Topic 4: Motion

Question 1

Where is motion processed?

Answer 1

• No motion receptors in the retina or LGN
• Processed in the Dorsal pathway
• V1, V2, V3, V5/MT, MST, V6

Question 2

How does V1 and V2 detect motion?

Answer 2

• Small receptive fields
• Respond to simple stimuli (lines) and linear motion (left/right)
• Not tuned for speed
• Not SPECIALISED

Question 3

How does V3 detect motion?

Answer 3

• Larger receptive fields
• Specialised for motion of complex stimuli (texture)

Question 4

How does V5/MT detect motion?

Answer 4

• Large receptive fields
• Responds to any motion (including random dots)
• Direction and speed tuned
• Motion contrast cells
• Static images that imply motion

Question 5

How does MST detect motion?

Answer 5

• Very large receptive fields
• Responds to any motion (including random dots)
• Direction and speed tuned
• Static images that imply motion
• Vestibular cues (self-motion)

Question 6

How does V6 detect motion?

Answer 6

• Self-induced motion

Question 7

What is:
1. Helmholtz brain signal theory (outflow)
2. Sherrington eye muscle theory (inflow)
3. Which theory is right?

Answer 7

1. Efference copy of move muscle signal from brain
2. Motion from actual eye movement signal is sent
3. Helmholtz

Question 8

What is opponent motion?

Answer 8

Signals movement in either direction. The perceived direction depends on the balance of left and right signals

Question 9

What is apparent motion?

Answer 9

If an object appears at A and B and time 1 and 2, it appears to have moved
If the delay between time 1 & 2 is too long, or the physical gap between positions A & B too great, then we won’t see motion: just displacement
If both the time delay and the distance travelled are sufficiently short we will see motion

Question 10

How are TV and movies designed around human vision?

Answer 10

• One image every 40ms fools the brain into seeing motion
• TV image updated every 20ms - just fast enough
• Can be issues with high frame rate TVs

Question 11

How does the wagon wheel illusion work?

Answer 11

• Apparent motion
• The spokes have moved through a small angle = slow moving
• The spokes have moved through a large angle = fast moving

Question 12

How does motion perception develop when you’re born

Answer 12

• Sensitivity to motion develops 10-12 weeks
• Rudimentary visual flow 6-8 weeks (unable to discriminate motion direction / can perform smooth pursuit)
• Rapid improvements between 6-14 weeks
• However, sensitivity to certain types of motion may develop sooner.
  Looming stimuli – i.e., things that might collide with the child – are detected at a very early age and may even be present at birth.
  However, some abilities that children are born with are later lost and have to be re-learnt.

Question 13

What happens if you damage MT?

Answer 13

Akinetopsia - motion blindness, world is perceived as a series of still photographs
- But… objects can still appear to move if the jump is short enough (i.e., apparent motion).
In akinetopsia the integration of smooth motion breaks down.

Question 14

What happens if you lose V1, V3, and MST/V6

Answer 14

V1: makes you functionally blind but you can still respond to some stimuli – especially movement
V3: impede your motion perception, but not destroy it
MST/V6: inhibit navigation, but not stop you seeing motion

Question 15

What does deg/s measure?

Answer 15

unit of angular speed.
Linear speed depends of viewing distance.
32 deg/s =
0.57 m/s (@ 1m)
2.85 m/s (@ 5m)
5.7 m/s (@ 10m)
57 m/s (@ 100m)

Question 16

How is speed perceived differently over space?
What does this mean?

Answer 16

Perceived speed the highest in fovea
Lower near periphery
Lowest far periphery
Speed judgements are more accurate before adaptation than afterwards
Motion in the periphery fades over time
Driving at 30mph feels slow after exiting the motorway where you were driving at 70mph

Question 17

What is the principle of univariate?

Answer 17

Cells cannot separate speed and contrast (or frequency and intensity)

• Motion cells respond more strongly to high contrast stimuli
• Also respond more strongly to faster motion.
• Thus, difficult to differentiate two stimuli with same output

Question 18

Does perceived speed reduce with reduced contrast?

Answer 18

Yes, at least for some stimuli including looming and receding objects. i.e cars!

Question 19

What is motion contrast in MT? What illusion do they cause?

Answer 19

• Cells that are motion contrast sensitive
• Snow Blind illusion

Question 20

What causes travel sickness?

Answer 20

• When we see a large area of moving stimulus we don’t know if it is us moving or the world
• Common when motion is slight and acceleration smooth as vestibular (ear/balance) cues don’t help
• Vection leads to cue conflict between visual and vestibular cues. Leads to sickness reflex

Question 21

What is illusory motion?

Answer 21

Sensation of motion in a static stimulus (e.g.: motion aftereffect

Pure illusory motion is the sensation of motion in an entirely static stimulus even without adaptation

MT Cells are active when viewing the Enigma image.
Possible explanation: Stripes over-excite orientation cells in V1, V2 or V3 and these cells are also motion sensitive so pass a motion signal to MT

Don’t confuse “apparent motion” and “illusory motion”

Question 22

What is the aperture problem?

Answer 22

If you view a moving stimulus through a small window (aperture) such that you only see one edge of the stimulus you will only see motion at right angles to the edge
Motion cells in area V1 have small receptive fields and only respond to short edges. So they can only signal motion at right angles to their preferred orientation (the mask mimics the small receptive fields of V1 cells).
Cells in MT must integrate these signals to derive the true motion. Hence they solve the aperture problem.
But MT cells can be fooled

Question 23

What is the pinna illusion?

Answer 23

The diamond patterns are good stimuli for V1 cells: essentially they are oriented edges.
As you rock back and forth the V1 cells signal motion orthogonal to the tilt of each diamond.
The aperture problem distorts the true direction of motion for every diamond.
This distortion produces a combination of a rotation and expansion (contraction) in MT.
But, your brain knows that you are rocking back and forth so it discounts the expansion / contraction signal.
You just see the rotation