Size Illusions

119
Organizing the Perceptual World
A FAILURE OF SHAPE CONSTANCY
When certain stimuli are viewed from an
extreme angle, the brain’s ability to maintain shape constancy can break down.
Traffic engineers take this phenomenon
into account in the design of road markings, as shown in these photos from a
London airport. The arrow in the top
photo appears to be about the same
height as the lettering below it, but it
isn’t. The arrow had to be greatly elongated, as shown in the side view, so that
approaching drivers would see its shape
clearly. If the arrow had been painted to
match the height of the accompanying
letters, it would appear “squashed,” and
only half as tall as the lettering.
the darkest object relative to its sunlit background and the paper is the brightest object
relative to its background of shade. As shown in Figure 3.23, the brightness of an object
is perceived in relation to its background.
Size Illusions
FIGURE
3.23
Brightness Contrast
At first glance, the inner rectangle on the left probably
by
looks lighter than the inner
rectangle on the right. But carefully examine the inner rectangles alone (covering
their surroundings), and you will see that
both are of equal intensity. The brighter
surround in the right-hand figure leads
you to perceive its inner rectangle as relatively darker.
doing
2
learn
Usually, the visual perceptual system works automatically and perfectly to create correct impressions of depth, distance, and size. Sometimes, though, it can fail, resulting
in size illusions such as the ones shown in Figure 3.24. Why does the monster that is
placed higher in Figure 3.24(A) look larger than the lower one, even though they are
exactly the same size? The converging lines in the tunnel provide depth cues telling
you that the higher monster is farther away. Because the retinal image of the “distant”
120
Chapter 3 Sensation and Perception
FIGURE
3.24
Three Size Illusions
These illusions are named for
the scientists who described
by
them. In Part A, a version of
the Ponzo illusion, the upper monster
looks bigger but is actually the same size
as the lower one. In the Müller-Lyer illusion (Part B), both vertical lines are actually of equal length; in the Ebbinghaus
illusion shown in Part C, both center circles
are exactly the same size. To prove that
you can’t always believe your eyes, measure these drawings for yourself.
doing
2
in review
learn
(A)
(B)
(C)
monster is the same size as the “closer” one, your perceptual system calculates that the
more distant monster must be bigger. This is the principle of size constancy at work:
When two objects have retinal images of the same size, you perceive the one that seems
farther away as larger. Now look at Figure 3.24(B). The two vertical lines are the same
length, but the one on the right looks longer. Why? One possible reason is that the perceived length of an object is based on what frames it. When the frame is perceived as
larger, as on the right side of Figure 3.24(B), so is the line segment within it (Rock,
1978). In Figure 3.24(C), the inner circle at the left looks smaller than the one at the
right because, like the brightness of the center rectangles in Figure 3.23, the inner circles are judged in relation to what surrounds them. Because perception is based on
many principles, illusions like these probably reflect the violation of more than one of
them. (See “In Review: Principles of Perceptual Organization and Constancy.”)
PRINCIPLES OF PERCEPTUAL ORGANIZ ATION AND CONSTANCY
Principle
Description
Example
Figure-ground
Certain objects or sounds automatically become
identified as figure, whereas others become
meaningless background.
You see a person standing against a building,
not a building with a person-shaped hole in it.
Grouping
Properties of stimuli lead us to automatically
group them together. These include proximity,
similarity, continuity, closure, texture, simplicity,
common fate, synchrony, common region, and
connectedness.
People who are sitting together, or who are
dressed similarly, are perceived as a group.
Depth perception
The world is perceived as three-dimensional,
with help from stimulus cues (such as relative
size, height in the visual field, interposition,
linear perspective, reduced clarity, light and
shadow, and gradients) and from visual system
cues (such as accommodation, convergence, and
binocular disparity).
A person who looks tiny and appears high in
the visual field will be perceived as being of
normal size, but at a great distance.
Perceptual constancy
Objects are perceived as constant in size, shape,
color, and other properties despite changes in
their retinal images.
A train coming toward you is perceived as
getting closer, not larger; an advertising sign is
perceived as rotating, not changing shape.
?
1. The movement we see in movies, videos, and DVDs is due to a perceptual illusion called
2. People who have lost an eye also lose the depth cue called
.
3. The grouping principle of
allows you to identify objects seen through a picket fence.
.