MOTION PERCEPTION : WHAT IS IT DOING ?
We obviously want to know what
objects are in view and where they’re located, but we also want to know what
these objects are doing. Are they moving or standing still, approaching slowly
or rapidly, racing toward the food we wanted for ourselves, or head-ing off in
some altogether different direction? These questions bring us to a different
aspect of perception—namely, how we perceive motion.
One might think that the
perception of motion is extremely simple: If an object in our world moves, then
the image cast by that object moves across our retinas. We detect that image
motion, and thus we perceive movement.
As we’ll soon see, however, this
account is way too simplistic. Still, it contains a key element of truth: We do
detect an image’s motion on the retina, and this is one aspect of the overall
process of motion perception. More specifically, some cells in the visual
cortex respond to image movements on the retina by firing at an increased rate
when-ever movement is present. However, these cells don’t respond to just any
kind of move-ment, because the cells are direction
specific. Thus, the cells fire if a stimulus moves across their receptive
field from, say, left to right; but not if the stimulus moves from right to
left. (Other cells, of course, show the reverse pattern.) These cells are
therefore well suited to act as motion
detectors (see, for example, Vaultin & Berkeley, 1977).
It’s clear, however, that retinal
motion is only part of the story. Suppose we turn on a light in one location in
the visual field, then quickly turn it off, and after an appropri-ate interval
(somewhere between 30 and 200 milliseconds) turn on a second light in a
different location. The result is apparent
movement. The light appears to travel from one point to another, even
though there was no motion and, indeed, no stimulation whatsoever in the
locations between the two lights (Figure 5.33). This phenomenon is perceptually
quite compelling; given the right timing, apparent movement is
indistin-guishable from real movement (Wertheimer, 1912). This is why the
images in movies seem to move, even though movies actually consist of a
sequence of appropriately timed still pictures (Figure 5.34).
Apparent movement might seem like an artificial phenomenon because the objects in our world tend to move continuously —they don’t blink out of existence here and then reappear a moment later there. It turns out, however, that, the motion we encounter in the world is often so fast that it’s essentially just a blur across the retina, and so triggers no response from the retinal motion detectors. Even so, we do perceive the motion by perceiv-ing the object first to be in one place and then, soon after, to be somewhere else. In this way, the phenomenon of apparent movement actually mirrors a process that we rely on all the time, thanks to the fact that our eyes often need to work with brief “samples” taken from the stream of continuous motion (Adelson & Bergen, 1985).
As you look around the world,
you’re constantly moving your head and eyes. This activ-ity creates another
complication for motion perception. Each movement brings you a somewhat
different view, and so each movement necessarily causes a change in the retinal
image. But, despite all this retinal motion, the world doesn’t seem to move
each time you shift your viewing position. Clearly, it takes more than motion
across the retina to produce a perception of motion in the world.
But how do you avoid becoming
confused about this retinal motion? How do you manage to separate the retinal
motion that’s caused by movement in the world from the retinal motion produced
by a change in your viewing position? The answer paral-lels our earlier
discussion of constancy. As we’ve seen, people take viewing distance into account when judging size, and that’s how they achieve size constancy. In the same way,
you seem to take your own movements
into account when judging the position
of objects in the world, and so you perceive the objects as having position constancy. How does this work?
Whenever you move your eyes or turn your head, you unconsciously compute the
shift in the retinal image that your own motion will produce, and you cancel
out this amount of movement in interpreting the visual input (Figure 5.35). The
result is constancy.
Here’s a specific example.
Imagine you’re about to move your eyes 5 degrees to the left. Even before
making the movement, you know that it will cause the retinal image to shift 5
degrees to the right. You can therefore compare this anticipated shift with the
shift that actually occurs; if they match, then you know that you produced all
of the reti-nal motion—and so there was no movement in the environment. In
algebraic terms, we can think of this canceling-out process this way: When you
move your eyes, there will be a 5-degree shift; but 5 degrees right of
anticipated change minus 5 degrees right of actual change yields zero change
overall. The zero change, of course, is what you per-ceive in this situation
—no motion (Bridgeman & Stark, 1991).
Evidence for this claim comes
from studies in which some heroic experimenters had themselves injected with
drugs that temporarily paralyzed their eye muscles. They reported that under
these circumstances, the world appeared to jump around whenever they tried to
move their eyes—just what canceling-out theory would predict. The brain ordered
the eyes to move, say, 10 degrees to the right; and so it anticipated that the
reti-nal image would shift 10 degrees to the left. But the paralyzed eyes couldn’t
follow the command, so no retinal shift took place. In this setting, the normal
cancellation process failed; as a result, the world appeared to jump with each
eye movement. (Algebraically, this is 10 degrees left of anticipated change
minus zero degrees of actual change, yield-ing 10 degrees left overall. The
visual system interpreted this 10-degree overall difference as a motion
signal.) These studies strongly confirm the canceling-out theory (Matin,
Picoult, Stevens, Edwards, & MacArthur, 1982).
Clearly, then, motion perception
depends on several factors. Movement of an image across the retina stimulates
motion detectors in the visual cortex (and elsewhere in the brain), and this
certainly contributes to movement perception. However, activity in these
detectors isn’t necessary for us to perceive motion; in apparent movement, the
detectors are silent but we perceive motion anyhow. Activity in the detectors
is also, by itself, not sufficient
for us to perceive motion: If those detectors register some move-ment, but the
nervous system calculates that the movement was caused by a change in the
observer’s position, then no motion is perceived.
Even with all of this said,
there’s a further step in the process of motion perception, because we not only
detect motion but also interpret it.
This interpretation can be demonstrated in many ways, including the phenomenon
of induced motion. Consider a ball
rolling on a billiard table. We see the ball as moving and the table at rest.
But why not the other way around? We can definitely see the ball getting closer
and closer to the table’s edge; but, at the same time, we can see the table’s
edge getting closer and closer to the ball. Why, then, do we perceive the
movement as “belonging” entirely to the ball, while the table seems to be
sitting still?
Evidence suggests that our
perception in this case is the result of a bias in our inter-pretation: We tend
to perceive larger objects as still and smaller objects as moving. In addition,
if one object encloses another, the first tends to act as a stationary frame so
that we perceive the enclosed object as moving. These biases lead to a correct
perception of the world in most cases, but they can also cause errors. In one
study, research partic-ipants were shown a luminous rectangular frame in an
otherwise dark room. Inside the frame was a luminous dot. As the subjects
watched, the rectangle moved upward while the dot stayed in place. But the
subjects perceived something else. They described the dot as moving downward,
in the direction opposite to the frame’s motion. Participants had correctly
perceived that the dot was moving closer to the rectangle’s bottom edge and
farther from its top edge. But they misperceived the source of this change.
The physical movement of the
frame had induced the perceived movement of the enclosed shape.
Similar effects can easily be
observed outside of the laboratory. The moon seems to sail through the clouds;
the base of a bridge seems to float upstream, against the river’s current. In
both cases, we see the surrounded object as moving and the frame as stay-ing
still—exactly as in the laboratory findings. A related (and sometimes
unsettling) phenomenon is induced motion
of the self. If you stand on the bridge that you perceive as moving, you
feel as if you’re moving along with it. The same effect can occur when you’re
sitting in a car in traffic: You’ve stopped at a red light and then, without
warning, the car alongside you starts moving forward. You can sometimes get the
feeling that for just a moment, you (and your car) are moving backward—even
though you’re sitting per-fectly still.
Induced motion provides one
illustration of how our perception of motion depends on interpretation. Another
illustration, and a different type of interpretation, is involved in the
so-called correspondence problem—the
problem of deciding, as you move from one view to the next, which elements in
the second view correspond with which elements in the first view (e.g., Weiss,
Simoncelli, & Adelson, 2002; Wolfe, Kluender, & Levi, 2006). To see how
this problem can arise, consider the stimulus pattern shown in Figure 5.36. At
one moment, you’re shown the pattern in panel A; then, a moment later, the
pattern in B; then the pattern in A again; and so on back and forth. What will
you perceive? With some adjustment of the timing, we can set things up so that
this display will produce apparent movement, but what will the movement be?
Will you perceive a red dot moving counterclockwise and turning blue, as shown
in panel C? Or will you perceive a dot moving clockwise, and chang-ing into a
square, as shown in panel D?
For the display shown in Figure
5.36, there’s no “correct” solution to the correspon-dence problem. The
solutions leading to panel C and panel D both make sense. Thus, the stimuli
shown in panels A and B are truly ambiguous; so it’s no surprise that some
people will perceive the motion illustrated in C and some will perceive the
pattern in D. Indeed, an individual can perceive one of these patterns for a
while and then abruptly shift her perception and perceive the other.
As we noted in our discussion of
apparent movement, many real-world circum-stances involve motion that’s fast
enough to be just a “blur” for the visual cortex’s motion detectors. The only
way we can detect this motion is to note that the stimuli were first here, and then there, and to infer from this that the stimuli had moved. It’s
exactly this situation that creates the correspondence problem—and so we often
encounter this problem in our everyday perception of the world. As in many
aspects of perception, our decision about the correspondence (i.e., what goes
with what) is made quickly and easily; and so we don’t realize that we have, in
fact, interpreted the input. But the role of our interpretation becomes clear
whenever our perception is mistaken. This
is, for example, why we perceive spirals that are merely spinning as“growing
outward” toward us, why we observe rotating barber poles to be moving upward,
and why a fast-spinning wagon wheel that’s turning clockwise can end up looking
like it’s actually spinning counterclockwise. In each case, the misperception
grows out of an incorrect solution to the correspondence problem, and hence a
con-fusion about which elements in the current view correspond with which
elements in the scene just a moment ago.
The correspondence problem is an
important aspect of motion perception. As we said at the outset, our perception
of the world usually seems immediate and effortless. We open our eyes and we
see, with no apparent need to interpret or calculate. Even so, our per-ception
of the world does involve a lot of steps and quite a few inferences. We become
aware of those steps and inferences when they lead us astray, such as when they
lead to an illusion of one sort or another. But it’s important to recognize
that the illusions arise only because interpretation is always a part of our
perceptual process. Otherwise, if the interpretation weren’t in place, then
there’d be no way for the interpretation to go wrong! Perception, in other
words, is always an active process—even if we normally don’t detect that
activity.
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