Lecture 5 - oculomotor systems Flashcards
1
Q
Eye movements
A
- Most common movement
- Important sensory and behavioural consequences:
- -> Diplopia (double vision)
- -> Drift (nystagmus)
- -> Vital for reading (may be contributor to dyslexia)
2
Q
Function of oculomotor systems
A
- To move eye across visual scene
- Allow interesting parts of image to fall onto high resolution part of retina (fovea)
- To converge eyes at different distance = when looking at something in front of room eyes converge and for back of room they diverge
- To stabilise the visual image on the retina
3
Q
Photoreceptor density
A
-Fovea has more photoreceptors than rest of retina = gives us central point of focus, high resolution and periphery in low resolution
-Daylight = only central fovea sees in fine detail and colour
-Night = only peripheral retina works, black and white, poor resolution
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4
Q
Gaze paths
A
- Spatial paths of eye as you move across an image
- Longer fixation times as try to process in detail
- Short words get skipped
- Longer words have more gaze points
- Saccades = jump between words
5
Q
Eye movements can be context dependent
A
- Experiment where picture showed father returning to family = ambiguous
- Eye movement made dependent on what questions subjects were asked
- If allowed to freely examine scene = looked at high contract points and faces of adult
- If asked to estimate age of adults = looked at individuals and ignored background
- If asked to estimate material circumstances of family = started scanning room rather than people
6
Q
Ocular muscles
A
- 6 muscles connected to each eye
- Medial and lateral rectus muscles = pull the eye towards the midline (nose) or away from midline towards ear = gives ability to look right and left
- Superior and inferior muscles = superior and inferior rectus = rotate eye upwards or downwards = do this by contact one of the muscles
- Superior and inferior oblique muscles = provide degree of torsion = allow eye to rotate in socket = provides flexibility for small adjustments
- Extra-ocular muscles innervated by specific cranial nerves
7
Q
Classification by direction of eye movement
A
- Ductions (adduction or abduction) = movement towards or away from midline, can involve either eye
- Version = both eyes moving in same direction
- Vergences = both eye moving in opposite directions
- Most gaze shifts involve version and mergence
8
Q
Functional types of eye movement
A
Gaze stabilising mechanisms (old systems):
- Make sure light entering eye gives stable image
- Optokinetic reflex (OKR) = driven by visual motion
- Vestibular-ocular reflex (VOR) = driven by head motion system
Gaze shifting mechanisms (new system):
- Only present if have fovea
- Saccades movements
- Smooth pursuit
- Vergence movement
Gaze fixation:
-Eye must hold stationary between movements = active process required to keep them still
9
Q
Gaze stabilisation: OptoKinetic Reflex
A
- Maintains gaze position
- Driven by whole field visual motion
- Assumes world is stationer
- Minimises visual slip of image on retina
- Takes in visual info and then tries to counteract any movement in scene
- Slow and incomplete since driven by error signal (negative feedback)
- BUT slow mechanism as needs you to process visual scene in order to react to it
10
Q
Nystagmus
A
- Eyes can only rotate with scene so far = will hit point where they need to re set = get tracking motion with saccade at the end = pattern called nystagmus
- The alternation of slow drift of the eyes and rapid saccades
- Normal when world drifts past you eyes e.g. looking out on train window
- Abnormal when caused by lesions
- Relatively slow process
11
Q
Vestibular Ocular Reflex
A
- Relies on input from inner ear and movement from head
- Fast = 14 ms (as only involves 3 neurons)
- Accurate = 90% head velocity can be counter-acted
- Maintains gaze position despite head movement
- Head velocity detected by semi-circular canals = info about head movement from inner ear
- 3 neurons relay to ocular motor neurons (OMN’s) to drive extracular muscles = all neurons within nuclei in brain stem
- When semi circular canals detect rotation of head –> sends signal to ocular motor neurones = can track muscles = causes eye to rotate in opposite direction
- At end of movement drive disappears to muscles return to natural length
12
Q
Vestibular ocular reflex: pathways
A
Direct pathway:
- From vestibular nucleus to the ocularmotor neuron
- By itself isn’t enough to make movement = why indirect needed
Indirect pathway:
- Same as direct but goes through another nucleus
- For memory of movement and maintains activation to keep visual scene static
- The indirect pathway through the nucleus PPH converts the phasic vestibular input into a tonic signal via a reverberating neural circuit
13
Q
Gaze shifting: smooth pursuit
A
- When you track object
- Slow simultaneous movement of both eyes in the same direction
- Evolutionarily new system to allow tracking of moving objects against stationary backgrounds
- Required suppression of optokinetic reflex (OKR)
- Limited to low velocities
Its a slow visual feedback mechanisms:
- Only possible in presence of visual moving target
- Driven by visual motion signals from MT to MST
14
Q
Gaze shifting: saccades
A
- Fast, simultaneous movements of both eyes in same direction
- Rapid gaze shifts
- Constant velocity
- Can reach 600 degrees
- During movement visual scene unavailable
- Can be involuntary
15
Q
Saccades
A
- 2-3 per second
- Feedback control uses signals from senses in muscles to control movement
- Feed-forward control (ballistic/open loop control) = when visual system detects error and causes a corrective saccade
