Eye Movement Flashcards
1
Q
why do we move our eyes
A
- to bring points of interest over the fovea
- to prevent blurring of the visual scene
2
Q
what are the types of eye movement
A
- 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
3
Q
what are saccades
A
- 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
4
Q
what are corrective saccades
A
normal saccadic eye movements often characterised by slight undershoot followed by a corrective saccade
5
Q
what are dysmetric saccades
A
- the cerebellum is important in tuning the gain of saccadic eye movements
- dysmetric saccades cause visual problems in cerebellar patients
6
Q
what is the vestibulo-ocular reflex (VOR)
A
- 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)
7
Q
how do you test for VOR function
A
- 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
8
Q
how is VOR adapted
A
- 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
9
Q
what happens when VOR goes wrong
A
- 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
10
Q
what is the optokinetic reflex
A
- OKN performs similar function to VOR but visually driven
- better for low frequency movements, whereas the VOR is adapted to high frequency
11
Q
what is velocity storage mechanism
A
- 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)
12
Q
what is smooth pursuit
A
- 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
13
Q
what is vergence
A
- ability to direct eyes toward the same point
- disordered vergence may underlive some types of dyslexia
14
Q
what are the methods of eye tracking
A
- 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
15
Q
what is the scleral coil
A
- very accurate eye position in all 3 axes - yaw, pitch and roll
- however, very uncomfortable and invasive (wire can scratch cornea)
16
Q
what is infrared reflectance
A
- 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
17
Q
what is electrooculography (EOG)
A
- 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
18
Q
what is video oculography and gaze tracking
A
simultaneous recording of the eye and scene allows point of gaze to be tracked
19
Q
how does gaze anticipate movements
A
eye movements tend to precede object manipulation by ~0.6s, moving on to the next object ~0.6s before the action is completed
20
Q
how does gaze position affected during locomotion
A
- 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
21
Q
what is action observation
A
- 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
