8: Motion Perception

Question 1

Why is motion perception important? Give examples.

Answer 1

Survival.

Helps detect predators and prey.

Question 2

Motion provides what three pieces of information about objects?

Answer 2

Depth, object shape, self-motion.

Question 3

List four benefits of visual motion.

Answer 3

Separates Figure from Ground (attentional capture).

Helps with object recognition (depth/shape/form from motion).

Helps with depth perception (motion parallax).

Self-motion.

Question 4

What is motion agnosia (akinetopsia)? Give an example of an individual with it and examples of abnormalities they face.

Answer 4

Damage to various modules along the dorsal stream impairs the perception of motion.

E.g., patient “L.M.” with stroke to middle temporal (MT) cortex.

Cannot perceive motion; when pouring tea, liquid appears frozen; visitors appear then disappear; cars suddenly appear while crossing the street.

Question 5

What is a problem with visual motion? What is the result of that?

Answer 5

Photoreceptors cannot code motion.

Motion signal must be calculated from the inputs of multiple neurons.

Question 6

Visual motion is not simply an image moving across the retina. List five pieces of evidence for this.

Answer 6

Apparent ,otion: movies - motion perceived in the absence of image movement across the retinA.

Induced Movement: object motion is perceived despite lack of image motion.

Motion aftereffects: motion perceived in the absence of image movement across the retina.

Tracking a moving object: motion perceived in the absence of image movement across the retina.

Self-motion: images move across the retina but object motion is not perceived.

Question 7

What happens if the interstimulus interval in apparent motion is too short? Too long?

Answer 7

Too short: appear simultaneous.

Too long: appear successive.

Question 8

Describe real motion vs. apparent motion? Why can we not tell the difference?

Answer 8

Real motion = object is physically moving.

Apparent motion = still images presented in rapid succession induces the perception of motion.

Cannot tell the difference because of the way visual neurons detect motion.

Question 9

What is an elementary motion detector?

Answer 9

Space between A and B photoreceptors have preferred feature size and preferred speed. When both are activated and reach threshold, fire to direction selective neuron.

Question 10

What is induced movement? Give an example.

Answer 10

Motion of one (usually larger) object causes a nearby (usually smaller) object to appear to move.

Spot in centre of frame. Move frame to the right, perceive motion of spot to the left.

Question 11

Give an example of motion aftereffect and explain it.

Answer 11

Waterfall illusion (negative motion aftereffect). Stare at moving stimulus for ~30s, look at a stationary stimulus. Will see motion in the opposite direction.

Question 12

What is the neural explanation for the waterfall illusion?

Answer 12

After removal of the continuous movement, “down” cell’s firing rate falls below the spontaneous rate.

Question 13

Motion coding in cortex affects perception. What did raising kittens in a stroboscopic environment find?

Answer 13

Eliminates direction-selectivity in striate cortex and the ability to determine the direction of a moving stimulus.

Question 14

Direction selective neurons spike most when motion in their receptive fields goes in what specific direction? What happens to receptive fields and motion coding?

Answer 14

Primary visual cortex (V1) → middle temporal cortex (MT) → medial superior temporal cortex (MST).

Receptive fields get larger, motion coding more complex.

Question 15

Ambiguity about stimulus direction is reduced by ____.

Answer 15

Population (distributed) coding.

Question 16

What is the aperture problem?

Answer 16

All movement of a bar/grating is seen as perpendicular to the long contour. Creates an ambiguous motion signal.

Question 17

Complex cells in V1 (and some in MT) only respond to the _____ motion.

Answer 17

Component.

Question 18

For the first ~70ms MT cells respond like _____, afterward MT cells prefer _____.

Answer 18

V1 cells; pattern motion.

Question 19

Some cells in MT respond to the overall pattern of _____.

Answer 19

Coherent motion.

Question 20

What is coherence? What neurons respond well to the associated stimulus?

Answer 20

Dot correlation (i.e., whether moving in same direction).

MT neurons.

Question 21

Lesion studies in MT discovered what about coherence?

Answer 21

Normal threshold = 1-2%

Threshold after lesions of MT = 10-20%

Question 22

Microstimulation studies in MT discovered what?

Answer 22

Electrical stimulation of down column = perception of dot direction shifted downward.

Question 23

Jeremy walks past Maria; Maria’s eyes are stationary. This does what?

Answer 23

Creates local disturbance in optic array.

Question 24

Jeremy walks past Maria; Maria’s follows him with her eyes. This does what?

Answer 24

Creates local disturbance in optic array.

Question 25

Maria walks through the scene. This does what?

Answer 25

Creates global optic flow.

Question 26

How do we get the perception of motion, when there is no motion across the retina?

Answer 26

Corollary discharge: a copy of the motor command.

Question 27

List four ways corollary discharge can cause the perception of movement when there is no movement on the retina.

Answer 27

Move eyes while viewing afterimage in the dark.

Push on eyeball while looking at a spot to keep eyes stationary.

Move eyes to follow a target.

Paralyze eye muscles and try to move eye.

Question 28

How is patient “R.W.” evidence for corollary discharge?

Answer 28

Damage to MST, vertigo. Cannot cancel visual motion with corollary discharge, therefore perceives world as moving, instead of eyes moving in a stationary world.

Question 29

What is a neuroscientific piece of evidence for corollary discharge? What happened during monkey studies?

Answer 29

Real Movement neurons in V3.

Cells in V3 of monkey cortex respond when bar moved across receptive field, but not when the eyes move and a stationary bar gets swept across the receptive field.

Question 30

What is the ecological approach to perception? Give an example.

Answer 30

Perception and action are always linked (e.g., visual perception and self-motion through world).

Question 31

The ecological approach to perception views traditional studies of visual perception how?

Answer 31

Traditional studies of perception may not reflect what observers actually do in the real world.

Question 32

What was one focus for J.J. Gibson?

Answer 32

“What information do perceivers use as they move through the environment?”

Question 33

What is optic flow?

Answer 33

As organisms move through an environment containing numerous visual stimuli, distinct and informative flowfields, called optic flow, occurs across the entire retina.

Question 34

What information do flowfields give?

Answer 34

Provide information with regard to your movement (self-motion) through the environment.

Question 35

What are two elements of optic flow?

Answer 35

Focus of expansion: where you are heading (no flow).

Gradient of Flow: slow near focus of expansion, faster away from the focus of expansion.

Question 36

Give two examples of how organisms use optic flow.

Answer 36

Airplane pilots use optic flow information to land airplanes.

Honeybees also use optic flow information to land.

Question 37

What can observers do very accurately with optic flow examples?

Answer 37

Estimate heading direction.

Question 38

Drivers do use optic flow to stay on the road, although they rarely look at _____.

Answer 38

Focus of expansion.

Question 39

Optic flow is self-produced. Explain.

Answer 39

Need to perceive to move, need to move to perceive. If want more optic flow information, just have to move more.

Question 40

What is optic flow a use for posture and balance?

Answer 40

If you want to stand still, eliminate optic flow. Lack of optic flow = lack of self-motion.

Question 41

What is vection?

Answer 41

Illusory self motion. Large moving optic flow stimuli (e.g., IMAX) can cause the percept that the stimulus is stationary and you are moving.

Question 42

Visual information can override _____ and _____ sensors that also control balance

Answer 42

Vestibular; muscle.

Question 43

In a swinging room, what happens to optic flow with a backward swing? A forward swing?

Answer 43

Backward swing: expanding optic flow = “I’m falling forward,” lean back.

Forward swing: contracting optic flow = “I’m falling backward,” lean forward.

Question 44

What cells respond to biological motion?

Answer 44

Cells in the Superior Temporal Sulcus (STS)

Question 45

Why is biological motion special?

Answer 45

Activates different areas than other motion - STS rather than MT/MST.

Question 46

Motion agnosia patients can still recognize _____ of human actions

Answer 46

Point-light displays.

Question 47

Describe trans-cranial magnetic stimulation (TMS) and its function.

Answer 47

Pulsating magnetic fields induce current in the brain, temporarily disrupt brain function.

Mainly used to inactivate brain regions.

Question 48

What happened when normal subjects had their STS targeted with TMS? When other brain regions targeted?

Answer 48

Biological motion became more difficult to see.

Other areas: did not affect perception of biological motion.

Question 49

What is the shortest path constraint? How does it happen and why?

Answer 49

During apparent motion, we tend to see motion following the shortest path.

Seen when second image presented rapidly - visual system has not processed meaning of scene. When first image longer, biological motion.

Question 50

What did a PET image study show regarding shortest path constraint vs. biological motion?

Answer 50

Both motions activated parietal cortex, but motor cortex was also activated when subjects saw the possible movement.

Question 51

What is implied motion and why does it happen?

Answer 51

Still pictures that depict motion. View pictures with implied motion, extend the motion in mind.

Question 52

fMRI studies indicate pictures with implied motion do what?

Answer 52

Activate areas MT and MST more than pictures without implied motion.

Question 53

Objects in pictures with implied motion have representational momentum. What does this mean?

Answer 53

Continue along a path dictated by their perceived properties.