Download Chapter 8 – Perceiving Motion

Chapter 8 – Perceiving Motion
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We perceive motion when we are stationary and when we are ourselves are moving
Function of Motion Perception
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Many different functions: from providing us with updates of what is happening to helping us
perceive things such as shape, moods etc.
Perception of motion is intimately linked with survival
Motion helps us Understand Events in our Environment
Important function of motion perception  ability to determine what is happening
- As a person moves forward, objects move relative to the person in the opposite direction 
optic flow
- Optic flow provides information about the walker’s direction and speed.
Ex. Women with motion agnosia could not perceive the fluid rising in a coup and trouble knowing
when to stop pouring. It was difficult for her to follow dialogue because she couldn’t see motions of
speaker’s face and mouth. She also could not see people walking or cars moving thus she perceive
things as sudden appearance or disappearance. This shows the importance of motion perception.
Motion Attracts Attention
Attentional capture: the ability of motion to attract attention
- This can occur not only if you are looking for something but also if you are not
- Plays an important role in animal survival.
 ex. A mouse freezes in place when it sees a cat. This eliminates the attention attracting
effects of movement and it makes it harder for the cat to differentiate between the cat
and mouse
Motion Provides Information about objects
Demonstration  proves idea that not moving can help an animal blend into the background
- our motion relative to objects is constantly adding to the information we have about the object
 ex. Moving around the “horse” reveals its shape
- Observers perceive shapes more rapidly and more accurately when an object is moving.
Studying Motion Perception
When do we Perceive Motion?
Real motion: actual motion of an object
Illusory motion: perception of motion when there actually is none
 Apparent motion – chapter 5 Max Wertheimer showed two stimuli at different
locations that are alternated with correct timing show motion
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basis for the motion we perceive in movies, on television, or moving signs in
advertisement
 Induced motion occurs when motion of one object (usually a larger one) causes a
nearby stationary object (smaller) to appear to move.
 ex. If clouds move past the moon, the moon appears to be racing through the
sky
 Motion aftereffects occur after viewing a moving stimulus for 30 to 60 seconds and
then viewing a stationary stimulus, which appears to move.
 Waterfall illusion – if you look at a waterfall and then look off to a side at a part
of the scene that is stationary, you will see everything you are looking at appear
to move for a few seconds.
Comparing Real Motion and Apparent Motion
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Two types of motion have much in common
Axel Larsen presented 3 types of displays to a person in an fMRI scanner
Control condition- two dots in a slightly different position were flashed simultaneously
Real motion display in which a small dot moved back and forth
Apparent motion display in which dots were flashed one after another so that they appeared to
move back and forth
The activation associated with apparent motion is similar to the activation for the real motion
display
Because of the similarities btw perception of real and apparent motion researchers study both
types of motion together
What we want to Explain
1. An object moves, and the observer is stationary
– Movement creates an image that moves on the
Observer’s retina
2. An object moves, and the observer follows the
Object with his or her eyes
– Movement is tracked so that the image is
Stationary on the retina
3. An observer moves through a stationary
environment
– Image of environment moves across retina but
environment is perceived as stationary
Motion Perception: Information in the Environment
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Information for perception is “out there” (Gibson)
Optic array – the structure created by the surfaces, textures and contours of the environment
and he focused on how the movement of the observer causes changes in the optic array
From above:
1. As Jeremy walks he covers some of the optic array and creates a local disturbance in the
optic array. This provides information that Jeremy is moving relative to the environment
2. The same level disturbance information that was available before is available now
3. As Maria moves everything around her moves. The fact that everything moves at once is
called global optic flow. This signals that Maria is moving but that the environment is
stationary.
-doesn’t explain physiologically!
Neural Firing to Motion across the Retina
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The physiol9iogical approach to motion focuses on determining the connection btw neural firing
and motion perception
Motion of a Stimulus Across the Retina: The Aperture Problem
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How can we explain how neural firing signals that direction that an object is mobbing
o Answer: as the stimulus sweeps across the retina, it activates directionally selective
neurons in the cortex that respond to orientated bars that are moving in a specific
direction
 The response of single directionally selective neuron does not provide sufficient
information to indicate the direction in which an object is moving!
 Neurons which only sees movement through the narrow view of its receptive
field only receives information about the rightward movement and cant
distinguish for example from rightward movement or right and upward
movement.
 Movement of an edge across an aperture occurs perpendicular to the direction
in which the edge is orientated
Aperture problem – the fact that viewing only a small portion of a larger stimulus can result in
misleading information about the direction in which the stimulus is moving/
o Solution: pooling the response of a number of neurons like our complex cell. This may
occur in the medial temporal (MT) cortex( an nucleus in the dorsal [action] stream)
 Christopher Back and Richard Born determined how neurons in the monkeys MT
cortex responded to moving orientated lines.
o Initial response of neurons were determined by the orientation of the bar
o However after 140ms the neurons starting responding to the actual
direction
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MT neurons receive signals from a number of neurons in the striate cortex
and then combine these signals to determine the actual direction of motion
Solution (aperture problem) – a neuron could use informational bout the end of a
moving object to determine its detection. Found neurons in the striate cortex that do
this.
Motion of Arrays of Dots on the Retina
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A type of movement that makes it possibly to vary how difficult it is to determine the direction
of movement
Neural Firing and the Perception of Moving-Dot Stimuli
- William Newsome used ac computer to create moving- dot displays in which the direction of
motion of individual dots can be varied
Coherence: the degree to which the dots move in the same direction
 I.e. 100% coherence- all the dots are moving in the same direction
- Used stimuli to determine the relationship btw
1. Monkeys ability to detect the direction in which the dots were moving
2. The response of a neuron in the monkey’s MT cortex
- Found that as the dots coherence increases:
1. The monkey detected the direction of motion more accurately
2. The MT neurons fired more rapidly
*Newsome directly measured the relationship between physiology and perception
Effect of Lesioning and Microstimulation
- The role of the MT cortex has been studied by determining how the perception of motion is
affected by
1. Lesioning some or all the MT cortex
2. Electrically stimulating neurons in the MT cortex
- intact MT cortex can detect the direction dots are moving when coherence is as low as 1-2%
- Lesioned MT the coherence must be 10-20%
Microstiumlation – lower a small wire electrode into the cortex and pass a weak electrical charge. This
weak shock stimulates neurons that are near the electrode tip and cause them to fire
- Moshvon and Newsome used microstiumlation to activate neurons in a column that responded
best to a particular direction of motion while a monkey was judging the direction of dots that
were moving in a different direction
- The monkey suddenly shifts its judgment toward the direction signaled by the stimulated
neurons
Taking Eye motions Into Account: The corollary Discharge
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How does the perceptual system indicate that the stimulus is moving even though there is no
movement on the retina
Corollary discharge theory – the perceptual system uses a signal called the corollary discharge to
take into account the fact that the observer’s eye is moving.
Corollary Discharge Theory
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If you are following someone with your eyes while keeping your head stationary your eyes move
because motor signals (MS) are being sent from the motor area of your brain to your eye
muscles.
- Another signal corollary discharge signal (CDS) split of form the motor signal. The signal which
occurs anytime motor signal is sent to the eye muscles indicates that a signal has been sent from
the brain to move the eye.
- The cds reached a hypothetical structure the comparator which relays information back to the
brain that eye is moving
*if there is no movement of an image across the retina but the comparator is receiving information
indicating that eye is moving, then the observer perceives motion
- if a stimulus is moving across the retina, the movement activates the retinal receptors and sends a
signal out the optic nerve, image displacement signal (IDS).
* the perception of movement occurs if the comparator receives EITHER a signal that the eye is moving
(CDS) or a signal that an image is displace across the retina (IDS)
*when both CDS and IDS reach the comparator no single is sent to the brain
- comparator probably located in many different places in the brain
Behavioural Demonstration of Corollary Discharge Theory
Demonstration – after image moves in synchrony with your eye motion
Why does the afterimage move when you move your eye?
- The image is not moving across the retina
- However there is a CDS sent with your motor signals sent your eye muscles
Demonstration – gently push your eyeball and the world seems to move
Why do you see motion when you push your eyeball?
- Lawrence Stark&Bruce Bridgeman did this experiment. The eye muscles keep your eye in place
therefore there is a CDS sent out
Physiological Evidence for Corollary Discharge Theory
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R.W. who experiences vertigo (dizziness) anytime he moves his eyes or experience motion when
he looked out the window of a moving car
- RW had lesions in the MST (above MT)
- When he moves his eyes there was an IDS because images were moving across his retina, but
the damage to his brain eliminated the CDS
- Since only he IDS reached the comparator RW saw motion when there actually was none
- Response from motion-sensitive neurons in the monkey’s extrastriate cortex. Neurons response
strongly when monkey looked at a fixation point while a bar sweeps the receptive field.
- Neurons don’t response when monkey follows a moving fixation point and the bar remains
stationary
Real-motion neuron- it responds only when the stimulus moves and doesn’t respond when the eye
moves. It must be receive information like the corollary discharge
Perceived Biological Motion
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Information about people’s action, intentions and moods can be determined solely on motion
information
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Gunnar Johansson created point-light walker stimuli by placing small lights on peoples joints and
filming the patterns created
Biological motion – the motion of a person or other living organism
Brain Activation by Point-Light Walkers
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Can perceptually organize dots into a person because we see biological motion all the time
Emily Grossman and Randolph Blake provided evidence supporting the idea of a specialized area
in the brain for biological motion by measuring observers brain activity as they viewed the
moving dots created by a point-light walker
- Activity in the superior temporal sulcus was greater for biological motion
- FFA were activate more by biological motion
- EBA did not distinguish between these motions
*conclude that there is a network of areas including the STS and FFA that are specialized for the
perception of biological motion.
Linking Brain Activity and the Perception of Biological Motion
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Need to prove not only that a certain structure is activated by a stimulus but that is also part of
perception of the stimulus
Method: Transcrainial Magnetic Stimulation (TMS)
- Temporarily disrupt the functioning of a particular area by applying a pulsating magnetic field
using a stimulating coil placed over the persons skull
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Grossmans experiment (transcraniall…) viewed point light stimulus. Their task was to determine
whether a display was a biological motion or scrabbled motion. Made it more difficult by adding
noise. The key result was that presenting TMS to the area of the STS that is activated by
geological motion caused as significant decrease in the observers’ ability to perceive biological
motion
*normal function of the biological motion area, STS, is necessary for perceiving biological motion
Going beyond the Stimulus
Implied Motion
Implied motion – a still picture depicts a situation involved motion
- Jennifer Freyd did an experiment involving implied motion pictures. After a pause she showed
her observes the same picture or a picture slightly in time or slightly backward in time. The
observers task was to indicate whether the second picture was the same or different
- Observer anticipate the motion that was going to occur , if this occurs the observer might
remember a picture as depicting a situation that occurred slightly later in time.
- The observer took longer to decide whether time time forward picture was different from the
original picture
Representational momentum: the idea that the motion depicted in a picture tends to continue in
the observers’ mind.
- Catherine Reed and Norman Vinson studied the effect of experience on representational
momentum by presenting a sequence of still pictures
- A memory picture was presented and then a test picture. The observers task was to indicate
whether they were in the same or different positions
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Rocket pictures showed greater representational moment effect than the weight pictures. The
observer were more likely to say that the test picture of the rocket that appeared in a position
higher than the memory picture was in the same position
*conclude that the representational moment effect is affected by the person’s expectation about the
motion of an object and that learned properties of objects contribute to these expectations.
- Zoe Kourtzi and Nancy Kanwisher measured the fMRI response areas of MT and MST to pictures they
found that the area of the brain that responds to actual motions also respond to pictures of motions and
that implied motion pictures caused a greater response than rest picture or non-implied motion.
- thus activity occurs in the brain that corresponds to the continued motion that implied- motions
pictures create
Apparent Motion
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When Ramachandran and Stuart Anstis flashed two dots on the left flowed by a single dot on
the right their observers saw the top dot move horizontally to the right and the bottom dot
move horizontally so both dots seemed to move to the same dot
- But adding a square caused a change in perception. Observers viewed both dots move
horizontally
*this perception occurs because of our past experience in seeing objects disappear behind other
objects.
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