Post on 23-Jun-2020
Eye Movements – VOR Stabilization of Gaze (Ch 20)
fixation point
saccade
I) The main point: eye movements try to stabilize image on the retina
Fig. 20. 1
II) Different Types of Eye Movements
A) Saccades
1) Time course – fast
2) Vision is blanked during saccade
Fig. 20.4
B) Vestibulo-ocular reflex (VOR) and optokinetic (OK) reflex
1) Stimulus:VOR – head rotationOK – head fixed, motion of entire visual field
2) VOR and OKR work together to stabilize visual image on retina
C) Smooth Pursuit:
1) Small moving stimulus against stationary background
2) Works for slowly moving stimuli
3) Present only in foveate animals
4) Brain centers involved: cerebellum, frontal eye fields
D) Vergence:
1) Motion in depth
III) Eye-head coordination
A) Definition of gaze
Gaze = E + H
E = Eye in headH = Head in Space
B) Eccentric targets – move eyes and head –saccade first, then the head moves more slowly
H E
G
(Light) (Dark)
Shows you don’t need vision to get stable gaze
Dichgans et al. Exp Brain Res 18: 548-562, 1973
anterior
posterior
medial rectus
lateral rectus
horizontal canal
superior rectus
inferior rectus
anteriorcanal
superior oblique
inferioroblique
posteriorcanal
IV) Eye muscles and their relations to semicircular canals
A) Three pairs of eye muscles – each pair works in push-pull fashion
B) Each pair is aligned with the plane of one semicircular canal
1) Horizontal canal:medial & lateral recti
2) Anterior canal:superior & inferior recti
3) Posterior canal:inferior & superior oblique
Abducens
Trochlear
Oculomotor
Anterior CanalPosterior Canal
Horizontal Canal
LR
LR
C. Motor nuclei of eye muscles:
1) Abducens(VI cranial nerve) lat rectus
2) Trochlear(IV cranial nerve)
3) Oculomotor nucleus (III cranial nerve)med rectus MR
MR
Abducens
Trochlear
Oculomotor
Anterior CanalPosteriorCanal
Horizontal Canal
D) Neural Pathway for VOR
1) 3-neuron arc
a) semicircular canal afferent to vestibular nuclei
b) 2nd Order neurons – vestibular nuclei to oculomotor nuclei via the MLF
c) motor neurons to eye muscles Lat rectus
Med rectus
LR
MR
Example: horizontal canal to lateral and medial rectus muscles
IV The Physiology of the VOR in Detail
A) VOR Response to constant angular velocity in the dark
Hea
d R
otat
ion
(Ang
ular
Vel
ocity
)H
ead
Rot
atio
nA
ngul
ar A
ccel
erat
ion
(Degrees/second)
(Degrees/second )2
Sem
icirc
ular
Can
alA
ffer
ent
Adapted from Purves, et al., Fig. 13.9, adapted in turn from Goldberg & Fernandez
1) Semicircular canal afferents respond to angular acceleration – therefore, they adapt
a) Time constant is about 6 sec
Hea
d R
otat
ion
(Ang
ular
Vel
ocity
)H
ead
Rot
atio
nA
ngul
ar A
ccel
erat
ion
(Degrees/second)
(Degrees/second )2
Sem
icirc
ular
Can
alA
ffer
ent
Eye Displacement
VelocitySlow Phase Eye
Post-rotatory Nystagmus
Adapted from Purves, et al., Fig. 13.9, adapted in turn from Goldberg & Fernandez
Adapted from Raphan and Cohen, In: Adaptive Mechanisms in Gaze Control
2) Eye movements elicited by rotation: Nystagmus = alternating saccades & slow phase eye movements
a) Slow phase is compensatory – rotation is opposite to direction of head rotation
b) Time constant – slow phase velocity decays ~ 15 sec > Time constant of canals
3) Postrotatory Nystagmus – reflects canal response to acceleration in opposite direction
0
10
20
30
Cum
ulat
ive
Eye
Posi
tion
Eye
Posi
tion
Eye
Velo
city
Hea
dVe
loci
ty
Head Rotation
SaccadeSlow Phase
The beginning of rotation at a constant speed in the dark
4) Gain – defined as ratio of eye velocity/head velocity
HE /
a) Gain is usually close to 1.0
Eye Displacement
Eye Velocity
Optokinetic Nystagmus
B) Optokinetic Nystagmus
1) Evoked by motion of entire visual field – head fixed
a) Time course – build up slowly, but persistsb) OKAN – optokinetic afternystagmus
Eye Displacement
Eye Velocity
Optokinetic Nystagmus
Rotation in Light
VOR and OKN
VOR
C) Conclusion - The VOR and OKN complement each other – acting together, they stabilize gaze