theeyesmustturntowardeachotherquiteabitforthesameimagetofallon
both foveae. You can actually see the eyes converge if you watch a friend focus
first on a distant object and then on one a foot or so away. Your brain uses in-
formation from your eye muscles to make judgments about depth. However,
convergence information from the eye muscles is useful for depth perception
only up to about 10 feet. At greater distances, the angular differences are too
small to detect, because the eyes are nearly parallel when you fixate on a dis-
tant object.
To see howmotionis another source for depth information, try the following
demonstration. As you did before, close one eye and line up your two index
fingers with some distant object. Then move your head to the side while fixat-
ing on the distant object and keeping your fingers still. As you move your head,
you see both your fingers move, but the close finger seems to move farther and
faster than the more distant one. The fixated object does not move at all. This
source of information about depth is calledrelative motion parallax.Motion par-
allax provides information about depth because, as you move, the relative dis-
tances of objects in the world determine the amount and direction of their
relative motion in your retinal image of the scene. Next time you are a passen-
geronacartrip,youshouldkeepawatchoutthewindowformotionparallax
at work. Objects at a distance from the moving car will appear much more sta-
tionary than those closer to you.
Pictorial Cues But suppose you had vision in only one eye. Would you not be
able to perceive depth? In fact, further information about depth is available
Figure 7.26
Convergence cues to depth.
168 Philip G. Zimbardo and Richard J. Gerrig