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