Chapter 8 Flashcards
(40 cards)
Functions of Motion Perception
Motion perception has a number of different functions, rang- ing from helping us perceive things such as the shapes of ob- jects to providing us with updates about what is happening. Perhaps most important of all, especially for animals, the perception of motion is intimately linked to survival.
Motion Provides Information About Objects
Motion is an important aspect of object recognition because it reveals information about objects that might otherwise be dif- ficult to discern.
a frightened field mouse will freeze in the hope that stillness will make it more difficult for a hawk to see it against the surrounding landscape
Observers perceive shapes more rapidly and accurately when an object is moving
Motion Attracts Attention
Movement also plays an important role in animal survival be- cause motion attracts attention. You may have experienced this yourself in several ways. For example, as you try to find your friend among a sea of faces in the stadium, you realize you have no idea where to look. But suddenly you see a person waving and recognize that it is your friend.
attentional capture
Motion is a very salient aspect of the environ- ment, so it attracts our attention
Motion Helps Us Understand Events in Our Environment
The gestures of the people in the group indicate the intensity of their conversation; the motions of the salesperson indicate what she is doing, and changes in her motion indicate when she has shifted to a new task; and motion indicates, even in the absence of sound, that something important is happen- ing in the game
y Fritz Heider and Marianne Simmel (1944), who showed a 21⁄2-minute animated film to subjects and asked them to describe what was happening in the movie.
These three geometric objects moved around both inside and outside the house
Events in the enviro
reaching out to accept the cup of coffee is an event, dropping change in the tip jar is an event, and so on. The point in time when each of these events ends and the next one begins is called an event boundary.
The connection of events to motion perception becomes obvious when we consider that events almost always involve motion, and that changes in the nature of motion are often associated with event boundaries. One pattern of motion occurs when placing the order, another when reaching out for the coffee cup, and so on.
event boundar- ies were more likely to occur when there was a change in the speed or acceleration of the actor’s hands
Life Without Motion Perception
through disease or trauma, suffer from damage to parts of the brain responsible for per- ceiving and understanding movement. When this happens, a person is said to suffer from a condition called akinetopsia or “motion blindness,” where motion is either very difficult or impossible to perceive.
is happens, a person is said to suffer from a condition called akinetopsia or “motion blindness,” where motion is either very difficult or impossible to perceive.
The most famous and well-studied case of akinetopsia is that of a 43-year-old woman known as L.M
LM
Without the ability to perceive motion following a stroke, L.M. was unable to successfully complete activities as simple as pouring a cup of tea
people suddenly appeared or disappeared because she couldn’t see them approaching or leaving.
Thus, her disability was not just a social inconvenience but enough of a threat to the woman’s well-being that she rarely ventured outside into the world of moving—and sometimes dangerous—objects
When Do We Perceive Motion
Actual motion of an object is called real motion. Perceiving a car driving
by, people walking, or a bug scurrying across a tabletop are all
examples of the perception of real motion.
Perception of motion when there actually is none is called illusory
motion. The most famous, and best studied, type of illusory motion is
called apparent motion
Wertheimer’s observation that
when two stimuli in slightly different
locations are alternated with the correct timing, an observer perceives
one stimulus moving back and forth smoothly between the two
locations
apparent motion
This is the basis for the motion we perceive in movies, on television,
and in moving signs that are used for advertising and entertainment
induced motion
Induced motion occurs when motion of one object (usu- ally a large
one) causes a nearby stationary object (usually smaller) to appear to
move
ie. moon
motion aftereffects
Motion aftereffects occur when viewing a moving stimu- lus causes a
stationary stimulus to appear to move
ie. waterfall illusion
Comparing Real and Apparent Motion
there is ample evidence that these two types of motion have much in
common
the activation associated with ap- parent motion is similar
to the activation for the real motion display
Table 8.1 Conditions for Perceiving and Not Perceiving Motion
Depicted
Review
Gibson, who founded the ecological approach to perception
This information for perception, according to Gibson, is located not on
the retina but “out there” in the environment
information in the environment in terms of the optic array—the
structure created by the surfaces, tex- tures, and contours of the
environment—and he focused on how movement of the observer
causes changes in the optic array.
local disturbance in the optic array
when Jeremy walks across Maria’s field of view, portions of the optic
array become covered as he walks by and then are uncovered as he
moves
the optic array occurs when Jeremy moves relative to the environment
A similar situation would occur if Maria were to walk through the
scene. The fact that everything moves at once in response to
movement of the observer’s eyes or body is called global optic flow;
this sig- nals that the environment is stationary and that the observer
is moving
Thus, according to Gibson, motion is perceived when one part of the
visual scene moves relative to the rest of scene, and no motion is
perceived when the entire field moves, or remains stationary
One such approach was a neural circuit proposed by Werner
Reichardt (1969), which has come to be called the Reichardt detector.
This circuit consists of two neurons, A and B, which send their signals
to an output unit which compares the signals it receives from neurons
A and B. The key to the operation of this circuit is the delay unit that
slows down the signals from A as they travel toward the output unit. In
addition, the output unit has an impor- tant property: It multiplies the
responses from A and B to create the movement signal that results in
the perception of motion
If the timing is right, the delayed signal from A (record
3) reaches the output unit just when the signal
from B (record 2) arrives
but does not create a signal for movement from right to left
by the time the response from A passes through the delay unit and
reaches the output unit, the response from B has dropped to zero
Besides enabling us to determine the direction of motion, the
Reichardt detector provides an additional benefit: it lets us determine
speed
The duration of the delay
is therefore crucial. If
Detectors that include a short delay will be se- lective for fast
movement while detectors that include a long delay will be selective
for slow movement
Corollary Discharge Theory
Corollary discharge theory
takes eye move- ments into account.
1. An image displacement signal (IDS) (Figure 8.10a) occurs when an
image moves across receptors in the retina, as when Jeremy walks
across Maria’s field of view while she stares straight ahead.
2. A
motor signal (MS) (Figure 8.10b) occurs when a signal is sent from
the brain to the eye muscles. This signal occurs when Maria moves
her eyes to follow Jeremy as he walks across the room.
3. A corollary
discharge signal (CDS) is a copy of the mo- tor signal that, instead of
going to the eye muscles, is sent to a different place in the brain
focusing on the two signals that are trans- mitted toward the brain: the
image displacement signal (IDS) and the corollary discharge signal
(CDS)
But if both sig- nals occur, as happens in situation 3, when an
observer scans the room
then no motion is
perceived. This solution is, in fact, the basis of corollary discharge
theory
the brain con- tains a structure or mechanism called the comparator
that receives both the IDS and the CDS
What would happen if there were no corollary discharge signal but
there was an image displacement signal? That is apparently what
happened to R.W.
a 35-year-old man who experienced vertigo
(dizziness) anytime he moved his eyes or experienced motion when
he looked out the window of a moving car.
R.W. had lesions in an area of his cortex called the medial superior
temporal (MST) area
important role in the control of eye movements
the damage to his brain had apparently eliminated the CDS. Because
only the IDS reached the comparator
This neuron is called a real-motion neuron because
it re- sponds only
when the stimulus moves and doesn’t respond when the eye moves,
even though the stimulus on the retina— a bar sweeping across the
cell’s receptive field—is the same in both situations
This
real-motion neuron must be receiving information like the corollary
discharge signal, which tells the neuron when the eye is moving
Motion Perception and the Brain
middle temporal (MT) area, which plays an important role in the
perception of motion.
The perception of motion begins
begins in the striate cortex, the region of the occipital lobe where
information from the retinas
first reaches the cortex (Figure 3.21, page 59). It is here that Hubel
and Wiesel (1959, 1965) discovered neurons called com- plex cells
that respond to bars that move in a specific direction
Another area that contains many directionally sensitive cells is the
middle temporal (MT) area
William Newsome and coworkers (1995) used the term
coherence to
indicate the degree to which the dots move in the same direction.
When the dots are all moving in random directions, coherence is 0
percent
They found that as the dots’ coherence increased, two things
happened: (1) the monkey judged the direction of motion more
accurately, and (2) the MT neuron fired more rapidly.
researchers could predict one from the other
But at a coherence of 12.8 percent—so, out of 200 moving dots,
about 25 were moving in the same direction—the monkey judged the
direction of the dots that were moving together correctly
Newsome has done is to measure relationship C: the physiology–
perception relationship
