Week 6 : Motion Processing

Question 1

Motion

Answer 1

• a relative change in position overtime
• perceiving motion allows us to move ourselves as well as to time the arrival of incoming objects

Question 2

3 features our visual system must be able to perceive

Answer 2

1. We need to know if moving objects are approaching us or heading away from us
2. We must be able to perceive this motion across 3 dimensions
3. We must be able to estimate the speed of moving objects

Question 3

Motion thresholds

Answer 3

• some motion is so slow/fast that we cannot perceive it
• motion thresholds are a function of what parts of the retinae are seeing the motion
• we have poor motion thresholds in the foveal regions of our retinae, it is much better in the periphery
• a moving stimulus in the periphery grabs attention + may direct the fovea over to the stimulus
• to detect movement, object must be moving at least 1 minute of 1 degree across the retina
• the brightness, the size and the amount of time the object is visible influences motion perception

Question 4

Real vs apparent motion…

Answer 4

• we watch someone run across the sidewalk… this is real motion
• illusory motion… apparent motion (appearance of real motion from a sequence of still images)

Question 5

Apparent motion = beta + phi

Answer 5

• beta motion… an object is perceived as realistically moving on the basis of what is acc a series of images presented sequentially
• phi motion… the basis of billboard displays

Question 6

What occurs when the eye is perfectly stationary while an object moves through space (retina)

Answer 6

• as an object moves across the visual field, an image of that object falls on the retina and also moves across the back of the retinal surface
• the visual angle denoted as d over which the object moves, corresponds to the visual angle over which the retinal image travels
• the visual system must be able to correlate an object at T1 in one position P1 with the same object at T2 in another position P2
• it also must be able to distinguish this motion from motion going in the opposite direction (P2 at T1 reaching P1 and T2)

Question 7

Reichardt detectors contain…

Answer 7

• a small circuit composed of 2 adjacent photoreceptors, each with their own receptive field
• a motion sensitive neuron that receives information from both

Question 8

How Reichardt detectors work

Answer 8

• as a light stimulus travels across the visual field it first generates a signal in photoreceptor A and later stimulates photoreceptor B
• due to a delay in the signal arriving from receptor A, these 2 impulses arrive at the motion sensitive neutron at the same time
• this creates a dramatic increase in the motion sensitive neurons output
• motion of different speeds would be detected by circuits that maintain different delays in the input from receptor A
• so this can account for both speed (measured from delay) and direction (measured by which receptive field is activated first)
• if we did not have this delay, the lack of coordination means a substantial increase in neural output is not produced

Question 9

Corollary discharge theory developed because…

Answer 9

to address the fact that in most cases, our eyes, head and body may be in motion while we are trying to assess the motion of another object in the environment

Question 10

Corollary discharge theory

Answer 10

• there are a series of small muscles that are responsible for rotating the eyeball
• the theory proposes that the feedback from these muscles can be combined with visual information about motion to develop a more complete understanding of object motion
• when we track an object across visual space, the motor cortex sends a signal to the muscles that control eye movement to move accordingly
• at the same time, a copy of that signal known as the corollary discharge is relayed to a comparator cell
• the comparator cell also receives info from the eye about how an object is sweeping across the retina, referred to as image movement signal
• ultimately we use these 3 sources to estimate something about an objects movement and relay that info up to the visual cortex

Question 11

Evidence of Corollary discharge theory…

Answer 11

1. movement of afterimages… look directly at bright line & close ur eyes you see afterimage of that light
   - when you move your eyes, the afterimage stays on the exact same part of the retina, but we sense that it is moving
   - it must be from feedback of a corollary discharge loop
2. physiological evidence… discovery of real motion neurons which respond only to movements of objects but not movements of eyes
   - to determine the difference, these neurons first must get feedback from eye movement signals so they can ‘know’ the difference

Question 12

Saccades

Answer 12

• most common eye movements
• look from one object to another
• very rapid, less than 50 milliseconds and can make abt 3 every second
• need up to 1 second to plan a saccade but once initiated the movement is super quick
• we do it when we read
• vision is surpassed during the actual movement

Question 13

Smooth-pursuit movements

Answer 13

• smooth-pursuit eye movements are the voluntary movements we use to track moving objects
• can only make them when there’s an acc moving object in environment

Question 14

What is the Motion Sensitive Cortex called…

Answer 14

• V5 or MT (medial temporal)
• named for its location adjacent to the medial temporal lobe
• MT receives input from V1, V2 + superior colliculus (eye movements)
• MT is sensitive to both direction of motion and speed of motion

Question 15

MT neurons and motion coherence study…

Answer 15

• study of the sensitivity of the MT to global field motion (perception that a significant region of the visual field is moving in tandem)
• experiment involved having monkey watch moving dots
• the number of dots moving together in the same direction could be changed from 0% coherence to 100% coherence

Question 16

What were experimenters able to show…

Answer 16

• experimenters were able to show that MT neurons were encoding coherence
• increase in neural output generated by an increase in dot coherence
• when monkeys were fully trained they could detect the general pattern of motion even when only 3% of the dots were moving in the same direction
• also showed this area to be directionally sensitive… the output of these neurons is specific to coherent motion of a particular direction

Question 17

Experimenters trick

Answer 17

• experimental team switched their set up so that instead of recording from neurons, they were acc stimulating neurons in MT of a known directional tuning
• they showed an animal coherent movement to the right while stimulating output from MT neurons that were tuned for downward motion
• the result was that the animal reported a perception of dots that moved in a diagonal direction downward and to the right

Question 18

Neuropsychology of MT… akinetopsia (motion blindness)

Answer 18

• when MT is damaged, a condition called akinetopsia or motion blindness can happen
• a patient is unable to detect motion despite intact visual perception of stationary stimuli
• perception resembles a series of still photos moving one to the next
• no loss of object recognition, colour vision or visual acuity
• difficulty with simple action tasks (filling glass of water or crossing a street)

Question 19

Motion after-effects…

Answer 19

• waterfall illusion
• motion-based visual illusion In which a stationary object is seen as moving in the opposite direction of real/apparent motion just observed
• suggests that motion neurons in the occipital cortex may have an opponent system
• consider 2 motion-sensitive neurons in MT…
  one neuron N1 sensitive to motion upward, N2 sensitive to motion downward… activity in N1 may prompt inhibition in N2 but when N1 is no longer activated and the inhibition is turned off, we will sense the downward motion that has been inhibited

Question 20

Biological human motion…

Answer 20

• our motion processing systems seem to be particularly good at interpreting patterns of motion that can be attributed to the movement of a living object
• example… point light displays w/ sensors on joints
• humans can judge age, gender and emotional state just from the point light displays
• more activity in the posterior superior temporal sulcus for biological motion than non-biological motion

Question 21

Lateral Intraparietal area (LIP)…

Answer 21

• area with the control of eye movement for assessing moving objects
• this is a brain region in the parietal lobe where signal from across sensory motor systems are known to be integrated
• the bulk about what is known about their area’s function comes from single unit studies in non-human primates…….

Question 22

Affordance…

Answer 22

• An affordance is the info in the visual world that specifies how that info can be used (e.g. see a piano = afford playing music)
• perception is about finding these affordances in the world
• we see an ever-changing world while constantly moving themselves
• affordances mean that perception is determined partially by meaning/function
• an object that has no function may also not draw our attention… function drives perception

Question 23

LIP area study + key features…

Answer 23

• LIP neurons are spatially tuned and involved in the planning of eye movements
• monkey was trained to fixate at point on screen while experimenters recorded from a LIP neuron w/ a known receptive field
• they played a stimulus that was in the receptive field of the LIP neuron then made it disappear, the after is disappeared they asked the monkey to make a saccade to look where it was
• the activity within the LIP neuron was maintained up to and during the eye movement which suggests it plays a role in maintaining the target position during motor planning + executing the movement
• therefore… LIP area of the parietal lobe is involved w/ both the anticipation of the eye movement and the eye movement itself

Question 24

Visually guided grasping

Answer 24

• our visual systems and our motor systems have coevolved to allow us precise control over movement on the basis of continual feedback from our visual system (grasping)
• an area of the posterior parietal lobe called the medial intraparietal (MIP) area… planning + control of reaching movements of the arms, goal reacted arm movements
• Nearby is anterior intraparietal area (AIP)… grasping
• once your arms are close enough to an object that your fingers can start grasping the item, activation shifts from MIP area neurons to AIP area neurons

Question 25

Apparent motion…

Answer 25

• motion processing systems that use our experience w/ the world best explain situations that we are fooled by illusory motion
• apparent motion is the phenomenon in which a percept of smooth motion is generated by a series of static images
• animation or as even 2 flashing lights that looks like a light bouncing across a screen
• our perceptual experience of the world builds up an expectation of how things move + this expectation can bias our perception giving rise to illusion of movement

Question 26

Motion Correspondence

Answer 26

• illusory percepts are one of the substantial challenges to the visual system when tracking motion
• the correspondence problem refers to how the visual system knows if an object seem at time 1 is the same object as time 2
• there must be a way in which we keep coherence in motion perception
• how to determine which objects within a complex visual scene should be bound when sampling at two different time points in order to assemble an accurate representation of motion
• pattern of underlying motion

Question 27

Induced motion (vection)

Answer 27

• induced motion percept like when you are sitting in a car and someone backs out beside u and it feels like u are the one moving but its the other vehicle
• one moving object may cause another object to look like its moving

Question 28

Rotating snakes

Answer 28

• the explanation of the motion induced revolves around eye movements… micro-saccades
• micro saccades prevent visual fading during fixation