Lecture 4

Question 1

is motion evolutionary?

Answer 1

Seemingly yes, the ability to accurately perceive motion is a survival necessity (not only as an evolution legacy)

Question 2

where is motion processed?

Answer 2

Humans have no motion processing in the retina and LGN, it is only when you reach the cortex (V1 and above) that we can sense motion

draw diagram for retina, LGN, and V1,2,3 etc…

Question 3

what is the blindsight phenomenon?

Answer 3

The phenomenon of blindsight - happens when you have damage to your primary visual cortex

So if your V1 is damaged, stroke or brain trauma, you are practically blind, but you can sense motion because of that direct projection from LGN to V5/MT

Question 4

what are the features of V1 & V2?

Answer 4

• Small receptive fields
  
  • Cells respond to simple stimuli (e.g., lines)
  • Cells respond to linear motion, left-right…
  • Not tuned for speed
    *Not specialised for motion

Question 5

what are the features of V3(VP,V3,V3B,V3A)?

Answer 5

Larger receptive fields

Specialised for motion of complex stimuli (texture)

Question 6

what are the features of V5/MT?

Answer 6

• Large receptive fields
• Responds to motion of anything, including random dots
• Direction and speed tuned
• Simple patterns of motion (expansion-contraction)
• Motion contrast cells
  Responds to static images that imply motion

Question 7

what are the features of MST?

Answer 7

• Very large receptive fields
• Responds to motion of anything, including random dots (not as critical as V5/MT)
• Direction and speed tuned
• Complex motion patterns (such as generated by locomotion)
• Static images that imply motion
• Responds to vestibular cues (self-motion)

Question 8

what are the features of V6?

Answer 8

Responds to self-induced motion

Question 9

how does motion on retina differ from motion of retina?

Answer 9

• When you keep your eyes still on a stationary blue star in the centre of a screen, the red dot moves across your retina - and you see it MOVING. The blue star stays in the same place on your retina - and you see it as STATIONARY
• When you track the red dot with your eyes it DOES NOT move on your retina - but you still see it as MOVING! The blue star MOVES on your retina - but you see it as STATIONARY!
• How? Your brain knows when you move your eyes (or head) intentionally and codes all movement relative to this

Question 10

what is the eye muscle signal theory?

Answer 10

Sherrington suggested we take the signal from the eye muscles, when the eye is moving, the muscles are contracting, and we can get the signal from the muscle and then use that to interpret the signal we get from the retina

motion from actual eye movements (slow)

Question 11

what is the brain signal theory?

Answer 11

Helmholtz suggested that was way too slow - we should rather take the intention signal from the brain

So the intention to move your eyes, that’s the intention we need to take

Question 12

which theory turned out to be more correct/valid?

Answer 12

Helmholtz outflow brain signal theory

Question 13

what is the baseline way humans CANNOT detect motion?

Answer 13

A and b represent two separate receptive fields, we want to be able to detect motion from A to B

Light activates A and then activates B

Cannot detect motion

Question 14

how do human detect motion?

Answer 14

We need to be able to somehow combine these two signals in time to detect motion indicating that something happens from A to B - we do this by adding a time delay to A

So that when the item goes through A it will not trigger it immediately, it will put a delay on that trigger so that when it arrive to B, the signals will coincide

Will signal if light moves from A to B during the time interval specified by the time delay. Note the time delay is inside the brain. Detects rightward motion

Question 15

can we reposition the time delay?

Answer 15

yes! by adding it to receptor B to detect motion in the opposite direction

Question 16

what is opponent motion? how does it work?

Answer 16

Opponent motion detectors respond to the balance between motions cells preferring opposite directions

Question 17

how do speed cameras work? relate it back to TV and animations?

Answer 17

A speed camera essentially takes two snapshots (of the same vehicle), at two different time points (A and B), it he compares the displacement from time A to time B, and because it knows how much time passed from these two points, you can deduct the speed of the car

• If an object appears in position A at time 1 and position B at time 2 it has moved from A to B, even if it did not appear at all the points in between
• 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
• This is the basis for seeing motion in all movies, TV programmes and animations

Question 18

what are the issues with both too slow flicker rates and too fast flicker rates?

Answer 18

• One image every 40ms (fools the brain into seeing motion!)
• Too slow for human vision - would see flicker
• Film - open shutter on projector 2 or 3 times per image. Increases flicker rate
• TV - display image in as two sets of strips, one set updated every 20ms - just fast enough
• However, problems arise for fast moving objects
• High frame rate TVs (120hz) avoid this… sort of

when the rate is too fast you can end up with reverse motion which looks like the Wagon Wheel illusion

Question 19

what is apparent motion?

Answer 19

• A series of still images showing displacement of an object over time will appear as fluid motion (i.e., apparent motion)
• Apparent motion is just as real to the visual system as ordinary (smooth) motion
• Only occurs if the time difference and displacement are sufficiently small
• Problems arise when the delay between images interacts with the speed of the object
• This applies especially to objects with repetitive patterns (e.g., wheel spokes)

Question 20

explain how apparent motion works

Answer 20

a series of snapshots (imagine a cross on each circle)

when the wheel is moving slowly

the spokes (cross on the circle) move through a small angle and appear close together

when the wheel is moving fast

the spokes have moved through a large enough angle that it can be mistaken as the previous position

when the spokes have moved into exact quarters, humans perceive no motion and it will look like nothing is moving (think the helicopter clip)

Question 21

how does motion perception develop?

Answer 21

Sensitivity to motion seems to develop at around 10-12 weeks
* Rudimentary visual flow 6-8 weeks (unable to discriminate motion direction / can perform smooth pursuit)
* Rapid improvements between 6-14 weeks

sensitivity to certain types of motion may develop sooner

Some studies suggests that 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

No one reports developmental motion blindness

Question 22

what is motion blindness?

Answer 22

Similar but opposite to blindsight

damage to V5/MT causes motion blindness where you can still see due to your primary visual cortex still being intact, but we are unable to sense motion

Question 23

How do patients with akinetopsia perform on coherence tasks?

Answer 23

Patients with akinetopsia could only perform well on a coherence task at 100% coherence (all dots moving in the same direction)

Question 24

how might a person with akinetopsia describe their motion blindness?

Answer 24

• Loss of area MT makes you motion blind
• The world perceived as a series of still photographs
• Loosing V1 makes you functionally blind but you can still respond to some stimuli - especially movement
• Loosing V3 would impeded your motion perception, but not destroy it
• Loosing MST/V6 would inhibit navigation, but not stop you seeing motion

“I have to look down the road and note where the cars are. I then wait a while and look again. If none have changed position then its safe to cross… but I prefer not to cross roads.”

Question 25

how can we adapt to faster movements?

Answer 25

We can manipulate the time delay to make it shorter so it is better suited for faster movements

Question 26

what is speed selectivity?

Answer 26

In the same manner cells are driven by specific directions, the cells in the above diagram are driven by specific speed of motion

Question 27

what is an example for the waterfall (notion-after) effect?

Answer 27

when staring at a moving image, and then asked to stare at a still image afterwards, it appears to be moving

Question 28

explain how the motion-after effect occurs?

Answer 28

Normally we'll have cells that respond to upwards motion and cells that respond to downwards motion Typically we combine the signals from all those cells to identify what motion we see So if nothing moves, then both of these cells, the upwards sensitive and the downward sensitive, will fire at the baseline But from out perspective, it will be stationary movement because the two cells cancel each other out When watching the waterfall going down, the downward cell will become very excited and signal to us that this substantial downward motion, and we will perceive that as downward motion As we continue watching this motion the cells will adapt to the downward motion, and after a long period of time they will reduce their activity They tend to reduce their activity so much that if we now remove the input, the cells will be inhibited, so they will fire below the rate At that point we will have an adaptive cell that fires below the baseline, and a non adapted cell that just fires at the baseline, and the combination of these two will now be in favour of the non adapted cell, so the cell that is sensitive to the other opposite motion direction, and that’s why one sense motion upwards

Question 29

explain motion after effect simply?

Answer 29

* Perceived motion is encoded by a population of cells (neural code) * For static stimuli all these cells respond at about the same - low - rate * After prolonged adaption to a given direction cells responsive to that direction will reduce their output - adaptation * The neural code for a subsequent static stimulus will be biased in favour of cells that normally respond to the opposite direction of movement

Question 30

what are the implication of perception of speed over time and space?

Answer 30

* 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 The peripheral is very useful for sensing a sudden change in movement, they will signal there is movement

Question 31

what is the principle of univariance?

Answer 31

The principle of univariance = cells cannot separate speed and contrast (or frequency and intensity) They can only produce a single output (the firing rate)

Question 32

describe the principle of univariance

Answer 32

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 33

does perceived speed reduce with reduced contrast?

Answer 33

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

Question 34

summarise speed

Answer 34

* Motion cells (at least in area V1) respond to both image contrast (difference between light and dark regions) and speed of stimulus * The principle of Univariance says that as individual cell can only have one output so it cannot signal two things: they will be confused * In colour vision a similar problem is resolved by comparing across many cells. Does this happen in the case of motion * It seems not, or at least not perfectly * For some stimuli, changes in contrast are confused with changes in perceived speed

Question 35

what is motion contrast in MT?

Answer 35

are there cells in the vision system that are motion contrast sensitive? cell responds to wide field of motion centre-surround cell for motion recap if a light shines on the centre, it excites the cell If a light shines on the surrounding, it inhibits the light question of whether we have similar cells that are sensitive to motion contrast cells that will for instance be driven by direction of motion in the centre, but will be inhibited by the same direction of motion with the surround but then maybe excited when the direction of motion is different

Question 36

what is the snow blind illusion?

Answer 36

once the blind were put on (horizontal lines) then some of those centre-surround cells, the surround will fall on those lines (and be inhibited) therefore the centre will be more excited because the surround does not include movement in the same direction as the centre and of course when this happens we perceive the motion as quicker

Question 37

explain induced motion and vection

Answer 37

Experiment when you put a person in the drum's centre, stationary just sitting down The cylinder has stripes, has texture on the cylinder and then starts turning When asking the participant whos moving? Is it them or the walls? Found that in the first 10 seconds, most people will say yes, it’s the walls turning After 10 or 20s this sensation completely reverses, now the person tells you that it's them moving rather than the walls (whilst being completely stationary) When we are moving there are vestibular cues to the movement, e.g. on a plane taking off, however when you’ve reached cruising altitude there's no more sensation of movement Because this is happening very gradually, we don’t have these vestibular cues because the change is gradual and small, this creates a sense of mismatch People can feel sick due to this, alludes to travel sickness

Question 38

summarise induced motion and vection

Answer 38

* 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 * Happens a lot on trains. Can think platform is moving when train is or that your train is moving when another train is pulling off * In drum experiment vection leads to cue conflict between visual and vestibular cues. Leads to sickness reflex (travel sickness)

Question 39

what is illusory motion?

Answer 39

* Illusory motion is the sensation of motion in a static stimulus (static meaning zero movement) * The motion after-effect is one kind of illusory motion * 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 40

what is the aperture problem?

Answer 40

* 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 edges * 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 41

what is the pinna illusion and how does it relate to the aperture problem?

Answer 41

* The diamond patterns are good stimuli for V1 cells: essentially they are oriented edges They are good due to the high contrast of the edges, the periphery translates this into 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

Question 42

summarise this whole lecture

Answer 42

* Motion after-effect – perceived speed reduces with time. * Motion contrast – alters speed perception. * Principle of Univariance reduced perceived speed in low contrast stimuli. * Large scale motion make you think you are moving (vection). * If MT cells are active we see illusory motion. They can be activated by static stimuli possibly by inappropriate excitation between cells sensitive to orientation and motion. * The aperture problem leads to errors of motion perception.