Transcript Document

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-ocular - 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
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
Eye
Position
fast phase
slow phase
Left
Time
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
Eye
Position
fast phase
slow phase
Left
Time
Here the OKR interprets visual motion as head movement.
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
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).
is present.
Smooth pursuits CANNOT be done voluntarily unless a target
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.
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.
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
CONTROL OF HORIZONTAL EYE MOVEMENTS
lateral rectus
medial rectus
medial rectus
oculomotor
nucleus
abducens
nucleus
semicircular
canal
Vestibular Nucleus
lateral rectus
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
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
Vestibular
Oculomotor
Nuclei
Nucleus
Semicircular
Canals
Cervical
Vertebrae
Eye
Inferior
Olive
COR
VOR
OKR
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!!!