Motion + Depth

Question 1

Optic flow

Answer 1

• The thing looking right at doesn’t move at all
• Things on edges do move
• Thing your heading towards doesn’t move
• Motion used to identify prey/predator
• Cattlefish plays pattern on skin thats opposite as it approaches predator, prey doesn’t move coz thinks its moving away but then gets eaten

Question 2

Uses for motion perception

Answer 2

• Breaking/camoflague (object segmentation) –> if things don’t move very hard to see edge but minute it moves you can see its edges
• Detecting things coming towards us
• Navigation through world - so don’t bump into things
• Interacting with world - catching, intercepting

Question 3

How can brain detect motion?

Answer 3

• Barlow + Levick (1965)
• 2 neurons lead onto single neuron and is only excited if occurs at same time
• 2 receptive fields
• Detect rightwards motion because there is a delay of firing from lefrt eye as item hasn’t appeared in RF yet

Question 4

Sutherland (1961) - Up + down motion

Answer 4

• Detect difference between 2 things
• Resulting motion is difference between up and down movement
• Look at something moving up you drive up motion system + this gets tired so it can’t signal at all and neuron becomes fatugued
• Afterwards the downwards system is firing more so see downward movement
• Opponent mechanism

Question 5

Motion after effects

Answer 5

• If you look at a moving thing for a long time your motion neurons adapt (gain control)
• If you look at something stationary it seems to move
• We have to model ‘resulting motion’ to explain this as being the opponent process
• 2 sets of neurons are pulling against each other
• When one weakens the other wins

Question 6

How does brain process emotion?

Answer 6

• Eye-specific organisation disappears in superficial layers of V1
• Send signals to segregated regions so if there is cross-adaptation
• Regions receive info from left + right eye
• Cross-adaptation suggests adpatiaion happens across cortex
• Some evidence there is motion detecting ganglion cells in retina

Question 7

Barlow + Hill (1963) - After effects of motion

Answer 7

• May result from temporary imbalance of maintained discharges of cortical cells responsive to opposite directions

Question 8

Compensating for eye movement

Answer 8

• Somehow brain compensates for big movement of eye when you are looking around
• When you move your eyes the world stays still

Question 9

Compensating for eye movement - Outflow theory

Answer 9

• Helmholtz
• Efferent signal moves eye muscle
• Signal copies to comparator
• Comparator subtracts efferent copy signal from retinal movement and compensates for it (ignores motion of eyes)
• Brain sends 2 signals when it tells eye to move: 1 to eye muscle, 1 to visual cortex
• Visual cortex then knows aboht eye movement and cancels them out somehow

Question 10

Compensating for eye movement - Inflow theory

Answer 10

• Sherrington
• Efferent signal moves eye muscle
• Eye muscle movement signal to comparator
• Comparator subjtracts eye muscle movement signal from retinal movement
• Brain sends 1 signal to eye muscle
• When contracts, moves eye + sends signal to visual cortex
• 2 signals cancel out

Question 11

Testing inflow outflow theories

Answer 11

• Helmoltz outflow theory is overall winner
• Test 1: Move eyes over stationary space - Both believe efferent copy is cancelled by retinal movement = both correct
• Test 2: Observe moving objects with stationary eyes - both say that no signal is sent to cancel the movement = both correct
• Test 3: Afterimage - no cancelling signal = both correct
• Test 4: move eye by poking with finger - Retinal movement not cancelled - Helholtz correct
• Test 5: Afterimage in total darkness - no retinal movement to cancel efferent copy - Helmholtz correct
• Test 6: Prevent eye from moving by putty or curare - efferent copy sent but no movement from retina to cance it = Helmholtz correct

Question 12

How do we process motion

Answer 12

• Motion signals get sent to MST quickly + Integrates dots of motion to one big motion signal
• Areas in cortex specialised for motion: MT, MST

Question 13

Motion Blindness

Answer 13

• Damage in MT areas
• See motion as stills
• Can’t tell when to stop filling cup
• Can’t read facial expressions
• Moving objects don’t attract patients attention
• Stroke on MT region

Question 14

Motion + consciousness

Answer 14

• Correlation isn’t causation
• Single neuron activity in MT correlates with perfoance

Question 15

Motion summary

Answer 15

• Motion extracted from simple neuronal model
• Although neuronal representations may be more complex
• Motion extracted in cortex in primates
• Opponent mechanism can give rise to motion after effect
• Eye movements are nulles via an efferent copy mechanism
• Cortical areas MT is necessary for motion perception + neurons accessed by conscious perception

Question 16

Depth

Answer 16

• Measure of how far away something is
• Lots of monocular cues to depth
• Things far away are: smaller, occluded, blueish, move slower, different focal plane

Question 17

Uncrossed disparity

Answer 17

• Objects positioned further away from us than the horopter have uncrossed disparity
• Representation on the retinas are closer together than the foveas

Question 18

Crossed disparity

Answer 18

• Objects positioned closer than the horopter have crossed disparity
• Their representations on the retina are fartehr apart than the foveas

Question 19

Stereopsis

Answer 19

• Disparity based on placement on retina is known as stereopsis
• Very good within certain range
• At 1 metre distance, we can tell which of 2 objects is 1mm closer to us
• Too close = double vision/diplopia
• Too far = disparities become too small

Question 20

The correspondence problem

Answer 20

• Have to match the disparities in both eyes
• One way to solve is sot identify the object first using outline
• Match later in visaul system, matching corresponding objects

Question 21

Random dot stereograms

Answer 21

• Ask people to go crosseyed when presenting 2 similar stimuli of group of dots with few dots out of place
• Dots not in same place for both eyes pop out when cross eyed
• Solve correspondence problem for hundreds of random dots at the same time
• Impressive example of parallele processing in cortex
• Brain matches everything to understand if there is a disparity

Question 22

Stereo vision in V1

Answer 22

• To compute stereo depth you need to compute disparity
• Need cells that have input from both eyes
• Riechardt detector tells you when 2 inputs aren’t lined up in time –> motion
• Disparity selective cells tell us when 2 things aren’t lined up between eyes –> depth

Question 23

Ocular dominance columns

Answer 23

• In centre of ocular dominance columns, cells are monocular
• Not all cells are in centre some are binocolular
• Some binocular cells have receptive fields on non-corresponding points on 2 retinas

Question 24

Disparity selective cells on visual cortex

Answer 24

• Binocular cells have receptive field in each eye
• Come with range of different seperations between receptive field centres
• Cells are selective for different disaprities
• Signal relative depth

Question 25

Amblyopia

Answer 25

- Born with - 1 eye is lazy - If don't catch early on then the brain just won't respond to the lazy eye anymore - Eye shuts down - Typically put patch over good eye to force use of lazy eye - Biggest source of visual loss in under 15s - Causes issues with depth perception - Columns for bad eye start to wither + die if not patched - Neurons responding to good eye compete with bad eye unless treated theres no way to get neurons back for bad eye

Question 26

Depth perception generally

Answer 26

- Want to know how neurons respond to loss of input - Important for sensory loss (blindness, deaf) - Also for things like stroke, trauma - Can we make brain go back to normal state?