Motion Perception

Question 1

What is the evolutionary importance of motion perception?

Answer 1

• Movement = life
• Predators that can detect movement of prey more likely to catch it
• Prey that can detect movement of predators are more likely to survive
• Many animals have poor depth, shape and colour perception
• NONE lack the ability to perceive movement

Question 2

What are the functions of motion?

Answer 2

• Movements attracts our attention (wave) (active or passive)
• Movement of an object relative to an observer provides information object’s 3D shape
• Movement provides information that helps segregate figure from ground and perceptual organisation (common fate)
• Movement breaks camouflage (freeze reflex)
• Movement provides information that enables us to actively interact with the environment. Ball games
• Informs of your heading and time to collision, your movement as well as other objects

Question 3

Do we need to be able to recognise an object in order to see it move and do we match edges and contours between successive views of an object? What provides evidence for this?

Answer 3

• Random dot kinematograms suggest not (the above) – motion analogs of Random dot stereograms
• Instead of presented each simultaneously to the right and left eye, we now present the first and then the second after a short time lag
• Here we perceive a central square to move rightward, even though we cannot perceive a square in either frame alone
• The correspondence problem highlighted by RDKs suggest that motion detection is direct

Question 4

What are 5 ways to make a spot of light move?

Answer 4

1. Real movement
2. Apparent movement
3. Induced movement
4. Autokinetic movement
5. Movement aftereffects

Question 5

What is real movement?

Answer 5

• Light physically moves, i.e. from place A to place B
• We perceive movement when the eyes are stationary, so that the image moves across the retina
• When an image moves across the retina, it stimulates a series of receptors
• There are neurons in the visual system that respond best when a stimulus moves in a particular direction

Question 6

What are movement detectors?

Answer 6

 When an image moves across the retina, it stimulates a series of receptors
 Excitation and inhibition interact to create a cell that responds only to movement from right to left but does not respond to movement from left to right
- Receptor cells can also detect movement of a specified direction and speed – if it receives two signals in close enough succession it will fire

Question 7

Give features of the speed movement detectors

Answer 7

 When something moves in the wrong direction there will be no response
 Bigger separation of receptors detects faster motion
 Detectors such as these have been found in insect sand frogs
 We have something similar
 Cells in the cortex are sensitive to different orientations, speed and direction of movement

Question 8

What is the aperture problem?

Answer 8

means output of all detectors must be integrated at some stage. This is a problem as there are lots of types of motion that can cause the same receptors to be activated. Brain must integrate activity of all the detection receptors.

Question 9

what is the threshold for detecting movement in real movement and what does it depend on?

Answer 9

• How quickly an object needs to be moving for you to be able to detect it
• Depends on object and surrounding

Question 10

What is the perception of velocity in real movement affected by?

Answer 10

Affected by surrounding plus the size of both the moving object and the framework through which it moves

Question 11

What can’t movement detectors explain?

Answer 11

• Cannot explain movement perception when:
  1. There is no movement on the retina – as when you follow a moving object with your eyes so your eye movements keep the object’s image stationary on fovea
  2. When you perceive no movement when these is movement on retina – as when you move your eyes to look at different parts of the scene or as you walk through a scene

Question 12

What is Helmholtz’s outflow theory?

Answer 12

• Efferent (outgoing) signal to eye muscles from brain
• Efferent copy of eye muscle commands
• Afferent (ingoing) movement signal from retina
• Brain comparator: if afferent signal cancels out efferent signal ‘no movement’ of the world has occurred; if there is a mismatch, movement is perceived.
• If there is a difference between muscle movement command and movement of image across the retina then we perceive movement

Question 13

How does the outflow theory work in regards to tracking the motion of a car and when we look around the world?

Answer 13

• when tracking car, eyes move but retinal signal remains stationary, therefore perceive movement of the car
• When we look around the world, eye movement command and retinal image movement are equal so we perceive NO movement

Question 14

Give evidence for Helmholtz’s outflow theory

Answer 14

1. Afterimages move when we move our eyes (eye muscle movement signal no retinal movement)
2. The world moves when we passively wobble our eyes (retinal movement, no eye muscle movement signal)
3. Immobilizing eye-ball results in attempted eye-movement leading to apparent movement of world in opposite direction (eye movement signal, no retinal movement)

Question 15

What is apparent movement?

Answer 15

• Illusion of movement between two light by flashing one light on and off, waiting between 40 & 299 msec, then flashing other light on and off
• Perception of movement in film = series of static images
• Less than 30 msec = no movement, simultaneously
• Above 30=60 msec = partial movement
• About 60 msec = optimum movement
• About 60-200 msec = beta and phi movement (phenomenon) phi: while movement appears to occur between the two lights, it is difficult to actually perceive an object moving across the space between them. Beta perceive an object between
• Above about 200 msec = no movement, successive
• Slow apparent motion can be ambiguous

Question 16

Apart from the time between light what else can affect apparent movement?

Answer 16

• Distance between two lights also affects perceptual apparent movement
   As distance increases, either the time interval or the intensity of the flashes must be increased to maintain the same perception of movement

Question 17

What is induced movement?

Answer 17

• Surround spot with another object and then move this object
• Sitting on train – feel it move backward, only to realise your train is actually standing still, and the one next to you is moving forward

Question 18

What is autokinetic movement?

Answer 18

Turn out all room lights. When the surrounding framework of the room is not visible, the small stationary light appears to move, usually in an erratic path

Question 19

What are movement after-effects?

Answer 19

• If an observer first views a pattern moving in one direction, and then views the spot of light, the spot (and surroundings) will appear to move the opposite direction.
• Waterfall illusion

Question 20

What is the ratio hypothesis of after-effects?

Answer 20

 Hubel and Wiesel (1959) identified directionally specific motion detectors
 Sutherland (1961) argued that motion after-effects arose from an imbalance in the ratio of activities from two sets of directionally-tuned receptors, each sensitive to the opposite directions of motion
 Barlow & Hill (1963) provide direct evidence to support ratio hypothesis

Question 21

What is event perception?

Answer 21

• Movement provides information about 3D shapes, segregate figure from ground and interact with the environment
• E.g.
• Creating structure from motion