Gaze shifting & gaze holding ocular motor functions

Gaze shifting & Gaze
holding Ocular motor
functions

Gauri S. Shrestha, M.optom, FIACLE

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

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)


Change the Angle of Gaze
 Align fovea with the object of interest

 Eye movements used to change gaze angle
 Saccades
 Pursuits
 Vergence


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


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


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

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


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?


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)


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


Eye Movement Control

Anatomical and Neural Factors


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


The Afferent System
 Provides information about eye position to the
brain
 How is this achieved?
 Two sources (subsystems)
 Proprioception
 Efference copy/Corollary discharge


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


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


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


Innervation of the EOMs
 Oculomotor (Cranial Nerve 3)
 MR, SR, IR, IO
 Trochlear (Cranial Nerve 4)
 SO
 Abducens (Cranial Nerve 6)
 LR


Review of these systems
 Afferent System
 Proprioception
 Corollary Discharge or Efference Copy
 Efferent System
 Sends pulse and step neural signals to the EOM


The EOM Fibers
 Two Major Function
 Move the eyes (quickly or slowly)
 Change eye position
 Keep the eyes relatively stationary
 Maintain new eye position


Physiological Types of Fibers
 Twitch Fibers (burst)
 All or none action potential

 Non-Twitch Fibers (tonic)
 Graded contractions

 Few fibers are a combination


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


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


Fiber Positions


Actions of EOMs
Cardinal Positions


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


 Five movement subsystems:
1. Saccadic systems
2. Pursuit systems
3. Vestibulo-ocular reflex
4. Optokinetic reflex
5. Vergence


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.

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


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


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


 Tonic neuron
∀ Responsible for maintaining muscle tone
∀ Its activity increase after saccade to maintain gaze
position represents the step


∀ 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.


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)


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.


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


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.


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

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.


∀ 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


∀ 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)


Anatomical pathways

 Labyrinth--> CN VIII nucleus –
PPRF & CN III, IV &VI nucleus


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)


∀ 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


∀ 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


∀ 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


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


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


Vergence
 The supraoculomotor
nucleus contains burst
and tonic cells as well
as phasic-tonic cells
that are characteristic
of the saccadic
pathways.


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.


Supranuclear disorders

∀ Gaze palsies – disorders of saccade amplitudes
and appropriateness
∀ Saccadic dysmetria
∀ Vertical gaze palsy
∀ Internuclear ophthalmoplegia


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