following experiment. First, close your left eye and use the right one to line up
your two index fingers with some small object in the distance, holding one
finger at arm’s length and the other about a foot in front of your face. Now,
keeping your fingers stationary, close your right eye and open the left one
while continuing to fixate on the distant object. What happened to the position
of your two fingers? The second eye does not see them lined up with the distant
object because it gets a slightly different view.
This displacement between the horizontal positions of corresponding images
in your two eyes is calledbinocular disparity. It provides depth information be-
cause the amount of disparity, or difference, depends on the relative distance of
objects from you (see figure 7.25). For instance, when you switched eyes, the
closerfingerwasdisplacedfarthertothesidethanwasthedistantfinger.
When you look at the world with both eyes open, most objects that you see
stimulate different positions on your two retinas. If the disparity between cor-
responding images in the two retinas is small enough, the visual system is able
to fuse them into a perception of a single object in depth. (However, if the
images are too far apart, as when you cross your eyes, you actually see the
double images.) When you stop to think about it, what your visual system does
is pretty amazing: it takes two different retinal images, compares them for hori-
zontal displacement of corresponding parts (binocular disparity), and produces
a unitary perception of a single object in depth. In effect, the visual system
interprets horizontal displacement between the two images as depth in the
three-dimensional world.
Other binocular information about depth comes fromconvergence.Thetwo
eyes turn inward to some extent whenever they are fixated on an object (see
figure 7.26). When the object is very close—a few inches in front of your face—
Figure 7.25
Retinal disparity. Retinal disparity increases with the distance, in depth, between two objects.
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