54 • CHAPTER 3 Perception
overlaps the tracks. Thus, the relationship of the creatures to the railroad
tracks provides information about their relative sizes. The perceptual sys-
tem’s use of information about the creatures’ distance and their size rela-
tive to the tracks illustrates how information in addition to the size of the
image on the retina helps determine the perception of their size.
Here’s a demonstration that shows how information provided by the
retinal image does not necessarily correspond to what we perceive.DEMONSTRATION Two Quarters
Hold two quarters as shown in ● Figure 3.11a, with one far away and one closer
(at about half the distance). Then close one eye and view the two quarters, keep-
ing them at the same distances and positioning them so their edges appear to be
touching (Figure 3.11b). Notice how you perceive the sizes of the two quarters
under these conditions. Then open your other eye and view the quarters with
both eyes so they no longer appear to be right next to each other. How does that
aff ect your perception of the sizes of the two quarters?It is likely that in the fi rst part of the demonstration, when you viewed
the quarters next to each other with one eye, the farther quarter appeared
smaller. This perception corresponds to the fact that the farther quarter cre-
ates a smaller image on the retina (Figure 3.11c). It is also likely that in the
second part of the demonstration, with both eyes open, the quarters appeared
more similar in size. This occurs because opening both eyes increases your
ability to perceive depth, or the relative distance of the two quarters. The
perceptual system can then take into account the quarters’ distance, and this
added information enables you to perceive their sizes more accurately.
Taking distance into account occurs all the time in real life. For exam-
ple, as a person who is standing near you begins to walk away, he doesn’t
appear to shrink as his distance increases. A person who appears to be
6 feet tall when he is nearby also appears to be 6 feet tall when he is stand-
ing across the room, even though the size of his image on your retinas
(as with the far quarter in the demonstration) is much smaller when he is
farther away. This phenomenon is called size constancy—we tend to per-
ceive objects as remaining the same size even when they move to different
distances. All of the examples above, which are summarized in Table 3.1,
lead to the same conclusion: Perception of the size of an object does not
depend solely on the size of the object’s image on the receptors.● (^) FIGURE 3.8 These two creatures are at
diff erent distances, but the farther one is larger.
Both creatures cover the same amount of the
observer’s fi eld of view (measure them!).
(Source: William Vann/www.edupic.net.)
● (^) FIGURE 3.9 The two creatures on the railroad tracks cover the same area
in the fi eld of view and cast the same-sized images on the retina because one
is small but close and the other is larger but farther away.
Size of both
creatures’
images on
retina
● (^) FIGURE 3.10 Like the two creatures on
the railroad tracks, the top part of the nearby
planter and the faraway building are the same
size in the observer’s fi eld of view.
Bruce Goldstein
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