot. The eye then fuses these two different images into one dot.
Changing the horizontal distance between the two colored dots will change the perceived
distance of the dot. This is because it simulates what happens when objects are closer or
farther away from your two eyes---Binocular Disparity. Now look at the dots that
are connected by the straight lines. What do you see?
Binocular vision requires that the brain integrate information from both eyes to determine true stereoscopic depth. As in the binocular disparity example, the brain matches up images in the left eye with corresponding images in the right eye. Binocular rivalry dramatically demonstrates what would happen if the image in the left eye was different from the image in the right eye. When the you view the grid above with the blue/red glasses, the blue eye sees the vertical lines, and the red eye sees the horizontal lines. Because the brain cannot find any similar objects or matching patterns, it does not combine the images into a grid. What happens is the brain attends to the image in one eye for a while, and then it attends to the image in the other eye for a while, looking for the most interesting image.