Ocular Motility Module 2 Fixation

Fixation

Image is held steadily on the fovea by the eye and head

Purpose of fixation

to see a stationary object

for clear 20/20 vision

to study targets on high spatial frequency

For accurate fixation to occur, motion of the iamge on the fovea no more than

5 degrees per second

if faster --> blur!

For accurate fixation to occur, image should ie within _____ of the center of the fovea

0.5 degrees

The visual system creates minuate eye movements to

prevent fading

These movements include

Drifts

Tremors

Micro saccades

Troxler effect

The fading of peripheral images when the eye is fixated on a central object. The small, involuntary micro-eye movements that occur during fixation help minimize this effect.

Minature eye movements

1. Tremor

2. Micro-saccades

3. Slow drifts

tremor

High frequency

Low amplitude (less than the size of one PR)

Direction not correlated in the two eyes

Peripheral origin

Mechanical noise in the ocular motor system

Is not influence by visual conditions or attention factors

Microsaccades

occurs between 1 to 5 times/ sec all direction

In primary gaze amplitude is less than 0.1 deg, .25 deg/sec

*Correct for drifts* f the image away from the point of fixation

Microsaccades are _____ in peripheral postion of gaze

Larger by 4x in peripheral position of gaze (bc od rods)

The further away the drift is from the point of fixation the -----the amplitude of the micro-saccade will be

larger

Unlike Tremor, Micro saccades are correlated between the two eyes in

time of onset

direction

But may not be correlated in amplitude

Microsacades can be suprressed with

concentration/attention

Drifts

deviation from the point of fixation of about 0.1 deg (6 min)

Velocity of drifts 0.35 deg/se

drifts can occur in opposite direction in the two eyes

Drifts: Aplitude of drift away from image depends on

attention

In darkness the velocity of drifts i

increases 4x

Field holding reflex

visual cues to fixate

important for steady control of gaze

Which is the only minautre eye movement yoked between eyes?

microsaccades

Fixation differs from pursuit!

Pursuits have small oscillations that are

not present when fixating on a stationary target

Patients with pursuit disorders ____ have fixation disorders

do not always

Certain parietal lobe neurons fire during steady fixation but

not during smooth pursuit

ie. Reading vs just following an obk=ject and not comprehending

fixation suprresses saccades

Stimulating the MST and DLPN puriots neurons does not

produce pursuit during fixation of a stationary targer

Fixation behaviors

1. Combining saccades and fixation while reading

2. Combining pursuits and fixation while scanning (lingest fixation duration on relavant defing detail)

3. Muller Lyer illusion (larger eye movement amplitude when appear longer)

Infantile Motor Nystgmus

- malfunction in neuro pathway

- horizontal

- decreases on convergence, +nul; point, increases on oclcusion, VA corresponds to amplitude, varibale amplitude and frequency

Infantile sensory nystagmus

- usually central ocular defect in both eyes

AMplitude corresopnds to visual impairment

- whne no obvious ocualr defect, need VEP an dbrain imagingn

Acquired Nystagmus: Symptoms vary with onset of age

Acquired Nystagmus

needs neuro workuo

vertical nystgmus is a red flag

exception: known drug side efects induce vertocal

PART 2

gaze holding

hold gaze steady when the eyes are turned away from primary position

=

gaze holding requires integration of the

velocity (pulse) signal into position (step) signal

To move, the eye most over come 2 forces

viscous drag of the orbit

elastic restoring forces of the orbital tissue

2 types of neural activity are required to overcome these forces

conversion of pulse signal to step signal by the brain

pulse = position/time

step - posion

Vertical conjungate gaze holding center

INC (interstitial nucleus of Cajal)

Horizontal conjugate gaze holding

NPH nucleus prepositus hypoglossal

MVN (medial vestibular nucleus)

The cerebellum is also important for gaze holding

Flocculus !

important for balance

Neural substrates for gaze holding (Cerebellum, INC, NHP, MVN) encode

position and velocity signals

These substrates utilize

ACH, GABA, and NMDA

If there are antagonists to these neural transmitters it will create

leaky neural integrators and we dont want that!

Pause cells send out a constant stream of

inhibitory signals to the burst cells

prevents unwanted saccades from interfering with fixation once the saccade has moved the eye to its new position

Except...

during a saccade or during blinking

Burst cells

move the eys

tonic cells

hold new (eccentric) position

Without pulse

eye movement is slow

Without step

eccentric eye postion cannot be maintained following the movement

Pulse and step must ___ for accurate eye movement and maintenance of steady fixation subsequent to the movement

Leak neural integrator ir mismatch is

BAD

When there is a problem with Neural Inregration

eye is carried to new position by the pulse but the fauilt step cannot hold the eye there

eye drifts back to central position

Pulse signal keeps moving eye to the eccentric postion but the eye keeps drifting back

Faulty gaze holdig!

cause gaze evoked nystagmus!

with eventual adaption leading to rebound nystagmus !!

Brun's Nystagmus (he prob wont ask this)

slow, large amplitude nystagmus towards lesion

Rapid small amplitude nystagmus away from lesion

occurs in 11% of large acustic tumors

3. Pulse and Step must match!!!

Internuclear ophthalmoplegia (INO)

MLF Lesion resulting in

ipsilateral adduction deficiet

contralateral nystagmus

PLEASE DRAW THIS

INO is commonly caused by

MS

Normal Physiological end point nystagmus

in extreme end gaze horizontal or vertical

possible even at 20 degrees eccentricity

dampens after several beats

*begins soon after eccentric position reached*

usually beats in direction of gaze

may be assymettric

a different normal form of fatigue nystagmus exists after sustained fixation in extremes

End point nystagmus is greater in the _____ eye

abducting

End point nystagmus clinical features

1. Low amplitude and frequency

2. Horizontal on far lateral gaze; upbeating on far upgaze

3. unsustained

4. on lateral gaze, nystagmus is horizontal without vertical component

5. rebound nystagmus is transient or absent

6. Ocular motor examination is otherwise normal

Most common cause of gaze evoked nystagmus is

drug side effect

sedativesm tranquilizers, antoconvulsants, and alcohol

cerebellum or vestibular nuclei are affected

cerebellar disorders also cause gaze evoked nystagmus

alchohol does not cause pinpoint pupils!

Clinical findings with lesions causes Gaze evoked nystagmus

1. impaired smooth pursuit and combined eye head tracking (VOR suppression)

2. Impaired ability to suppress caloric nystagmus by fixating a stationary target

3. Impaired gaze holding function, leading to gaze evoked nystagmus; centripetal and rebound nystgmus

4. Downbeat nystagmus, ften greates on looking laterally and downward and with convergence

5. impaired ability to adapt to the VOR to changing visual needs (new rx)

6. post saccadic drift

PART 3

Optokinetic Nystagmus OKN

1. Occurs naturally during sustained self rotation in the light (if you spin and stop the world keeps spinning)

OR

2. Stimulated by motion of the visual field

Purpose of OKN

- stabilize images on the retina

Most retinal image motion is generated by

self motion !

the world is mostly stabile

exception: Watching passing scenery from a train window

Or stabilizing when large objects move across VF

OKN functions as a supplement to

VOR during locomoation

OKN requirements

Stimulus fills the field for best OKN

Sitting in a large rotating patterned drum

Use handeld OKN drum or tape or virtual reality goggles

Rotate the aptient at constant velocity,stop them, as teh labyrhinge signal sies down the sustained nystagmus is due purely to visual input

Measurement with ONK with drum or tape

- not ideal because it tends to stmualte intentional following movements which amsk the reflex

- VA cannot be quantified bc the distance of teh drum to the eye varies and speed of drum varies

ONK can be used to confirm

an intact visual pathway from retina to LGN to OM nuclei in non verbal subjects such as infants, non verbal, unresponsive pts !!!!

Dynamics of OKN

gain

amplitude

frequency

gain

ratio of slow Phase velocity to stimulus velocity

amplitude

extent of each slow phase excursion

Frequency

the rate at which the gaze is reset by the fast faze

These depend on instructions, because

voluntary pursuits and saccades can contribute to OKN

adults TRY to follow the lines

Two methods

1) Stare blankly at drum

2) Look OKN (active) (look at moving target)

Dynamics of OKN: On sustained rotation of a stimulus

there is a slow phase following movement interrupted by a fast phase returns to primary gaze (nystagmus)

Dynamics of OKN: after builduo if the lights are turned off

it continues in the dark for several seconds

Dynamics of OKN: then a persist effect called OKAN occurs via

velocity sotrage mechanism

Optokinetic After Nystagmus OKAN

after stimulus ceases signal remains in the brain because velocity storage mechanism continues to give output

OKAN response continues in

same direction

OKAN is observed only

in darkness with IR

not in light due to fiation cues

Purpose of OKAN

is to help neutralize post rotational nystagmus, PRN, after self rotation stops

OKAN is a weak variable measure

not used clinically!

Velocity Storage mechanism

once OKN in the light is established a velocity storage mechanism keeps the ONK persist after the lights are turned off

Circularvection in OKN

the sensation of self rotation

You cant tell of its your train or another train thats moving

Circularvection becomes more sensitive (lower threshold higher sensitivity)

with age when sense like proprioception are blunted

balance issues?

Factors that affect ONK

- Target luminance and spatial frequencu

- velocity of target

- vertical, horizontal and torsional (vertical and torsional is slower)

TOrsional: wathcing a revliving disk

Fain is highest for

horizontal ONK

PUTTING IT ALL TOGETHER

Rotation of the head --> retinal slip --> motion smear

Generates VOR in OPPOSITE direction of head movement

compensaroey eye movement

not intitated by retinal slip

Rotation of OKN Drum -- > retinal slip --> mortion smear

generate OKN in the same direction as target movement

compensarotry eye movement

SEE DIAGRAM