Depth Perception
Until now, we have considered only two-dimensional patterns on flat surfaces.
Everyday perceiving, however, involves objects in three-dimensional space.
Perceiving all three spatial dimensions is absolutely vital for you to approach
what you want, such as interesting people and good food, and avoid what is
dangerous, such as speeding cars and falling comets. This perception requires
accurate information aboutdepth(the distance from you to an object) as well as
about itsdirectionfrom you. Your ears can help in determining direction, but
they are not much help in determining depth.
When you think about depth perception, keep in mind that the visual system
must rely on retinal images that have only two spatial dimensions—vertical
and horizontal. To illustrate the problem of having a 2-D retina doing a 3-D job,
consider the situation shown in figure 7.24. When a spot of light stimulates the
retina at point a, how do you know whether it came from position a 1 or a 2 ?In
fact, it could have come fromanywherealong line A, because light from any
point on that line projects onto the same retinal cell. Similarly, all points on line
B project onto the single retinal point b. To make matters worse, a straight line
connecting any point on line A to any point on line B (a 1 to b 2 or a 2 to b 1 ,for
example)wouldproducethesameimageontheretina.Thenetresultisthatthe
image on your retina is ambiguous in depth: it could have been produced by
objects at any one of several different distances.
ThetwopossibleviewsoftheNeckercubefromfigure7.6resultfromthis
ambiguity in depth. The fact that you can be fooled under certain circum-
stances shows that depth perception requires aninterpretationof sensory input
and that this interpretation can be wrong. (You already know this if you’ve
ever swung at a tennis ball and come up only with air.) Your interpretation of
depth relies on many different information sources about distance (often called
depth cues)—among them binocular cues, motion cues, and pictorial cues.
Binocular and Motion Cues Have you ever wondered why you have two eyes
instead of just one? The second eye is more than just a spare—it provides some
of the best, most compelling information about depth. The two sources of bino-
cular depth information arebinocular disparityandconvergence.
Because the eyes are about two to three inches apart horizontally, they re-
ceive slightly different views of the world. To convince yourself of this, try the
Figure 7.24
Depth ambiguity.
166 Philip G. Zimbardo and Richard J. Gerrig