Eye Movement

Question 1

why do we move our eyes

Answer 1

• to bring points of interest over the fovea
• to prevent blurring of the visual scene

Question 2

what are the types of eye movement

Answer 2

• fast
  
  • saccades - brings the area of interest onto the fovea
  • resetting of eye position during VOR and OKR
• slow
  
  • vestibulo-ocular reflex (VOR)
  • optokinetic reflex (OKR)
• smooth pursuit - tracks moving objects
• vergence - point the eyes in the same direction

Question 3

what are saccades

Answer 3

• ballistic
  
  • up to 900deg/s
  • last 40-200ms (too fast for sensory feedback)
• roughly 3 saccades per second
  
  • more frequent than heart beats
  • ~10% of waking hours spent making saccades
• two types
  
  • reflex (stimulus driven)
  • voluntary (no stimulus necessary)
• vision actively suppressed during a saccade
  
  • try seeing your eyes move in the mirror; you can’t
• observing a scene is characterised by alternating fixate and saccade pattern
• fixations last roughly 300ms
• saccadic eye movements useful for revealing cognitive and motor processes

Question 4

what are corrective saccades

Answer 4

normal saccadic eye movements often characterised by slight undershoot followed by a corrective saccade

Question 5

what are dysmetric saccades

Answer 5

• the cerebellum is important in tuning the gain of saccadic eye movements
• dysmetric saccades cause visual problems in cerebellar patients

Question 6

what is the vestibulo-ocular reflex (VOR)

Answer 6

• 3 semicircular canals - detect head rotation
• 2 otolith organs - detect tilt (gravity) and linear acceleration
• rotates the eyes to compensate for head movement
• helps to stabilise the visual image
• often need to suppress VOR
• basic brainstem circuit with only 3 neurons
• therefore very fast (~15ms from head to eye movement)

Question 7

how do you test for VOR function

Answer 7

• rotation in darkness used to test VOR function
• provides an alternating pattern of fast and slow eye movements called Nystagmus
• quick phase (saccades) resets the position of the eye in the head
• if the VOR is working perfectly, the slow phase eye rotation and head rotation should cancel out
  
  • thus equates to a ‘gain’ of 1
• VOR gain may be less than 1 if the vestibular apparatus is damaged

Question 8

how is VOR adapted

Answer 8

• different relationship between head and scene movement requires different VOR again
• adaptation can be demonstrated experimentally with rotating chair and curtain
• also happens in ‘real life’ - if you get a stronger pair of glasses, you need to adapt to your VOR
• cerebellar disease impairs VOR adaptation

Question 9

what happens when VOR goes wrong

Answer 9

• anything which affects vestibular function can compromise the VOR
  
  • vestibular loss: e.g. viral infection, head injury, surgical intervention. symptoms include loss of balance, disorientation and oscillopsia (blurring of visual field)
  • ageing: vestibular hair cells are gradually lost with age
  • alcohol: alcohol nystagmus caused by changes in specific gravity of the fluid in the canals - hence the spinning room sensation

Question 10

what is the optokinetic reflex

Answer 10

• OKN performs similar function to VOR but visually driven
• better for low frequency movements, whereas the VOR is adapted to high frequency

Question 11

what is velocity storage mechanism

Answer 11

• during continuous rotation, the vestibular signal delays away much earlier than the eye movement (6s versus 15s time constant)
• the brainstem has a ‘velocity storage’ mechanism to prolong gaze stabilisation
• however, during prolonged rotation this signal ultimately fails - this is when the optokinetic reflex takes over (assuming vision is available)

Question 12

what is smooth pursuit

Answer 12

• ability to track a moving object with the eye
• pursuit must involve prediction - visual feedback is too slow
• hence the brain must predict the future flight of the object
• eyes continue moving after object disappears

Question 13

what is vergence

Answer 13

• ability to direct eyes toward the same point
• disordered vergence may underlive some types of dyslexia

Question 14

what are the methods of eye tracking

Answer 14

• scleral oil
  
  • contact lens with embedded wire coils
• infrared reflectance
  
  • beam of infrared reflected from cornea
• electro-oculography (EOG)
  
  • retina produces measurable electric charge
• video-oculography (VOG)
  
  • use software to track pupil and / or reflection

Question 15

what is the scleral coil

Answer 15

• very accurate eye position in all 3 axes - yaw, pitch and roll
• however, very uncomfortable and invasive (wire can scratch cornea)

Question 16

what is infrared reflectance

Answer 16

• as the eye rotates the beam is reflected in a different direction
• this change in position is detected by the IR detector
• can be used in the dark

Question 17

what is electrooculography (EOG)

Answer 17

• permanent potential difference between the cornea and retina
• this sets up an electrical field in the surrounding tissue which we can measure
• as the eye rotates, the voltage between each pair of electrodes changes
• can measure horizontal and vertical eye movement
• commonly used in neurology clinics to test VOR function (cheap, easy and reliable)
• works with the eyes closed
• can provide gaze information, if head is stationary

Question 18

what is video oculography and gaze tracking

Answer 18

simultaneous recording of the eye and scene allows point of gaze to be tracked

Question 19

how does gaze anticipate movements

Answer 19

eye movements tend to precede object manipulation by ~0.6s, moving on to the next object ~0.6s before the action is completed

Question 20

how does gaze position affected during locomotion

Answer 20

• when walking in difficult terrain, gaze typically stays 2 steps ahead
• roughly 2 fixations per step
• hence, foot placement is generally planned around 2 steps in advance
• older adults end to show more predictive sampling, at the expense of the current step

Question 21

what is action observation

Answer 21

• when observing movement, gaze is predictive not reactive. mimics the gaze pattern observed when actually moving
• supports the hypothesis that action understanding is based on a direct matching mechanism that maps the visual representation of the observed action onto a motor representation of the same action