Eye movements (wk 9)

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

Describe the 4 types of eye movements:

Answer 2

• Fast -> Saccades – brings the area of interest onto the fovea. Resetting of eye position during VOR and OKR
• Slow -> Vestibular-ocular reflex (VOR) and Optokinetic reflex (OKR) – Maintains a stable image on the retina. (you cannot voluntarily create a slow eye movement)
  -Smooth pursuit -> tracks moving objects
  -Vergence -> point the eyes in the same direction

Question 3

What are saccades?

Answer 3

Observing a scene is characterised by alternating fixed and saccade pattern. Saccadic eye movements useful for revealing cognitive and motor processes

Question 4

Describe saccades?

Answer 4

• Ballistic – Up to 900 deg/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 supressed during a saccade – Try seeing your eyes move in the mirror; you can’t

Question 5

What are corrective saccades?

Answer 5

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

Question 6

What are dysmetric saccades?

Answer 6

The cerebellum is important in tuning the gain of saccadic eye movements. Dysmetric saccades cause visual problems in cerebellar patients.

Question 7

What is the vestibulo-ocular reflex (VOR)?

Answer 7

Have to move head to see images near you with your head. 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 (~15 ms from head to eye movement

Question 8

How do you test VOR function?

Answer 8

Rotation in darkness used to test VOR function. Produces 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. This equates to a ‘gain’ of 1. VOR gain may be less than 1 if the vestibular apparatus is damaged.

Question 9

What is VOR adaptation?

Answer 9

-> Different relationship between head and scene movement requires different VOR gain. Adaptation can be demonstrates experimentally with rotating chair and curtain. Also happens in ‘real life’ – if you get a stronger pair of glasses, you need to adapt your VOR. Cerebellar disease impairs VOR adaptation.

Question 10

What happens when the VOR goes wrong?
-vestibular loss, ageing, alcohol

Answer 10

• Vestibular loss -> e.g. viral infection, head injury, surgical intervention. Symptoms include loss of balance, disorientation and oscillopsia (blurring of the visual field)
• Ageing -> Vestibular hair cells are gradually lost with age
• Alcohol -> Alcohol nystagmus caused by changes in specific gravity of fluid in the canals – hence the spinning room sensation

Question 11

What is the velocity storage mechanism?

Answer 11

-> During continuous rotation, the vestibular signal decays 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 the optokinetic reflex?

Answer 12

-> OKN performs similar function to VOR, but visually driven. Better for low-frequency movements, whereas the VOR is adapted to high frequency.

Question 13

What is smooth pursuit?

Answer 13

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 the object disappears

Question 14

What is vergence?

Answer 14

Ability to direct eyes towards the same point. Disordered vergence may underlie some types of dyslexia.

Question 15

What is scleral coil?
-eye tracking technique

Answer 15

-> Contact lens with embedded wire coils. 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?
-Eye tracking technique

Answer 16

-> Beam of infrared reflected from cornea. 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 electro-oculography (EOG)
-Eye tracking technique

Answer 17

-> Retina produces measurable electric charge. Permanent potential difference between the cornea and retina. This sets up an electrical field in the surrounding tissue which 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). Can provide gaze information of head is stationary.

Question 18

What is video-oculography (VOG)?
-Eye tracking technique

Answer 18

-> Use software to track pupil and/or reflection. Simultaneous recording of the eye and scene allows point of gaze to be tracked.

Question 19

How does gaze anticipate movement and describe gaze position during locomotion:

Answer 19

-Gaze anticipates movement -> Eye movements tend to precede object manipulation by ~0.6s, moving on to next object ~0.6s before the action is completed.
-Gaze position during locomotion -> 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 tend to show more predictive sampling, at the expense of the current step (Young & Hollands, EBR, 2010)

Question 20

Describe cricket and the eye movements in sporting performance:

Answer 20

-Batsmen’s eye movements monitor the moment when the ball is released, make a predictive saccade to the place where they expect it to hit the ground, wait for it to bounce, and follow its trajectory for 100-200ms after the bounce.
-The downward saccade is earlier in better batsmen and also better predicts the type of ball delivery.
-Cricket -> Visual and spatial occlusion studies show that additional information is gained before the ball leaves bowler’s hand. Information from the hand/ arm is very important.

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.