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Gaze shifting & gaze holding ocular motor functions
1. Gaze shifting & Gaze
holding Ocular motor
functions
Gauri S. Shrestha, M.optom, FIACLE
2. What do Eye Movements Do?
Keep visual images relatively stable on the retina
Change the angle of gaze
Prevent the fading of retinal images
Gauri S. Shrestha, M.optom
3. Keeping Visual Images Stable
Prevents retinal slip which leads to motion smear
5 deg/sec can cause motion smear
Example: head movement
Motion Smear causes blur and the inability to accurately
judge object location in space
Eye movements used to prevent motion smear
Vestibular Ocular Reflex (VOR)
Optokinetic Nystagmus (OKN)
Gauri S. Shrestha, M.optom
4. Change the Angle of Gaze
Align fovea with the object of interest
Eye movements used to change gaze angle
Saccades
Pursuits
Vergence
Gauri S. Shrestha, M.optom
5. Prevent Image Fading
Perfectly stabilized images fade
Troxler’s Effect
Eye Movements used to prevent image fade
Drifts
Tremors
Microsaccades
Miniature ocular movements that constantly occur
Gauri S. Shrestha, M.optom
6. Eye Movement Development
From birth and continues to develop
VOR, OKN
From birth and continues to develop
Saccadic movements
From 6-8 weeks and continues to develop
Pursuit movements
From 3 months and continues to develop
Vergence, accommodation
BINOCULAR VISION
Gauri S. Shrestha, M.optom
7. How does the eye move?
The eye is tightly packed in the orbit
2 forces must be overcome
Viscous drag of the orbit
Elastic restoring forces of the orbital tissues
(primary resting position)
2 types of neural activity required to overcome
these forces
Gauri S. Shrestha, M.optom
8. Neural Activity Required
Velocity (pulse) Signal
Phasic increase in neural activity allowing the
EOM to move quickly
Overcomes the viscous drag of the orbit
Position (step) Signal
Sustained increase in neural activity allowing the
eye to maintain the eye at the new position
Overcomes the elastic restoring forces of the
orbital tissues
Gauri S. Shrestha, M.optom
9. Velocity and Position Signals
Without pulse (velocity), eye movement is slow
Without step (position), eccentric eye position
cannot be maintained
Pulse and step signals must match for accurate eye
movements and steady fixation
How do we achieve this?
Gauri S. Shrestha, M.optom
10. Neural Integration
The brain converts the pulse (velocity) signal into
the step (position) signal
What happens if the system fails?
Eye is carried to new position, but drifts back to the
central position
Causes nystagmus (jerky eye movements)
Gauri S. Shrestha, M.optom
11. Motor Plasticity
Eye movements can undergo adaptation changes
When a patient receives new glasses
Larger movements needed for plus lenses
Cerebellum has important role in plasticity
Flocculus and nodulus
Dorsal vermis
Gauri S. Shrestha, M.optom
13. What is Needed for Accurate Eye Movements
The brain needs to know where the eye is located with
respect to the head and orbit
THE AFFERENT SYSTEM
The brain needs to know where and by how much to move
the eye
THE EFFERENT SYSTEM
Clinical Application: Disorders can cause the inability to
accurately judge visual space
Inaccurate ‘positional sense’ in targeting task
Gauri S. Shrestha, M.optom
14. The Afferent System
Provides information about eye position to the
brain
How is this achieved?
Two sources (subsystems)
Proprioception
Efference copy/Corollary discharge
Gauri S. Shrestha, M.optom
15. Afferent System-Proprioception
Includes Muscle Spindles in the human EOM that
respond to stretch
Includes the Palisade Tendon Organ in the human
EOM that responds to tension
These signals are sent to the brain through the
trigeminal nerve
Gauri S. Shrestha, M.optom
16. The Afferent System-Efference Copy
AKA: Corollary Discharge
This is a ‘copy’ of the motor signal to move the
eye that is sent back to the brain
Anatomical origin: unknown
Also involved in distinguishing between real
motion or self motion
Gauri S. Shrestha, M.optom
17. The Efferent System
Provides brain with information about how much
and where to move the eye
Neural information from this system provides
innervation to the ocular motor nuclei to move
the EOM
Gauri S. Shrestha, M.optom
18. Innervation of the EOMs
Oculomotor (Cranial Nerve 3)
MR, SR, IR, IO
Trochlear (Cranial Nerve 4)
SO
Abducens (Cranial Nerve 6)
LR
Gauri S. Shrestha, M.optom
19. Review of these systems
Afferent System
Proprioception
Corollary Discharge or Efference Copy
Efferent System
Sends pulse and step neural signals to the EOM
Gauri S. Shrestha, M.optom
20. The EOM Fibers
Two Major Function
Move the eyes (quickly or slowly)
Change eye position
Keep the eyes relatively stationary
Maintain new eye position
Gauri S. Shrestha, M.optom
21. Physiological Types of Fibers
Twitch Fibers (burst)
All or none action potential
Non-Twitch Fibers (tonic)
Graded contractions
Few fibers are a combination
Gauri S. Shrestha, M.optom
22. Twitch Fibers (Burst)
All or None response
Fast-fatiguing fibers
Known as global fibers
Fibers that are closer to the eyeball
Good for rapidly moving the eye to a new position
Gauri S. Shrestha, M.optom
23. Non-Twitch Fibers (step)
Receive the step signal
Graded contraction
High oxidative capacity
Known as orbital fibers
Fibers that are closer to the orbit
Good for maintaining the new eye position
Gauri S. Shrestha, M.optom
26. Actions of EOMs
From Primary Position
Primary Secondary Tertiary
MR Adduction
LR Abduction
IR Depression Extorsion Adduction
SR Elevation Intorsion Adduction
IO Extortion Elevation Abduction
SO Intorsion Depression Abduction
Gauri S. Shrestha, M.optom
27. Five movement subsystems:
1. Saccadic systems
2. Pursuit systems
3. Vestibulo-ocular reflex
4. Optokinetic reflex
5. Vergence
Gauri S. Shrestha, M.optom
28. Saccadic systems
Saccade named after the French
word describing the rapid turning of
a horse's head
Saccades are very fast yoked eye
movements that have a variety of
function
Speed of saccade – 400-700/s
Saccades produce the quick phase
of vestibular & OKN to avoid
turning the eyes to their mechanical
limitations
Saccades also occur withS.head M.optom
Gauri Shrestha,
movements.
29. Saccadic systems
∀ Undershoot or overshoot during saccades is corrected by
micro saccades or glissade
∀ Saccadic waltz (pulse-slide-step) called glissade (pulse) &
tonic (step) innervations of the saccade
∀ Saccade in neonates is inaccurate . Developed by 1 yr.
∀ Saccade respond very quickly because of their burst or
pulse innervations
∀ Saccadic system involves pulse step mechanism.
∀ Burst of electrical activity is required to move the eye to
the desired position-pulse
Gauri S. Shrestha, M.optom
30. Saccadic systems
∀ After pulse, further energy required to maintain the eye in
desired position & counter elastic recoil –step
∀ Speed of saccade is greatest midway between 1/3 &
halfway of saccade movement -max. Velocity peak (MVP)
∀ Larger the saccade greater the MVP
Gauri S. Shrestha, M.optom
31. Pulse –Step theory
∀ Pulse –step theory is due to 3 groups of neuron.
∀ Burst neuron
∀ Cause rapid electrical discharge with rapid acceleration
medium
∀ At the end stage of the saccade inhibitory burst neuron
stimulate antagonist muscle to stop the movement.
Pause neuron
∀ Inhibit firing of burst neuron until initiation of the saccade
∀ Its activity is suspended immediately after the start of the
saccade
Gauri S. Shrestha, M.optom
33. Pulse –Step theory
Tonic neuron
∀ Responsible for maintaining muscle tone
∀ Its activity increase after saccade to maintain gaze
position represents the step
Gauri S. Shrestha, M.optom
34. Pulse –Step theory
∀ Saccade are regulated by Neural integraters (NI).
NI converts velocity command into appropriate
position command – step (pulse step mechanism)
i.e. pulse is integrated to produce steps.
∀ The pulse innervation produced by the burst cell,
controls the velocity of the saccade and the step of
innervation produced by tonic cell, controls the
final position of the eye upon completion of the
saccade.
Gauri S. Shrestha, M.optom
35. Anatomical pathways
Frontal eye field (frontal cortex,
area 8)-->
ant. Limb of internal capsule-- >
decussate in lower midbrain-- >
synapse at horizontal gaze center
(PPRF-Paramedian Pontine
Reticular Formation) -- >
CN VI nucleus-->
motor neurons to LR and
Interneurons to MR
subnucleus(CNIII)
Gauri S. Shrestha, M.optom
36. Anatomical pathways
Motor neurons of MR i.e. right
frontal cortex initiates saccades to
left
Also superior colliculus initiate
the contralateral saccade in
response to novel visual stimuli
Cerebellum also plays a major role
in controlling saccadic pulse size
and so aids in co-ordinated eye
movements.
Gauri S. Shrestha, M.optom
37. Smooth pursuit
Tracking or following movement
∀ Much slower than saccadic with maximum speed
at 400/s, if speed higher than that??
∀ Latency- 100- 125msec
∀ Slow eye movements are also generated by
vestibules
In neonates tracking usually accompanied by
series of saccades. Full development by 3-4
months of age
Gauri S. Shrestha, M.optom
38. Smooth pursuit
Functions
∀ Cancellation of VOR during head tracking foveas
∀ Cancellation of OKN during fixation and tracking
∀ Foveate moving isolate targets (stabilizes moving objects
on retina when background.
Gauri S. Shrestha, M.optom
39. Smooth pursuit
Control of pursuit
∀ Parietal cortex (medial,
temporal..)
∀ MT, MST
∀ Horizontal pursuit is initiated by
ipsilateral occipito-paritetal
cortex--> PPRF--> motor
neurons of LR & MR i.e. left
occipital cortex is responsible for
left pursuit
∀ Vertical pursuit originate in
occipito parietal region,
interstitial nucleus of cajal
(INC)--> CN IV, III Gauri S. Shrestha, M.optom
40. Vestibulo-ocular reflex (VOR)
∀ The first class of stabilizing eye movement that
components for brief head and body rotation
∀ Generates slow eye movements in response to head
movement maintaining steady eye position
∀ Semicircular canals of the vestibulo labyrinth
signals how fast the head is moving and oculomotor
system responds to this signal by rotating the eyes
in an equal and opposite velocity.
Gauri S. Shrestha, M.optom
41. Vestibulo-ocular reflex (VOR)
∀ Stabilizes the eyes relative to the external world and
keep visual images fixed on the retina
∀ Works in total darkness responds to acceleration and
deceleration but not to constant velocity
∀ Control initial image stabilization
∀ Otoliths (saccula) compensates for head tilt movement that
cause torsional eye movement
∀ Proprio-receptors in neck muscles also contribute towards
VOR
Gauri S. Shrestha, M.optom
42. Vestibulo-ocular reflex (VOR)
∀ Can be tested with Doll head movement and
inducing vestibular nystagmus using swinging
baby test or caloric test
∀ Caloric test --> Pt head inclined at 600 so that
Horizontal Semicircular canals lies vertically-->
∀ COWS (Cold water – fast phase of nystagmus
towards opposite labyrinth, warm water – fast
phase towards same labyrinths)
Gauri S. Shrestha, M.optom
44. Optokinetic Reflex (OKN)
∀ 2nd ocular stabilization system that responds to
currents of image motion
∀ Also referred as railway / parade nystagmus
∀ OKN supplements VOR in several ways
∀ OKN responds to constant velocity
∀ Both OKN & VOR exhibit jerk nystagmus
(following movement and then saccade)
Gauri S. Shrestha, M.optom
45. Optokinetic Reflex (OKN)
∀ Active OKN or pursuing, the phase follows target
towards the periphery away from primary gaze
∀ Passive OKN or starring, the fast phase (saccade)
to where the target is emerging from and then has
a slow phase back to primary gaze.
∀ OKN is responsible to large fields 20- 500
∀ Max. Velocity rarely exceeds 500/sec usually in
close to the stimulus velocity below 300/sec
Gauri S. Shrestha, M.optom
46. Optokinetic Reflex (OKN)
∀ Can be demonstrated with OKN drum, look at the
stripes, pursuit and saccades occurs --> refers
OKN
∀ Slow target velocities --> good correspondence
∀ >30-1000/s velocities- eye movement lags behind
target movement
∀ Beyond 1000/s – OKN can't be demonstrated
Gauri S. Shrestha, M.optom
47. Optokinetic Reflex (OKN)
∀ OKN development in an infant from the sub-
cortical crossed input—stimulates a nasal ward
slow phase of OKN (Both eyes move smoothly
towards covered eye)
∀ After 3 months, infants cortical projection
predominate and horizontal OKN responds to both
temporal ward and nasal ward image motion.
∀ Vertical OKN can also tested
Gauri S. Shrestha, M.optom
48. Vergence
∀ Disjunctive or Vergence eye movements are movements of
the eyes in opposite direction
∀ They can be horizontal , vertical and cyclo
∀ Vergence doesn't appear in animals with laterally placed
eyes
∀ Units of measurement- degrees, prism Diopters, meter
angle
∀ Maddox classification of Vergence- tonic, proximal,
disparity & accommodative
∀ Stimuli- retinal disparity, diplopia, accommodation,
convergence
Gauri S. Shrestha, M.optom
50. Vergence
∀ Development – in full term from neonates but un-
coordinated- accurate convergence developed by 2-3
months of age
∀ Slower than saccades
∀ Supra nuclear control of Vergence- unclear- frontal eye
fields and occipital regions produces convergence.
∀ Middle temporal region and parietal cortex discharge in
response to retinal disparity and objects moving in depth
∀ Neurons near oculomotor nucleus act as immediate pre-
motor neurons for Vergence
Gauri S. Shrestha, M.optom
51. Vergence
The supraoculomotor
nucleus contains burst
and tonic cells as well
as phasic-tonic cells
that are characteristic
of the saccadic
pathways.
Gauri S. Shrestha, M.optom
52. Vergence
It is thought that velocity signals related to disparity
stimlui innervate the bursters and that velocity information
is integrated to form the position signal that is processed
by the tonic cells.
Accommodative-vergence is already represented by these
cells so that the wiring for cross-coupling between
accommodation and convergence must occur more
centrally in the pathways.
Gauri S. Shrestha, M.optom
53. Supranuclear disorders
∀ Gaze palsies – disorders of saccade amplitudes
and appropriateness
∀ Saccadic dysmetria
∀ Vertical gaze palsy
∀ Internuclear ophthalmoplegia
Gauri S. Shrestha, M.optom