Fixation characteristics and abnormalities

FIXATION CHARACTERISTICS AND ANOMALIES

By: Noor Munirah binti Awang Abu Bakar

Optometrist (Moc No: O-0869)

OUTLINES:• Binocular vision • Eye movements• Types of eye movements• Eye Fixation- Concepts• Eye Fixation- Components• Fixation abnormalities

BINOCULAR VISION• Binocular vision is a sensorimotor process.• Eye movements to keep lines of sight pointing

same target.

(Otero-Millan et al, 2014)

EYE MOVEMENTSWhat? Refers to voluntary or involuntary movement of the eyes,

helping in acquiring, fixating and tracking the visual stimuli. Eye movements place the image of interest on the fovea

(part of retina with highest acuity) Controlled by 6 extraocular muscles innervated by cranial

nerves III, IV and VI.

EYE MOVEMENT CONTROLLED BY 6 EXTRAOCULAR MUSCLES (SUPERIOR VIEW)

TYPES OF EYE MOVEMENT

(Ciuffreda & Tannen, 1995)

EYE FIXATION Question:

Do our eyes move or still during fixating on a stationary object?

Answer:

Moving

EYE FIXATION-CONCEPT During attempted visual fixation at a stationary object,

our eyes continually move involuntarily.

The movement can be slow or rapid small amplitude- make us unaware of the movement.

The fixational eye movements change the degree of alignment between 2 eyes and continuously move the retinal images.

Even there is movement, the image of the object is still retained within an acceptable foveal retinal locus. (±30 minutes of arc)


EYE FIXATION-CONCEPT

EYE FIXATION-CONCEPT 1.Our eyes are focusing at a fly

EYE FIXATION-CONCEPT2. At macular area, there is a central foveal fixation point.

Macula

EYE FIXATION-CONCEPT3. At macular area, there is a central foveal fixation point.

EYE FIXATION-CONCEPT4. Central fovea is highly dense with cone photoreceptor.s

EYE FIXATION-CONCEPT5. During the eye fixation, the image moving in the central

fovea (across a cone, a dozen to hundreds of photoreceptors).


(Otero-Millan et al, 2014)

While our eyes are constantly moving, why we do not suffer DIPLOPIA?


(Martinez-Conde et al., 2004)

EYE FIXATION-CONCEPTALTERNATE EYE SACCADE AND FIXATION

• This is the typical pattern of eye movement during reading.

•The eyes never move smoothly over still text.

•Fixation lengths ~ 100 to 600 milliseconds length shows information processing.

• During this stop the brain starts to process the visual information received from the eyes

An example of fixations and saccades over text

EYE FIXATION-CONCEPTPATTERN FOR SHOWING FIXATIONAL EYE MOVEMENTS

To experience it, look at the central black dot for about a minute, then look at the white dot in the adjacent dark square.

The dark after-image of the white line pattern should be seen in constant motion owing to fixational eye movements.

EYE FIXATION-COMPONENTS

NEED TO UNDERSTAND…

EYE FIXATION COMPONENTS Microsaccades

( Martinez-Conde et al, 2004 & Otero-Millan et al, 2014)

EYE FIXATION COMPONENTS Tremors


EYE FIXATION COMPONENTS Drifts


TREMOR, DRIFT & MICROSACCADES ACROSS PHOTORECEPTORS

Curved lines: High frequency tremors & drifts

Straight lines: -Fast, jerk-like movements of microsaccades-Bring the image back towards the centre of vision

Fovea diameter: 0.05mm

EYE FIXATIONEYE MOVEMENTS DURING VISUAL FIXATION

• An observer views a picture (left) while eye positions are monitored (right).

• The eyes jump, seem to fixate or rest momentarily, producing a small dot on the trace, then jump to a new region of interest.

• However, even during these fixation, or ‘rest’ times, the eyes are never still, but continuously produce fixational eye movements — drifts, tremor and microsaccades.

EYE FIXATION

VISUAL FADING: TROXLER’S EFFECT

1. Fixate at red spot.

2. After few secs, bluish

annulus will disappear.

3. Red spot surrounded by

white field.

FIXATIONAL EYE MOVEMENT:MONOCULAR VS BINOCULAR VIEWING

Binocular performance can be superior to monocular performance of the same visual task. Why? Binocular summation – An ability of the brain to combine the

info from two eyes. Binocular summation predicts improved fixation stability

under binocular viewing as compared to monocular viewing.

Increased fixation instability during monocular viewing, especially for the occluded eye (González et al.,2012)

FIXATIONAL EYE MOVEMENT:NEAR VS FAR VIEWING

No differences in fixational eye movement characteristics between far and near viewing (Krauskopf et al., 1960)

FIXATION ABNORMALITIES








FIXATION ABNORMALITIES2. Saccadic intrusions


2. Saccadic intrusions



FIXATION ABNORMALITIES3. Nystagmus- Involuntary rhythmic oscillation of the eye


FIXATION ABNORMALITIESPe

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FIXATION ABNORMALITIESJe

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3. Nystagmus- Involuntary rhythmic oscillation of the eye


3. Nystagmus


REFERENCES Ciuffreda, K.J. 1995. Eye Movement Basics for The Clinician. Mosby. Martinez-Conde, S. Macknik, S.L. & Hubel, D.H. Role of Fixational Eye in

Visual Perception. Nat Rev Neurosci. 2004 Mar;5(3):229-40. Martinez-Conde, S. Fixational Eye Movements in Normal and Pathological

Vision. Prog Brain Res. 2006;154:151-76. Otero-Millan, J., Macknik, S. & Martinez-Conde, S. Fixational Eye Movements

and Binocular Vision. Front Integr Neurosci. 2014; 8: 52. Published online 2014 July 7. doi: 10.3389/fnint.2014.00052 PMCID: PMC4083562

Rucci, M. & Poletti, M. Control and Functions of Fixational Eye Movements. Annual Review of Vision Science, Vol. 1: 499 -518 (Volume publication date November 2015)

Shaikh A.G. , Otero-Millan J., Kumar P., Ghasia F.F. Abnormal Fixational Eye Movements in Amblyopia. PLoS One. 2016 Mar 1;11(3):e0149953. doi: 10.1371/journal.pone.0149953. eCollection 2016.

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Fixation characteristics and abnormalities