L19 - Action and Stability

Question 1

What happens when you have an after image?

Answer 1

• Image is glued onto the retina
• This image moves with your eyes and is not stationary
• But we still see the world as stationary even though eyes are continually moving

Question 2

Why do we move our eyes?

Answer 2

-Seeing: Images disappear if eye movements prevented
- Removal of image displacement, it disappears = requires dynamic change to function = called troxler fading
- Freezing: head and body movements cause lots of image motion, eye movements help to cancel out e.g if you were on a train = brain tries to fixate things causing following and flicking of eyes = cancels out unwanted image motion = useless and irritating
- Inspecting: keep objects of interest on fovea = very small and important, predators have foveas to find prey that have hidden

Question 3

What are the types of eye movements?

Answer 3

• Saccades: ballistic changes in fixations = sudden hopping of eyes
  Tracking motion: reflexive (freezing)
• Vestibulo-ocular reflex (VOR): gaze is kept stable by countering the eyes to the brain, depends on head movements and is controlled by the vestibular system and is fastest reflex in body (happens in the dark)
• Optokinetic nystagmus: in the light: VOR in balance and the scene in front of you to stop it moving = fixating = rotating eyes to look in the same place as head rotates = stimulated by large-field motion
  Tracking motion: intentional
• Smooth pursuit: object following

Question 4

What is motion like in image?

Answer 4

• When fixation: nothing changes but during pursuit, there are other things moving = eyes tend to focus and follow the moving thing = making the thing seem more stationary like recording a horse = smooth pursuit
• Both pursuit and saccades create changes to retinal image that the visual system must somehow compensate for
• Saccades show a lot of image motion
• Across saccades: there is a lot of fovea input that is constantly moving due to repeating change of image
• Brain has to reconstruct image in front us from dynamic input

Question 5

How do we perceive a stable world?

Answer 5

• All these motions are useless so you must suppress vision during a saccade = called Perisaccadic vision e.g cannot see your own eye movements in the mirror
• Inbetween Saccades: transaccadic integration = knowing where eyes are moving = integrating space with saccades

Question 6

What would Saccadic Supression look like?

Answer 6

• Time and visual sensitivity to things measured = visual sensitivity decreases in saccades but is higher in fixation
  Passive solution:
• Building motion mechanism for slow speeds, but motion moves fast in saccades, so system reacts the way it should
• This is true but only works for fine details
  STUDY:
• Use gratings: high spatial frequency gratings (thin lines and textured) or low, then you present them very fast to make it seem like a saccade
• The high frequency grey’s out and disappears but lower spatial frequencies you still see something
• Measured classic contrast sensitivity function: looking at if there is a change in sensitivity if there is movement of eyes
• Only need to suppress low spatial frequencies no need to suppress colour vision - not a good spatial sense (luminance has greater seeing)

Question 7

What is the overall active mechanisms for Saccadic Supression?

Answer 7

• Turn vision down when saccade occurs e.g by knowing when eye moved but clearly only parts of vision are suppressed via motor systems
• Masking: image after saccade overpowers image just before/during saccade

Question 8

What is Trans-saccadic integration?

Answer 8

• When you have different fixations due to saccades, the persons foot in the static scene has not moved although it seems like it has
• All simple cells in striate cortex are active = image-centred map
• Later visual cortex has a map that is spatiotopic
• Do this by knowing the movements from the motor system to visual system about eye position

Question 9

What is a study looking at trans-saccadic integration?

Answer 9

• Ppt makes saccade from something that is down and then up
• Each time you trigger that saccade, you have a flash that will appear before/during/after saccade
• Have a laser pointer to indicate perceived location
• When flash was 200ms beofre the saccade, there was little error in localisation
• Before the saccade, perceived location changes because motor system tells visual system it will change to where it will so you remap before the saccade
• Location misperceived around time of saccade so eye position signal used to create spatiotopic map starts just before eye movement and lasts sometime after