Chapter 8 Perceiving motion
Chapter 8 Perceiving motion Functions of motion Motion helps us understand events in our environment -
Motion perception Is also essential for our ability to move through the environment
-
This movement is called optic flow and it provides information about the walkers direction and speed
-
Motion agnosia: lost the ability to perceive motion o
Difficult to follow dialogue because she couldn’t see motions of a speaker face or mouth
o
Sudden appearance of people or objects
Motion attracts attention -
attention capture: The ability of motion attracts attention
-
This effect occurs when you are consciously looking for something, also while you are paying attention to something else
Motion provides information about objects -
Soon as all the elements of the bird begin moving in the same direction, the bird becomes visible
-
Moving relative to an objects have help perceive its shape more accurately
-
Motion relative to objects is constantly adding to the information we have about the object
Studying motion perception When do we perceive motion -
Real motion: actual motion of an object
-
Illusory motion: perception of motion when there is none o
-
Apparent motion study
Induced motion occurs when motion of one object usually a large one, causes a nearby stationary object (usually smaller) to appear to move (clouds moving makes the moon seem to move)
-
Motion after effects: occur after viewing a moving stimulus for 30 – 60 seconds and then viewing a stationary stimulus, when appear to move o
Waterfall illusion
Comparing real and apparent motion -
These two type of motion have much in common
-
Larsen: o
Presented three types of displays to a person in an fMRI scanner
o
Control condition: 2 dots in slightly different positions were flashed at same time
o
Real motion display: small dot moved back and forth
o
Apparent motion display: dots were flashed one after the other so they appeared to move back and forth
o
Activation associated with apparent motion was similar to real motion
Motion perception: information in the environment -
Motion perception cant be explained by considering just what is happening on the retina
-
Gibsons approach involves looking for information in the environment that provides information for perception
-
Optic array: the structure created by the surfaces, textures, and contours of the environment o
He focused on how movement of the observer causes changes in the optic array
-
Local disturbances in the optic array: occurs when one object moves relative to the environment , covering and uncovering the stationary background
-
Jermy’s image is stationary on the retina, the same local disturbance information that was available when Maria was keeping her eyes still remains available when she is moving her eyes, and this local disturbance information indicates that Jeremy is moving.
-
This local disturbance in the optic array provides information that Jeremy is moving relative to the environment
-
The fact that everything moves at once is called global optic flow, this signals that maria is moving but that the environment is not
Neural firing to motion across the retina Motion of a stimulus across the retina: the aperture problem
-
As the stimulus sweeps across the retina, it activates directionally selective neurons in the cortex that respond to oriented bars that are moving in a specific direction
-
The response of single directionally selective neurons does not provide sufficient information to indicate the direction in which an object is moving
-
As the pole moves to the right, to moves across the receptive field in the direction indicated by the red arrows and the neuron fires
-
If you were able to focus only on what was happening inside the aperture, you probably noticed that the direction that the front edge of the pencil was moving appeared the same whether the pencil was moving horizontal to the right or up and to the right
-
Movement of an edge across an aperture occurs perpendicular to the direction in which the edge is oriented ( pencil was vertical, the movement was horizontal)
-
So the activity of this neuron would not provide accurate information about the direction of the pencils motion
-
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
-
The visual system appears to solve the aperture problem by pooling the responses of a number of neurons like our complex cell
-
This occurs in the medial temporal cortex, a nucleus in the dorsal stream (where) which contains a large number of directionally selective neurons and which we will see is important for movement perception
-
Pack and Born: determined how neurons in the monkeys MT cortex responded to moving oriented lines like the poll or pencil o
They found that the MT neurons initial response to the stimulus, at about 70 msec after was determined by the orientation of the bar
o
140 msec after presentation of the moving bars, the neurons began responding to the actual direction in which the bars were moving
o
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
-
A neuron could use information about the end of a moving object to determine its direction of motion
-
Neurons that could signal this information, because they respond to the end of moving objects, are found in the striate cortex
-
The visual system apparently can solve this problem by o
Using information for neurons in the MT cortex that pool the responses of a number of directionally selective neurons,
o
Using information from neurons in the striate cortex that respond to the movement of the ends of objects
Motion of arrays of dots on the retina Neural firing and the perception of moving dot stimuli -
Newsome: created moving dot displays in which the direction of motion of individual dots can be varied
-
Coherence: to indicate the degree to which the dots move in the same direction
-
Newsome o
Determined the relationship between a monkeys ability to judge the direction in which dots were moving
o
The response of a neuron in the monkey MT cortex
o
As dots coherence increased
o -
The monkey judged the direction of motion more accurately
The MT neurons fired more rapidly
The MT neurons always fired faster than its baseline rate
This experiment is important because it directly measured the relationship between physiological processes and experience and action which is usually inferred
Effect of lesioning and micro stimulation -
Mt cortex is important for motion perception
-
A monkey with intact MT can begin detecting the direction of dots are moving when coherence is as low as 1-2 %
-
After MT lesion, its 10-20%
-
Micro-stimulation o
Achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode
o
This weak shock stimulates neurons that are near the electrode to and causes them to fire
-
Neurons are organized in orientation columns in the cortex, with neurons in the same column responding best to one specific orientation
-
Movshon and Newsome: o
Used micro-stimulation to activate neurons in a column that responded best to particular direction of motion while a monkey was judging the direction of dots that were moving in a different direction
o
The monkey suddenly shifted its judgment toward the direction signalled by the shock
Taking eye motions into account: the corollary discharge -
According to corollary discharge theory: perceptual system uses a signal called the corollary discharge to take into account the fact that the observers eye is moving
Corollary discharge theory -
Your eyes move because motor signals are being sent from the motor area of your brain to your eye muscles
-
According to corollary discharge theory, another neural signal, called the corollary discharge signal (CDS) splits off from the motor signal
-
The CDS which occurs anytime a motor signal is sent to the eye, indicates that a signal has been sent from the brain to move the eye
-
The CDS reaches a hypothetical structure called the comparator, which relays information back to the brain that the eye is moving
-
If there is no movement of an image across the retina, but the comparator is receiving information indicating that the eye is moving, then the observer perceives motion
-
The comparator not only receives the CDS, but also receives the signal that occurs when an image moves across the retina
-
This movement activates the retinal receptors and sends a signal out the optic nerve that we will call the image displace signal IDS, because it occurs when a stimulus is displaced across the retina
-
When the IDS reaches the comparator, the comparator sends a signal to the brain that results in the perception of motion
-
Corollary discharge theory is therefore a fairly simple idea, which can be summarized by saying that the perception of movement occurs if the comparator receives either o
A signal that the eye is moving CDS
o
A signal that an image is begin displaced across the retina IDS
-
What happens if both are firing ( you are moving your eye to inspect a stationary scene)
-
The CDS is generated because the eye is moving, and the IDS is generated because images of the scene are sweeping across the retina
-
Both CDS and IDS reach the comparator simultaneously, no signal is sent to the brain, so no motion is perceived
-
The collar discharge signal probably originates from a number of different places in the brain
-
CDS proposes that the visual system takes into account information about both stimulation of the receptors and movement of the eye determine our perceptions
Behavioural demonstrations of Corollary discharge theory -
Afterimage appear to move when you move your eyes
-
A corollary discharge signal accompanies the motor signals sent to your eye muscles as you move your eyes
-
Only the corollary discharge signal reaches the comparator, and you see the afterimage move
-
Stark and Bridgeman: o
Instructed observers to keep looking at a particular point while pushing on their eyelid
o
According to the Corollary discharge theory, the motor signal sent to the eye muscles to hold the eye in place created a corollary discharge signal, which reached the comparator alone
Physiological evidence for corollary discharge theory -
R.W. male who experienced vertigo (dizziness) anytime he moved his eyes or experienced motion when he looked out the window of a moving car o
He had lesions in area of his cortex called medial superior temporal area (MST)
o
As he moved his eyes, the stationary environment appeared to move with the velocity that matched the velocity with which he was moving his eyes
o
When he moved his eyes, IDS because images were moving across his retina, but the damage to his brain apparently eliminated the CDS
-
The neuron responds strongly when the monkey looks directly at the fixation point as a moving bar sweeps across the cells receptive field, but does not respond when the monkey follows a moving fixation point with its eyes as the bar remains stationary
-
This neuron is called real motion neuron, because it responds only when the stimulus moves and doesn’t respond went the eye moves , when though the stimulus on the retina, is the same
Perceiving biological motion -
Johansson: created point light walker stimulus by placing small lights on peoples joints and then filming the patterns created in the dark
-
The person looks like meaningless pattern when standing still
Brain activation by point light walkers -
Movement can create perceptual organization
-
We are particularly good at perceptually organizing the complex motion of an array of moving dots
-
Grossman and Blake: provided evidence supported the idea of a specialize area in the brain for biological motion o
Activity in a small area in the superior temporal sulcus (STS) was greater for biological motion than for scrambled motion in all eight observers
o
Other regions, as the FFA were activated more by biological motion than by scrambled motion
o
They concluded that there is a network of areas, which include the STS and FFA that are specialized for the perception of biological motion
Linking brain activity and the perception of biological motion -
Showing that a structure responds to a specific type of stimulus does not prove that the structure is involved in perceiving that stimulus
-
Newsome: o
-
Grossman: o
-
The MT cortex is activated by motion, he also showed that perception of motion is decreased by lesioning MT cortex and is influenced by stimulating neurons in Mt cortex
Disrupting operations of the STS in humans decreased ability to perceive biological motion
Presenting transcranial magnetic stimulation (stops function) to the area of the STS is activated by biological motion caused a significant decrease in the observers ability to perceive biological motion
Something to consider: going beyond the stimulus Implied motion -
A situation such as this in which a still picture depicts a stimulation involving motion
-
Freyd: o
showing observers pictures that depicted a situation involving motion, such as a person jumping off a low wall
o
-
After a pause, she showed her observers either
The same picture
A picture slightly forward in time
A picture slightly backwards in time
o
Task was to indicate whether the 2nd picture was the same or different
o
Observers took longer to decide if the time forward picture was different
o
Representational momentum: motion depicted in pictures tends to continue in the observers minds
Representational momentum effect is affected by a person’s expectations about the motion of an object and that learned properties of the objects (that rockets go up) contribute to these expectations
Apparent motion -
Flashes the two dots on the left in followed by the single dot on the right, their observers saw the top dot move horizontally to the right and bottom move diagonally
-
But added a square caused observer to perceive both dots moving horizontal (the bottom one gets covered up by the square)
Motion perception Is also essential for our ability to move through the environment
-
This movement is called optic flow and it provides information about the walkers direction and speed
-
Motion agnosia: lost the ability to perceive motion o
Difficult to follow dialogue because she couldn’t see motions of a speaker face or mouth
o
Sudden appearance of people or objects
Motion attracts attention -
attention capture: The ability of motion attracts attention
-
This effect occurs when you are consciously looking for something, also while you are paying attention to something else
Motion provides information about objects -
Soon as all the elements of the bird begin moving in the same direction, the bird becomes visible
-
Moving relative to an objects have help perceive its shape more accurately
-
Motion relative to objects is constantly adding to the information we have about the object
Studying motion perception When do we perceive motion -
Real motion: actual motion of an object
-
Illusory motion: perception of motion when there is none o
-
Apparent motion study
Induced motion occurs when motion of one object usually a large one, causes a nearby stationary object (usually smaller) to appear to move (clouds moving makes the moon seem to move)
-
Motion after effects: occur after viewing a moving stimulus for 30 – 60 seconds and then viewing a stationary stimulus, when appear to move o
Waterfall illusion
Comparing real and apparent motion -
These two type of motion have much in common
-
Larsen: o
Presented three types of displays to a person in an fMRI scanner
o
Control condition: 2 dots in slightly different positions were flashed at same time
o
Real motion display: small dot moved back and forth
o
Apparent motion display: dots were flashed one after the other so they appeared to move back and forth
o
Activation associated with apparent motion was similar to real motion
Motion perception: information in the environment -
Motion perception cant be explained by considering just what is happening on the retina
-
Gibsons approach involves looking for information in the environment that provides information for perception
-
Optic array: the structure created by the surfaces, textures, and contours of the environment o
He focused on how movement of the observer causes changes in the optic array
-
Local disturbances in the optic array: occurs when one object moves relative to the environment , covering and uncovering the stationary background
-
Jermy’s image is stationary on the retina, the same local disturbance information that was available when Maria was keeping her eyes still remains available when she is moving her eyes, and this local disturbance information indicates that Jeremy is moving.
-
This local disturbance in the optic array provides information that Jeremy is moving relative to the environment
-
The fact that everything moves at once is called global optic flow, this signals that maria is moving but that the environment is not
Neural firing to motion across the retina Motion of a stimulus across the retina: the aperture problem
-
As the stimulus sweeps across the retina, it activates directionally selective neurons in the cortex that respond to oriented bars that are moving in a specific direction
-
The response of single directionally selective neurons does not provide sufficient information to indicate the direction in which an object is moving
-
As the pole moves to the right, to moves across the receptive field in the direction indicated by the red arrows and the neuron fires
-
If you were able to focus only on what was happening inside the aperture, you probably noticed that the direction that the front edge of the pencil was moving appeared the same whether the pencil was moving horizontal to the right or up and to the right
-
Movement of an edge across an aperture occurs perpendicular to the direction in which the edge is oriented ( pencil was vertical, the movement was horizontal)
-
So the activity of this neuron would not provide accurate information about the direction of the pencils motion
-
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
-
The visual system appears to solve the aperture problem by pooling the responses of a number of neurons like our complex cell
-
This occurs in the medial temporal cortex, a nucleus in the dorsal stream (where) which contains a large number of directionally selective neurons and which we will see is important for movement perception
-
Pack and Born: determined how neurons in the monkeys MT cortex responded to moving oriented lines like the poll or pencil o
They found that the MT neurons initial response to the stimulus, at about 70 msec after was determined by the orientation of the bar
o
140 msec after presentation of the moving bars, the neurons began responding to the actual direction in which the bars were moving
o
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
-
A neuron could use information about the end of a moving object to determine its direction of motion
-
Neurons that could signal this information, because they respond to the end of moving objects, are found in the striate cortex
-
The visual system apparently can solve this problem by o
Using information for neurons in the MT cortex that pool the responses of a number of directionally selective neurons,
o
Using information from neurons in the striate cortex that respond to the movement of the ends of objects
Motion of arrays of dots on the retina Neural firing and the perception of moving dot stimuli -
Newsome: created moving dot displays in which the direction of motion of individual dots can be varied
-
Coherence: to indicate the degree to which the dots move in the same direction
-
Newsome o
Determined the relationship between a monkeys ability to judge the direction in which dots were moving
o
The response of a neuron in the monkey MT cortex
o
As dots coherence increased
o -
The monkey judged the direction of motion more accurately
The MT neurons fired more rapidly
The MT neurons always fired faster than its baseline rate
This experiment is important because it directly measured the relationship between physiological processes and experience and action which is usually inferred
Effect of lesioning and micro stimulation -
Mt cortex is important for motion perception
-
A monkey with intact MT can begin detecting the direction of dots are moving when coherence is as low as 1-2 %
-
After MT lesion, its 10-20%
-
Micro-stimulation o
Achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode
o
This weak shock stimulates neurons that are near the electrode to and causes them to fire
-
Neurons are organized in orientation columns in the cortex, with neurons in the same column responding best to one specific orientation
-
Movshon and Newsome: o
Used micro-stimulation to activate neurons in a column that responded best to particular direction of motion while a monkey was judging the direction of dots that were moving in a different direction
o
The monkey suddenly shifted its judgment toward the direction signalled by the shock
Taking eye motions into account: the corollary discharge -
According to corollary discharge theory: perceptual system uses a signal called the corollary discharge to take into account the fact that the observers eye is moving
Corollary discharge theory -
Your eyes move because motor signals are being sent from the motor area of your brain to your eye muscles
-
According to corollary discharge theory, another neural signal, called the corollary discharge signal (CDS) splits off from the motor signal
-
The CDS which occurs anytime a motor signal is sent to the eye, indicates that a signal has been sent from the brain to move the eye
-
The CDS reaches a hypothetical structure called the comparator, which relays information back to the brain that the eye is moving
-
If there is no movement of an image across the retina, but the comparator is receiving information indicating that the eye is moving, then the observer perceives motion
-
The comparator not only receives the CDS, but also receives the signal that occurs when an image moves across the retina
-
This movement activates the retinal receptors and sends a signal out the optic nerve that we will call the image displace signal IDS, because it occurs when a stimulus is displaced across the retina
-
When the IDS reaches the comparator, the comparator sends a signal to the brain that results in the perception of motion
-
Corollary discharge theory is therefore a fairly simple idea, which can be summarized by saying that the perception of movement occurs if the comparator receives either o
A signal that the eye is moving CDS
o
A signal that an image is begin displaced across the retina IDS
-
What happens if both are firing ( you are moving your eye to inspect a stationary scene)
-
The CDS is generated because the eye is moving, and the IDS is generated because images of the scene are sweeping across the retina
-
Both CDS and IDS reach the comparator simultaneously, no signal is sent to the brain, so no motion is perceived
-
The collar discharge signal probably originates from a number of different places in the brain
-
CDS proposes that the visual system takes into account information about both stimulation of the receptors and movement of the eye determine our perceptions
Behavioural demonstrations of Corollary discharge theory -
Afterimage appear to move when you move your eyes
-
A corollary discharge signal accompanies the motor signals sent to your eye muscles as you move your eyes
-
Only the corollary discharge signal reaches the comparator, and you see the afterimage move
-
Stark and Bridgeman: o
Instructed observers to keep looking at a particular point while pushing on their eyelid
o
According to the Corollary discharge theory, the motor signal sent to the eye muscles to hold the eye in place created a corollary discharge signal, which reached the comparator alone
Physiological evidence for corollary discharge theory -
R.W. male who experienced vertigo (dizziness) anytime he moved his eyes or experienced motion when he looked out the window of a moving car o
He had lesions in area of his cortex called medial superior temporal area (MST)
o
As he moved his eyes, the stationary environment appeared to move with the velocity that matched the velocity with which he was moving his eyes
o
When he moved his eyes, IDS because images were moving across his retina, but the damage to his brain apparently eliminated the CDS
-
The neuron responds strongly when the monkey looks directly at the fixation point as a moving bar sweeps across the cells receptive field, but does not respond when the monkey follows a moving fixation point with its eyes as the bar remains stationary
-
This neuron is called real motion neuron, because it responds only when the stimulus moves and doesn’t respond went the eye moves , when though the stimulus on the retina, is the same
Perceiving biological motion -
Johansson: created point light walker stimulus by placing small lights on peoples joints and then filming the patterns created in the dark
-
The person looks like meaningless pattern when standing still
Brain activation by point light walkers -
Movement can create perceptual organization
-
We are particularly good at perceptually organizing the complex motion of an array of moving dots
-
Grossman and Blake: provided evidence supported the idea of a specialize area in the brain for biological motion o
Activity in a small area in the superior temporal sulcus (STS) was greater for biological motion than for scrambled motion in all eight observers
o
Other regions, as the FFA were activated more by biological motion than by scrambled motion
o
They concluded that there is a network of areas, which include the STS and FFA that are specialized for the perception of biological motion
Linking brain activity and the perception of biological motion -
Showing that a structure responds to a specific type of stimulus does not prove that the structure is involved in perceiving that stimulus
-
Newsome: o
-
Grossman: o
-
The MT cortex is activated by motion, he also showed that perception of motion is decreased by lesioning MT cortex and is influenced by stimulating neurons in Mt cortex
Disrupting operations of the STS in humans decreased ability to perceive biological motion
Presenting transcranial magnetic stimulation (stops function) to the area of the STS is activated by biological motion caused a significant decrease in the observers ability to perceive biological motion
Something to consider: going beyond the stimulus Implied motion -
A situation such as this in which a still picture depicts a stimulation involving motion
-
Freyd: o
showing observers pictures that depicted a situation involving motion, such as a person jumping off a low wall
o
-
After a pause, she showed her observers either
The same picture
A picture slightly forward in time
A picture slightly backwards in time
o
Task was to indicate whether the 2nd picture was the same or different
o
Observers took longer to decide if the time forward picture was different
o
Representational momentum: motion depicted in pictures tends to continue in the observers minds
Representational momentum effect is affected by a person’s expectations about the motion of an object and that learned properties of the objects (that rockets go up) contribute to these expectations
Apparent motion -
Flashes the two dots on the left in followed by the single dot on the right, their observers saw the top dot move horizontally to the right and bottom move diagonally
-
But added a square caused observer to perceive both dots moving horizontal (the bottom one gets covered up by the square)
