Humans use numerous visual cues to perceive distance. One of the
most accurate of those cues is binocular disparity, which takes advantage
of the fact that most people 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 away an object
is 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 at the left of the figure below 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. The reason
you see only two dots when you are really looking at four is that the eye
looking through the blue filter sees only the red dots and the eye looking
through the red filter sees only the blue dots. The eyes fuse these four
different images into two dots. The reason why one set of fused dots appears
closer than the other is due to the difference in the horizontal distance
between the two sets of dots. This simulates binocular disparity,
a perceptual cue that uses the separation between the two eyes to judge
the distance of objects. 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 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.
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