Motion Perception

Question 1

What does motion perception require?

Answer 1

• Requires combo of spatial and temporal summation
• Detect motion when there is a change in position of luminance pattern over time
• Smallest detectable shift (motion detection threshold) is ~10 arc seconds

Question 2

What is motion perception usually decomposed into?

Answer 2

Motion Perception is usually decomposed into space & time for analysis
* Can illustrate motion as still images in a space-time diagram
* Car’s position is plotted on horizontal axis as a function of time, on the vertical axis with displacement in time, which is basically a velocity in the unit of degrees of visual angle per second

Question 3

How do we sense motion: Reichardt Detector?

Answer 3

• Reichardt Detector requires 2 spatially separated inputs & temporal filters to delay delivery of signal triggered by input to create a time difference & a comparator
• Comparator can evaluate spatial & temporal changes signalled by the inputs
• According to this model, motion & its direction can be signalled when 2 spatially separated inputs from adjacent retinal locations are initiated at slightly different times
• When original signal from one point in space coincides w/ the delayed signal from an adjacent point in space will signal motion & its respective direction

Question 4

What are the functions of motion perception?

Answer 4

• Detection & identification:
  o When something is moving, draws an immediate attention to it
  o More detailed info about a moving object can be extracted (e.g. 3D structure/biological motion)
  o Segregation of objects & scenes
• Navigation: compute distance & direction to various objects in the scene through self-generated motion (AKA optic flow) – helps avoid collision
• We can recognise actions of humans or other living organisms w/ v limited amount of motion info

Question 5

Describe scene segregation by motion?

Answer 5

• Many animals use colours & patterns on their bodies to help them blend into background & avoid attention of predators – called crypsis in Zoology  defined as strategies animals use to avoid detection by other animals
• Camouflage is well-known method of crypsis – many animals are evolved with
• Once the animal moves, the camouflage is broken & it becomes much easier to recognise them because their deceptive presence is now segregated from background by motion

Question 6

What is kinetic depth?

Answer 6

• Motion also provides info about depth & other 3D info about an object
• Kinetic depth effect: can perceive 3D structure of object when object is set in motion

Question 7

Describe biological motion?

Answer 7

• Movement of living organism
• Provides us w/ unique info that can be distinguished from motions from inanimate objects
• “Point-like walkers” – shows we are sensitive to our own motion – as if we have acquired knowledge of the motion
  o Nikolaus Troje’s biomotion lab – can manipulate many features of the walker such as gender, overall physique & emotional state of walker

Question 8

Describe optic flow?

Answer 8

• When diving a car forward, can sense size of road & above seems expanding towards you  called optic flow
• Gradient of the flow is represented by different arrow length & flow is more rapid near the moving observer & slower further away
  o There is a point on the horizon for which the observer is heading & out of which everything seems expanding w/ no flow is called focus of expansion
• As observer moves around the space, a characteristic, perceptual pattern of motion generated by observer’s self-locomotion is called optic flow
• Optic flow provides info about a layout of objects in space
• Gradient of flow will provide info about relative speed & direction of observer & distance of objects from focus of expansion (reference point in distance where there is no flow)
• Opposite direction: when everything moves AWAY from viewer & sucked into a point in distance called the focus of contraction (FOC)
• Optic flow is self-generated info
• When the observer moves, flow occurs by the movement of the observer & it keeps flowing as long as they are in motion. Once flow is created, then objects around observer move relative to observer – provides more info for next movement
• Optic flow is a good clue to determine where we are in the environment
  o Differs from other clues because it is determined by our actions

Question 9

Describe the 3 situations from which motion perception arise?

Answer 9

• Can arise from various situations
• 1. Retinal Motion:
     o 1st we sense motion when image of object moves in retina – eye is stationary – object moves from L to R, retinal image moves in opposite direction
     o When object moves on retina then that’s when sense motion & is called an oculocentric motion
• 2. Smooth Pursuit:
     o Sense motion through smooth pursuit eye movement – type of eye movement where eye smoothly follow moving target  egocentric or viewer centred motion
     o The image of the object will always fall on same retinal location when being pursued
     o No retinal motion here but we never fail to see the movement of the object
• 3. Retinal Motion by Eye Movement:
     o Create retinal motion over a static image by moving eye between 2 positions
     o We know that the image is not moving – leads to Q of how you know if motion across retina is generated by eye movement or actual moving object

Question 10

How is the problem from retinal motion, smooth pursuit and retinal motion by eye movement resolved?

Answer 10

To resolve this problem, hypothesised that there is a sensory area of visual system that receives copy of signal issued by motor system from the eye muscles when eyes move
This copied signal is referred to as corollary discharge signal.

Question 11

What are the 3 theeories of corollary discharge theory?

Answer 11

1. Image Movement Signal (IMS): movement of image stimulating receptors across retina (after the eye movement) – the comparator receives these IMS & the corollary discharge signal & then compare them to compensate for retinal motion due to eye movement (by subtracting them)
2. Motor Signal (MS): signal triggered by eye muscles
3. Corollary Discharge Signal (CDS): copy of motor signal

Question 12

Describe corollary discharge theory: Eye Movement on a static image?

Answer 12

• When make saccade from L to R or R to L over a static image
• According to the theory, when that happens then the motor signal is created because eye moved then motor signal is created
• Copy of motor signal sent to the comparator
• Then from retina, the image movement signal is also created because by moving eye over static image, artificially created retinal motion –> signal sent to comparator too
• Final output: comparing CDS against IMS and when subtract them (CDS-IMS) they cancel each other out so there is no real motion

Question 13

Describe corollary discharge theory: Real Motion over Fixation?

Answer 13

• Real movement over a fixation – you didn’t move your eye but outside there’s something that moved
• You didn’t move your eye so there’s no motor signal & no copy of this signal sent to the comparator
• But at the retinal level, there was actually a motion of that object so the IMS is sent to the comparator so net signal is IMS so there was a real motion detected

Question 14

Describe corollary discharge theory: Real Motion over Fixation?

Answer 14

• Real movement over a fixation – you didn’t move your eye but outside there’s something that moved
• You didn’t move your eye so there’s no motor signal & no copy of this signal sent to the comparator
• But at the retinal level, there was actually a motion of that object so the IMS is sent to the comparator so net signal is IMS so there was a real motion detected

Question 15

Describe corollary discharge theory: Real Motion over Fixation?

Answer 15

• Real movement over a fixation – you didn’t move your eye but outside there’s something that moved
• You didn’t move your eye so there’s no motor signal & no copy of this signal sent to the comparator
• But at the retinal level, there was actually a motion of that object so the IMS is sent to the comparator so net signal is IMS so there was a real motion detected

Question 16

Describe corollary discharge theory: smooth pursuit?

Answer 16

• When follow an object w/ a smooth pursuit then motor signal is generated from eye moving
• Copy of this motor signal is sent to the comparator but because there was no retinal motion w/ smooth pursuit
  o This is not sent to the comparator so net signal is left w/ the copy – the CDS –> so in this case, the real motion is also signalled

Question 17

Describe apparent motion?

Answer 17

• Unlike motion perception caused by a real movement of an object – no. of instances where we sense motion even when nothing’s really moving
  o This motion is called apparent motion
• This illusory impression of motion occurs when spatially separated stationary stimuli presented sequentially on a connected path over a short time interval in a quick subsection
• Beta movement: type of apparent motion where static spots are turned on & off in sequence w/ a frame rate of 10-12 frames per second between which the apparent motion typically arises
• A motion picture or moving billboard w/ LEDs are the real-life e.g.s of the apparent motion
• As a general rule of thumb, we perceive the shortest path of motion for the apparent motion w/ flashes like this

Question 18

Describe induced motion?

Answer 18

• Sometimes, motion perception of a static image is caused by relative movement of other objects
• Example: looking at moon on partly cloudy, windy night – when watch cloud passing by moon long enough, there’s a time when suddenly experience that it is the moon moving through the clouds, when in fact it’s other way around
  o This relative movement or sometimes called induced movement is influenced by frame of reference
• Usually a larger object appears stationary & smaller object appears to move when opposite is reality
• Another example: relative movement can be experienced when 2 trains going the opposite directions are stopping side by side at same time. Your train is still stopping & other starts to move then you suddenly feel like you’re moving backwards even though your train is still

Question 19

What is motion aftereffect?

Answer 19

• Make us see a static stimulus moving after prolonged exposure to a moving object
• Effect always gives rise to motion in opposite direction of adapting motion
• Example: waterfall illusion – stare at cross in centre of video of a moving waterfall – after a minute of looking at it – the waterfall video changes to a picture, but the water looks like it’s moving backwards

Question 20

What is the aperture problem?

Answer 20

• In computing direction of motion, visual system sometimes faces unique problem called aperture problem
• Occurs when a moving object is viewed through a smaller aperture than the entire size of the object where edges & terminals are hidden from view
• When a moving object is viewed through a smaller aperture than entire size of object & direction of motion of the local features or part of object in the aperture may become ambiguous
  o Visual system applies heuristics in this case assuming that the motion is perpendicular to the orientation of the edge or contour

Question 21

What are plaids (motion perception)?

Answer 21

• Tartan-like pattern where 2 gratings w/ different orientations are overlaid on top of each other
• Resulting plaid pattern appears to move in direction of a vector sum direction of each individual grating –> component motion

Question 22

Describe MT for Global Motion Perception?

Answer 22

• When a moving plaid grating pattern or random dots in motion is passed through receptive field of direction selective V1 neurons, the neurons will respond to the individual component motion
• However, our final percept is of a coherent plate drifting in a vector sum of 2-component motions so to see the global direction of the plaid pattern motion – need to leave V1 & go to extrastriate area called MT where direction signals coming from V1 are combined
• When a plaid pattern arrives at the receptive field of MT neurons, then the cell responds to the vector sum direction of the plaid which is the perceived direction of motion, no to the direction of each component gratings
  o These cells in MT are called pattern cells & the pattern cell can receive inputs from a series of component cells – each of which has a different preferred direction from V1
  o These different directional signals are combined in MT for the computation of the global direction of the plaid

Question 23

Describe starling murmuration and motion perception?

Answer 23

• Good illustration of the division of labour in detecting local & global motion
• When watch them flying around, can see both overall direction of motion & local motion from individual birds of which the global direction of motion is comprised as a group

Question 24

What is the random dot kinematogram (RDK)?

Answer 24

• To test functional role of MT & relationship between perception of local & global motion in mammals & humans a stimulus called a random dot kinematogram has been used
• One of most popular stimuli to study motion in psychophysics & neuroscience experiments as it is v easy to manipulate parameters of motion w/ which you can stimulate cells w/ the different directional selectivities

Question 25

Describe Saccadic Eye Movement?

Answer 25

• Eyes continuously execute rapid, ballistic eye movements called saccades that continuously change fixation from one location to another every ~200ms & it takes ~30-100ms to complete
• This eye movement is so fast that even a real motion is often perceived only in snapshots & velocity ranges from 70-700 degrees per second
• Between these saccadic eye movements - period of ~30-100ms – it is literally blinding & this phenomenon is called saccadic suppression or saccadic masking
  o This phenomenon is defined as a reduction in visual sensitivity that occurs during the saccadic eye movements
  o This kind of suppression is though to eliminate trails or smear from the retinal image motion during an eye movement
  o Can test this saccadic suppression w/ a mirror – look closely in mirror & shift gaze from one eye to other & you will never be able to see your eyes moving back & forth
   However, you will see the eye movement when watch your friend or family doing it