eye movements

Question 1

what are the 2 main reasons we move our eyes?

Answer 1

1. brings point of interest over fovea (foveate on this part of retina)
2. prevents blurring of visual scene (fixation)

Question 2

what are saccades?

Answer 2

• fast type of eye movement bringing area of interest to fovea

Question 3

what are the 4 slow types of eye movements?

Answer 3

1. vestibulo-ocular reflex (vor)
2. optokinetic reflex (okr)
3. smooth pursuit
4. vergence

Question 4

explain vestibulo-ocular reflex

Answer 4

• when you move your head

Question 5

explain OKR

Answer 5

• when you maintain a stable image on retina
• visually driven
• VOR high frwquency
• slow automated movement

Question 6

explain smooth pursuit

Answer 6

• tracks moving objects
• prediction as visual fields too slow
• Catch up saccade
• brain must predict future flight of object
• eyes tend to continue after objects disappears for 100ms

Question 7

explain vergence

Answer 7

• point eyes in same direction (close and divergence is moving out
• directs eyes forward towards same point
• disordered mergence may underlie dyslexia types

Question 8

characteristics of saccades

Answer 8

• ballistic (40-200ms)
• 2 saccades per sec
• 2 types: reflex and voluntary
• vision is actively suppressed during a saccade

Question 9

do normal saccades usually overshoot or undershoot

Answer 9

• undershoot followed by a corrective saccade

Question 10

how does the cerebellum affect saccades

Answer 10

• tuning gain of saccadic eye movements
• visual problems in cerebellar = dysmetric saccades
• hypermetric and hypometric (make small saccades to make target)

Question 11

how does the VOR work?

Answer 11

• 3 semi-circle canals detecting head rotation
• 2 otolith organs detecting wild and linear acceleration
• eye rotate compensating head movement
• suppresses vor
• constantly active
• only involves 3 neurons

Question 12

how do you test VOR function?

Answer 12

• rotation in darkness to produce patterns of fast and slow eye movements called nystagmus
• if vor is perfect the slow phase head and eye rotation should cancel out

Question 13

3 ways the VOR can go wrong

Answer 13

1. vestibular loss - due to infection, head injury and it causes loss of balance, disorientation and oscillopsia
2. ageing.- hair cells in vestibular system are gradually lost with age
3. alcohol - nystagmus caused by changes in gravity of build in canals. alcohol affects vestibular system throwing off balance liquid levels in ear causing dizziness

Question 14

what is a velocity storage mechanism

Answer 14

• vestibular signal decays earlier than eye movement so brainstem has a velocity storage mechanism to prolong gaze stabilisation
  if signal fails optokinetic reflex takes over

Question 15

4 methods of eye tracking

Answer 15

1. scleral coil
2. infrared reflectance
3. EOG
4. VOG

Question 16

scleral coil

Answer 16

• contact lense with wire coils
• very accurate but uncomfortable

Question 17

infrared reflectance

Answer 17

• beam of infrared reflected from cornea
• change in position detected by IR detector

Question 18

EOG

Answer 18

• electro-oculogrpahy
• retina produces measurable electric charge
• permanent potential difference between cornea and retina
• sets up electrical fields surrounding tissue to measure
• eye rotates and voltage between pair of electrodes changes

Question 19

VOG

Answer 19

• video-oculography
• use software to track pupil and reflection
• recording eye and scene simultaneously and allows point of gaze to be tracked