One very important difference is that the retinal image istwo-dimensional,
whereas the environment isthree-dimensional.This difference has many con-
sequences. For instance, compare the shapes of the physical objects in figure 7.2
with the shapes of their corresponding retinal images. The table, rug, window,
and picture in the real-world scene are all rectangular, but only the image of
the window actually produces a rectangle in your retinal image. The image of
the picture is a trapezoid, the image of the table top is an irregular four-sided
figure, and the image of the rug is actually three separate regions with more
than 20 different sides! Here’s our first perceptual puzzle: How do you manage
to perceive all of these objects as simple, standard rectangles?
The situation is, however, even a bit more complicated. You can also notice
that many parts of what you perceive in the room are not actually present
in your retinal image. For instance, you perceive the vertical edge between
the two walls as going all the way to the floor, but your retinal image of
that edge stops at the table top. Similarly, in your retinal image parts of the
rug are hidden behind the table; yet this does not keep you from correctly per-
ceiving the rug as a single, unbroken rectangle. In fact, when you consider all
the differences between the environmental objects and the images of them on
your retina, you may be surprised that you perceive the scene as well as you
do.
The differences between a physical object in the world and its optical image
on your retina are so profound and important that psychologists distinguish
carefully between them as two different stimuli for perception. The physical
object in the world is called thedistal stimulus(distant from the observer) and
the optical image on the retina is called theproximal stimulus(proximate, or
near, to the observer), as shown in figure 7.3.
The critical point of our discussion can now be restated more concisely: what
youperceivecorresponds to thedistal stimulus—the ‘‘real’’ object in the envi-
ronment—whereas the stimulus from which you must derive your information
is theproximal stimulus—the image on the retina. The major computational task
Figure 7.3
Distal and proximal stimulus. The distal stimulus is the pattern or external condition that is sensed
and perceived. The proximal stimulus is the pattern of sensory activity that is determined by the
distal stimulus. As illustrated here, the proximal stimulus may resemble the distal stimulus, but
they are separate events.
138 Philip G. Zimbardo and Richard J. Gerrig