Chapter 8: Perceiving Motion Functions of Motion Perception Motion ...
Chapter 8: Perceiving Motion Functions of Motion Perception Motion Helps Us Understand Events in Our Environment One source of information about where we are going and how fast we move is the way objects in the environment flow past us as we move Optic flow is when a person moves forward objects move relative to the person in the opposite direction; this provides information about the walker’s direction and speed Akinetopsia blindness to motion; woman had a stroke and couldn’t perceive motion, didn’t know when to stop pouring tea, couldn’t follow conversations because she couldn’t see facial movement Motion Attracts Attention Attentional capture is the ability of motion to attract attention Freezing in place eliminates the attention attracting effects Motion Provides Information About Objects The stationary bird is difficult to see when it is covered by the pattern because the bird and the pattern are made up of similar lines Once the bird starts moving it is easier to identify Our own motion relative to objects is constantly adding to the information we have about the objects and we receive similar information when objects move relative to us Studying Motion Perception When Do We Perceive Motion? Real motion is the actual motion of an object We perceive motion when something moves across our field of view Illusionary motion is perception of motion when there actually is none Apparent motion is an illusion of movement that occurs when two objects separated in space are presented rapidly one after another separated by a brief time interval Induced motion occurs when motion of one object (usually large) causes a nearby stationary object (usually smaller) to appear to move Ex. Clouds (large) moving across the moon (small) makes it look like the moon is moving when it is stationary Motion aftereffects occur when viewing a moving stimulus 3060sec causes a stationary stimulus to appear to move Waterfall illusion is an example of motion aftereffects because if you look at a waterfall for 3060seconds and then look off to the side at part of a scene that is stationary, you will see everything in the scene, such as rocks and trees, appear to be moving for a few seconds Comparing Real and Apparent Motion There is evidence that apparent motion and real motion have a lot in common
Larsen created an experiment to show using fMRI scans that during each motion, the same part of the brain was being used Because of the similarities, both motions are studied together as one
What We Want to Explain The goal is to understand how we perceive things that are moving As Jeremy walks past Maria, her eyes are stationary; His image sweeps across her retina As Jeremy walks past Maria, she follows him with her eyes; His image is stationary on her fovea As Maria scans the room by moving her eyes; The images of the room move left across her retina but no perception of the objects of the room moving Motion Perception: Information in the Environment Optic array is the structure created y the surfaces, textures, and contours of the environment How movement of the observer causes changes 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 on. This is called a local disturbance in the optic array Local disturbance in the optic array occurs when Jeremy moves relative to the environment, covering and uncovering the stationary background When Maria follows Jeremy with her eyes, the same local disturbance information that was available when Maria was keeping her eyes still remains available when she is moving her eyes, which indicates that Jeremy is moving When Maria scanned the room with her eyes, the fact that everything moved at once in response to movement of the observers eyes or body is called global optic flow; this signals that the environment is stationary According to Gibson, motion is perceived when one part of the visual scene moves relative to the rest of the scene and no motion is perceived when the entire field moves or remains stationary Motion Perception: Retina/Eye Information Consider neural signals that travel from eye to brain for moving person The Reichardt Detector The Reichardt Detector results in neurons that fire to movement in one direction Excitation and inhibition are arranged so that movement in one direction creates inhibition that eliminates neural responding, whereas movement in the opposite direction creates excitation that enhances neural responding When moving from left to right, receptor A is stimulated which excites receptor E, then receptor E inhibits receptor F. The light has moved to receptor B causing an excitatory response to F, but F is already inhibited by E so it does not fire, therefore when moving from left to right, neuron I does not respond
When moving from right to left, receptor D sends a signal to H causing it to fire and excite neuron I. Recptor C activates neuron G which sends inhibition back to H, however this is too late since H is already excited so G becomes inhibited Thus, the inhibition arrives too late to stop the signal from getting to neuron I Therefore, neuron I only fires to movement to the left, but not to the right
Corollary Discharge Theory Corollary discharge theory takes into account eye movement; explains motion perception as being determined by movement of the image on the retina and by signals that indicate movement of the eyes Signals From the Retina and the Eye Muscles An image displacement signal (IDS) 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 A motor signal (MS) 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 A corollary discharge signal (CDS) is a copy of the motor signal that, instead of going to the eye muscles, is sent to a different place in the brain. This is analogous to using the “cc” function when sending an email. The email goes to the person it is addressed to and is simultaneously sent to someone else at another address. When Maria follows Jeremy with her eyes The comparator receives both IDS and CDS. If just one type of signal reaches the comparator it relays a message to the brain that movement occurred and motion is perceived. But if both reach the comparator at the same time, the cancel out each other, so no signal is sent to the area of the brain responsible for motion perception The CDT proposes that the visual system takes into account both information about stimulation of the receptors and information about movement of the eyes Behavioural Evidence for Corollary Discharge Theory Why does the afterimage appear to move when you move your eyes? The circles image on the retina has created a circular area of bleached visual pigment which remains in the same place no matter where the eye is looking The motor signals sent to move your eyes are creating a corollary discharge signal which reaches the comparator alone so the afterimage appears to move Why do you see motion when you push your eyelid? In an experiment participants were asked to do this and stare at a specific spot but the push didn’t cause their eye to move because the eye muscles were pushing back against the force of the finger to keep the eye in place According to the CDT, the motor signal sent to the eye muscles to hold the eye in place created a corollary discharge signal which reached the comparator alone so observers saw the scene move Physiological Evidence for Corollary Discharge Theory R.W had lesions in the MST area in which he could not move his eyes or experience motion when he looked out the window of a moving car because it made him dizzy
When he moved his eyes to the left, there was an IDS because images were moving across his retina to the right but the damage to his brain eliminated the CDS. Because the IDS reached the comparator, he saw motion when there was actually none Looked at monkeys extrastriate cortex Neuron responds strongly when looking steadily at fixation point as a moving bar swoops across receptive field. If they follow the bar, the neurons sweep across the receptive field but no firing occurs This is a real motion neuron because it responds only when the stimulus moves and doesn’t respond when the eye moves, even though the stimulus on the retina is the same in both situation
Motion Perception and the Brain The Movement Area of the Brain The middle temporal area and medial superior temporal area are important Coherence indicates the degree to which the dots move in the same direction in Newsome’s experiment When dots are all moving in random directions, coherence is 0; when dots are all moving in the same direction there is 100% coherence Dots were used to determine the relationship between 1) a monkeys ability to judge the direction in which dots were moving, 2) the response of neuron in the monkey’s MT cortex As the dots coherence increased the monkey judged the direction of motion more accurately and the MT neuron fired more rapidly The stimulus – perception relationship: Presenting a stimulus and determining whether motion is perceived. Ex. When an object moves fast we perceive movement; when array of dots move in direction, we perceive movement in that direction The stimulus – physiology relationship: Presenting a movement stimulus and measuring neural responding. Ex. A moving bar caused a response in a monkey’s cortex The physiology – perception relationship: Measuring the relationship between physiological responding and perception. This is the relationship measured by Newsome because he measured the response of the MT neurons to the moving dots and also measured the monkey’s perception to moving dots Effect of Lesioning and Microstimulation The role of the MT cortex has been studied by determining how perception of motion is affected by lesioning (destroying or deactivating) some or all of the MT cortex or by electrically stimulating neurons in the MT cortex Monkeys can detect the dots at 12% coherence, but when MT is lesioned, is has to be 1020% coherence METHOD: Microstimulation is achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode. This weak shock stimulates neurons that are near the electrode tip and cause them to fire; measures the link between MT cortex and motion perception
When stimulation of downward MT neurons, monkeys responded as dots moving to the right an down, even though the dots were not moving downward
Motion From a Single Neurons Point of View Woman holding pole moves to the right, pole moves to the right, woman goes up stairs, pole goes up and to the right, but the neuron only receives movement of going to the right not up The movement of an edge across an aperture occurs perpendicular to the direction in which the edge is oriented (pencil is vertical, movement horizontal) Aperture problem is 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 systems solves this problem by pooling responses of a number of neurons with the help of MT cortex 70ms Neurons responded in the same way in both a vertical bar moving horizontally to the right and a vertical bar moving up and to the right 140ms after presentation of bar, neuron responded to the actual direction in which the bar was moving Receive signals from the striate cortex to determine actual direction Neurons respond to ends of moving objects to help determine direction of movement Motion and the Human Body Apparent Motion of the Body Shortest path constraint is the apparent movement tends to occur along the shortest pathway between two stimuli The shortest path constraint would indicate that the girls hand in the photo goes through her head from the front to the back when picture is flipped quickly When picture is flipped more slowly, observers perceived motion going around her head This is interesting because 1) they show that the visual system needs time to process information in order to perceive the movement of complex meaningful stimuli and 2) they suggest that there may be something special about the meaning of the stimulus that influences the way movement is perceived Movement thorugh head and around head showed activation in PET scan for the parietal cortex The motor cortex was activated as well for movement around the head Motor cortex is activated when things are humanly possible Motion of PointLight Walkers Pointlike walkers are created by placing small lights on peoples joints and then filming the patterns created by these lights when people walk and carry out other actions in the dark Perceptual Organization Biological motion is the selfproduced motion of a person or other living organisms
We see biological motion all the time so we know what it looks like Brain Mechanisms There is a specialized area in the brain for biological motion Small area is STS was more active when viewing biological motion than scrambled motion Disrupting the STS in humans decreases the ability to perceive biological motions; Used transcranial magnetic stimulation for this (TMS) Representational Momentum: Motion Responses to Still Pictures Implied motion is a situation in which a still picture depicts an action involving motion (like a picture of someone skiing) Subjects were shown a picture of a person in midair jumping, then shown a picture after of either an earlier time (backward) or a later time (forward) Found that subjects took longer to decide if the timeforward picture was the same or different as the original Representational momentum is the idea that the motion depicted in a picture tends to continue in the observers mind Experience influences our perception because it depends on our knowledge Found using fMRI scans that the area of the brain that responds to actual motion also responds to pictures of motion and the implied motion picture caused greater response than no implied motion pictures, at rest pictures, or house pictures Viewing implied motion in pictures decreased the activity of neurons selective to that direction of motion Implied Movement to the right, observed it going to the left, vise vera Event Perception Event is defined as a segment of time at a particular location that is perceived by observers to have a beginning and an end Event boundary is the point in time when one event ends and another begins
Larsen created an experiment to show using fMRI scans that during each motion, the same part of the brain was being used Because of the similarities, both motions are studied together as one
What We Want to Explain The goal is to understand how we perceive things that are moving As Jeremy walks past Maria, her eyes are stationary; His image sweeps across her retina As Jeremy walks past Maria, she follows him with her eyes; His image is stationary on her fovea As Maria scans the room by moving her eyes; The images of the room move left across her retina but no perception of the objects of the room moving Motion Perception: Information in the Environment Optic array is the structure created y the surfaces, textures, and contours of the environment How movement of the observer causes changes 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 on. This is called a local disturbance in the optic array Local disturbance in the optic array occurs when Jeremy moves relative to the environment, covering and uncovering the stationary background When Maria follows Jeremy with her eyes, the same local disturbance information that was available when Maria was keeping her eyes still remains available when she is moving her eyes, which indicates that Jeremy is moving When Maria scanned the room with her eyes, the fact that everything moved at once in response to movement of the observers eyes or body is called global optic flow; this signals that the environment is stationary According to Gibson, motion is perceived when one part of the visual scene moves relative to the rest of the scene and no motion is perceived when the entire field moves or remains stationary Motion Perception: Retina/Eye Information Consider neural signals that travel from eye to brain for moving person The Reichardt Detector The Reichardt Detector results in neurons that fire to movement in one direction Excitation and inhibition are arranged so that movement in one direction creates inhibition that eliminates neural responding, whereas movement in the opposite direction creates excitation that enhances neural responding When moving from left to right, receptor A is stimulated which excites receptor E, then receptor E inhibits receptor F. The light has moved to receptor B causing an excitatory response to F, but F is already inhibited by E so it does not fire, therefore when moving from left to right, neuron I does not respond
When moving from right to left, receptor D sends a signal to H causing it to fire and excite neuron I. Recptor C activates neuron G which sends inhibition back to H, however this is too late since H is already excited so G becomes inhibited Thus, the inhibition arrives too late to stop the signal from getting to neuron I Therefore, neuron I only fires to movement to the left, but not to the right
Corollary Discharge Theory Corollary discharge theory takes into account eye movement; explains motion perception as being determined by movement of the image on the retina and by signals that indicate movement of the eyes Signals From the Retina and the Eye Muscles An image displacement signal (IDS) 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 A motor signal (MS) 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 A corollary discharge signal (CDS) is a copy of the motor signal that, instead of going to the eye muscles, is sent to a different place in the brain. This is analogous to using the “cc” function when sending an email. The email goes to the person it is addressed to and is simultaneously sent to someone else at another address. When Maria follows Jeremy with her eyes The comparator receives both IDS and CDS. If just one type of signal reaches the comparator it relays a message to the brain that movement occurred and motion is perceived. But if both reach the comparator at the same time, the cancel out each other, so no signal is sent to the area of the brain responsible for motion perception The CDT proposes that the visual system takes into account both information about stimulation of the receptors and information about movement of the eyes Behavioural Evidence for Corollary Discharge Theory Why does the afterimage appear to move when you move your eyes? The circles image on the retina has created a circular area of bleached visual pigment which remains in the same place no matter where the eye is looking The motor signals sent to move your eyes are creating a corollary discharge signal which reaches the comparator alone so the afterimage appears to move Why do you see motion when you push your eyelid? In an experiment participants were asked to do this and stare at a specific spot but the push didn’t cause their eye to move because the eye muscles were pushing back against the force of the finger to keep the eye in place According to the CDT, the motor signal sent to the eye muscles to hold the eye in place created a corollary discharge signal which reached the comparator alone so observers saw the scene move Physiological Evidence for Corollary Discharge Theory R.W had lesions in the MST area in which he could not move his eyes or experience motion when he looked out the window of a moving car because it made him dizzy
When he moved his eyes to the left, there was an IDS because images were moving across his retina to the right but the damage to his brain eliminated the CDS. Because the IDS reached the comparator, he saw motion when there was actually none Looked at monkeys extrastriate cortex Neuron responds strongly when looking steadily at fixation point as a moving bar swoops across receptive field. If they follow the bar, the neurons sweep across the receptive field but no firing occurs This is a real motion neuron because it responds only when the stimulus moves and doesn’t respond when the eye moves, even though the stimulus on the retina is the same in both situation
Motion Perception and the Brain The Movement Area of the Brain The middle temporal area and medial superior temporal area are important Coherence indicates the degree to which the dots move in the same direction in Newsome’s experiment When dots are all moving in random directions, coherence is 0; when dots are all moving in the same direction there is 100% coherence Dots were used to determine the relationship between 1) a monkeys ability to judge the direction in which dots were moving, 2) the response of neuron in the monkey’s MT cortex As the dots coherence increased the monkey judged the direction of motion more accurately and the MT neuron fired more rapidly The stimulus – perception relationship: Presenting a stimulus and determining whether motion is perceived. Ex. When an object moves fast we perceive movement; when array of dots move in direction, we perceive movement in that direction The stimulus – physiology relationship: Presenting a movement stimulus and measuring neural responding. Ex. A moving bar caused a response in a monkey’s cortex The physiology – perception relationship: Measuring the relationship between physiological responding and perception. This is the relationship measured by Newsome because he measured the response of the MT neurons to the moving dots and also measured the monkey’s perception to moving dots Effect of Lesioning and Microstimulation The role of the MT cortex has been studied by determining how perception of motion is affected by lesioning (destroying or deactivating) some or all of the MT cortex or by electrically stimulating neurons in the MT cortex Monkeys can detect the dots at 12% coherence, but when MT is lesioned, is has to be 1020% coherence METHOD: Microstimulation is achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode. This weak shock stimulates neurons that are near the electrode tip and cause them to fire; measures the link between MT cortex and motion perception
When stimulation of downward MT neurons, monkeys responded as dots moving to the right an down, even though the dots were not moving downward
Motion From a Single Neurons Point of View Woman holding pole moves to the right, pole moves to the right, woman goes up stairs, pole goes up and to the right, but the neuron only receives movement of going to the right not up The movement of an edge across an aperture occurs perpendicular to the direction in which the edge is oriented (pencil is vertical, movement horizontal) Aperture problem is 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 systems solves this problem by pooling responses of a number of neurons with the help of MT cortex 70ms Neurons responded in the same way in both a vertical bar moving horizontally to the right and a vertical bar moving up and to the right 140ms after presentation of bar, neuron responded to the actual direction in which the bar was moving Receive signals from the striate cortex to determine actual direction Neurons respond to ends of moving objects to help determine direction of movement Motion and the Human Body Apparent Motion of the Body Shortest path constraint is the apparent movement tends to occur along the shortest pathway between two stimuli The shortest path constraint would indicate that the girls hand in the photo goes through her head from the front to the back when picture is flipped quickly When picture is flipped more slowly, observers perceived motion going around her head This is interesting because 1) they show that the visual system needs time to process information in order to perceive the movement of complex meaningful stimuli and 2) they suggest that there may be something special about the meaning of the stimulus that influences the way movement is perceived Movement thorugh head and around head showed activation in PET scan for the parietal cortex The motor cortex was activated as well for movement around the head Motor cortex is activated when things are humanly possible Motion of PointLight Walkers Pointlike walkers are created by placing small lights on peoples joints and then filming the patterns created by these lights when people walk and carry out other actions in the dark Perceptual Organization Biological motion is the selfproduced motion of a person or other living organisms
We see biological motion all the time so we know what it looks like Brain Mechanisms There is a specialized area in the brain for biological motion Small area is STS was more active when viewing biological motion than scrambled motion Disrupting the STS in humans decreases the ability to perceive biological motions; Used transcranial magnetic stimulation for this (TMS) Representational Momentum: Motion Responses to Still Pictures Implied motion is a situation in which a still picture depicts an action involving motion (like a picture of someone skiing) Subjects were shown a picture of a person in midair jumping, then shown a picture after of either an earlier time (backward) or a later time (forward) Found that subjects took longer to decide if the timeforward picture was the same or different as the original Representational momentum is the idea that the motion depicted in a picture tends to continue in the observers mind Experience influences our perception because it depends on our knowledge Found using fMRI scans that the area of the brain that responds to actual motion also responds to pictures of motion and the implied motion picture caused greater response than no implied motion pictures, at rest pictures, or house pictures Viewing implied motion in pictures decreased the activity of neurons selective to that direction of motion Implied Movement to the right, observed it going to the left, vise vera Event Perception Event is defined as a segment of time at a particular location that is perceived by observers to have a beginning and an end Event boundary is the point in time when one event ends and another begins
