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Binocular Disparity

Another important depth cue is binocular disparity: the slight difference in visual images reaching the two eyes. A close object produces greater binocular disparity than a distant one, as you can verify by covering first one eye then the other. Nearby objects will appear to jump as you switch from one eye to the other, showing that different images are reaching each eye. More distant objects will not appear to change position as much.

What is binocular disparity? How is it used in a stereoscope?

Binocular disparity is responsible for the illusion of depth in stereoscopes, a favorite source of amusement in the 1800s. A stereoscope holds a card that contains two images, one visible to each eye. Because the two images are photographed from slightly different angles, creating binocular disparity, the result is a 3-D image. In the 1800s, pictures of dramatic scenes such as the pyramids or Niagara Falls were popular. Today the same principle is used in the Viewmaster toy familiar to many children.

A stereoscope

Barlow and colleagues (1963) identified populations of disparity-detecting neurons that receive inputs from both eyes and respond vigorously only if a stimulus hits the two eyes in slightly different locations. Different neurons respond to different levels of disparity, thereby providing the brain with depth information.

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