OCULOMOTOR SYSTEM

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OCULOMOTOR SYSTEM
The function of the eye system is to acquire
visual targets rapidly and, once acquired, to
stabilize the image on the retina in spite of
relative movements between the target and the
observer.
Movements of the eye can be classified as
follows (Kandel, Schwartz, Jessel) Movements
that stabilize image when head moves Vestibulo-oc
ular - Uses vestibular input to hold images
during brief or rapid head rotation Optokinetic
- Uses visual input to hold images stable on
the retina during sustained or slow head
rotation Movements that keep the fovea on a
visual target Saccade Brings new objects of
interest onto the fovea Smooth pursuit Holds the
image of a moving target on the
fovea Vergence Adjusts the eyes for different
viewing distances in depth
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OCULOMOTOR SYSTEM II
Vestibulo-ocular reflex (VOR)
As we have seen, the semi-circular canals signal
the speed of rotation in any direction and the
oculomotor system responds by rotating the eyes
at an equal but opposite SPEED so that an
object of interest can be foveated. (Give
example) So if you were to rotate (in the dark)
at a constant rate to the left, your eyes would
be pinned to the right corner of your eyes
(cannthus) due to signals from the semicircular
canales. This does not occur due to quick
reset motions of the eyes (nystagmus).
NOTE movement of head is to the left
Right Left
fast phase
Eye Position
slow phase
Time
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OCULOMOTOR SYSTEM III
Optokinetic Reflex (OKR) As the eyes move in the
environment, the image of fixed objects move
across the retina in a direction opposite to the
that of the head. The optokinetic system drives
the eyes in the direction of visual field motion
(to try to foveate fixed objects) which is
opposite the head movements inducing that motion.
NOTE movement of striped drum is to the right
while the subject is still
Right Left
fast phase
Eye Position
slow phase
Time
Here the OKR interprets visual motion as head
movement.
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OCULOMOTOR SYSTEM IV
SO, the VOR and the OKR work in a complementary
fashion to keep an image foveated as the head
moves in space. The OKR is particularly
effective at very low frequency whereas the VOR
is not very effective at extremely low
frequency. The effectiveness of reflexes are
usually measured by the engineering technique of
gain measurement. The reflexes must be
modifiable because sometimes they are
counter-functional. (e.g., when an object of
interest is moving with you). NOTE gains
of the reflexes can be modified in interesting
ways
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OCULOMOTOR SYSTEM V
Smooth Pursuit The Optokinetic System tries to
stabilize the eyes in space when head movements
occur (involuntary). The Smooth Pursuit System
moves the eyes in space to keep a single target
on the fovea (voluntary). Smooth pursuits
CANNOT be done voluntarily unless a target is
present.
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OCULOMOTOR SYSTEM VI
Saccadic System When an image of interest which
is on the fovea suddenly moves to another part
of the visual field, the eyes remain in the
original position for about 200ms and then move
quickly to re-acquire the image. The speed
of the re-acquiring movement (saccade) is
dependent on the length of the eye movement
required, andthe initial eye position.
Corrections are made (mini-saccades) after
the initial saccade gets the eyes near the
visual target.
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OCULOMOTOR SYSTEM VI
Vergence When an object of interest moves toward
or away from us, our eyes must adjust to keep the
image foveated. This requires disconjugate
movement of the eyes,i.e., movements in
opposite directions. NOTE All previously
described eye movements were conjugate. The
mechanism for vergence depends upon the
blurring of the image as the target moves
toward (or away from) you. The ciliary
muscles contract to change the shape, and
therefore, the focal length of the lens to
focus the image.
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Modifiability of VOR
The job of the Vestibulo-ocular Reflex (VOR) is
to allow a visual image to remain fixated on the
fovea during movements of the head in 3-space.
This takes place through the activity of the
ocular motor neurons which reflect the velocity
and position of the eyes. The VOR can
be modified both functionally and
experimentally Functional modification -
fixation on an object moving through space with
you Experimental modification - goggles or
training
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CONTROL OF HORIZONTAL EYE MOVEMENTS
lateral rectus medial
rectus medial rectus
lateral rectus
oculomotor nucleus
abducens nucleus
semicircular canal
Vestibular Nucleus
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GAZE CONTROL
Because an animal may be moving with respect to
the environment AND the head may be moving
with respect to the body AND the eyes may be
moving with respect to the head, the problem of
finding the visual fixation point relative to the
environment is complex. Luckily, the brain
(cerebellum) is able to handle the necessary
coordinate transformations in terms of GAZE,
i.e., where the 12 muscles of the eyes set the
point of visual fixation. The signals needed
to perform this complex function come from -
vestibular system - senses body movement wrt the
Earth coordinates - neck receptors - sense
head rotations wrt the body coordinates -
optokinetic and retinal slip systems - sense eye
movement in head coordinates
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COR / VOR / OKR
Rotations of the head on the body are sensed by
mechanoreceptors in the first 3 cervical joints.
Interactions of the vestibular, cervical and
visual control of eye movements are below
Cerebellum
Semicircular Canals
Eye
Cervical Vertebrae
COR VOR OKR
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OCULARMOTOR SYSTEM SUMMARY
More is known about the ocularmotor system than
about any other motor system. But Several
important things are still not known For
example, ocular muscles are rich in muscle
spindles but ocular muscles do not exhibit
stretch reflexes so their function is not
known. Also, despite the fact that the signal
of each type of neuron is known wrt eye position
and velocity, the control of recruitment to
drive the eye muscles is unknown. So there is
lots of science and engineering left to do!!!