Day 11 (2): Binocular Single Vision

Question 1

What is binocular single vision?

Answer 1

• State of SIMULTANEOUS vision achieved by COORDINATED use of BOTH eyes so that separate images arising in each eye are appreciated as a SINGLE image by the process of FUSION

Components:
1. Motor Fusion: coordinated use of both eyes
2. Clear visual axis of BOTH eyes
3. Sensory Fusion: unification of images from both eyes

Question 2

What are the prerequisites for binocular vision?

Answer 2

Normal ANATOMIC components
- normal orbit: shape, size and depth
- normal soft tissues: EOMs, innervation
- BOTH eyes need to be properly placed and oriented within the orbit
- visual axis of BOTH eyes should be directed and focused on the same point

Properly developed PHYSIOLOGIC reflexes
A. Fixation Reflexes
1. Compensatory/Gravitational Reflex
- keeps both eyes in a fixed, frontally-directed position
- exertion of a basal muscle tone on the vertical rectus (by utricles) and oblique (by saccules) muscles
- reflects normal vertical axis

2. Orientation Reflex
   - slow movement of continued fixation by the eyes when following a moving object or panorama
   - reflects normal horizontal axis
3. Accommodation - Convergence Reflex
   - aimed at correctly aligning the eyes and keeping them focused on an object
   - components:
   + Vergence Fixation Reflex
   + Accommodation Reflex
   + Fusional Vergence Reflex

B. Refixation Reflexes
- relates the eye back to its original orientation point OR adjust to the new orientation point

Question 3

What are the advantages of binocular vision arising from using two eyes instead of one?

Answer 3

1. Spare eye if one is damaged
2. Ability to see behind an obstacle
   - due to depth perception
3. Wider field of view
   - 190 degrees: maximum horizontal field of view using both eyes:
   + 120 degrees: binocular field
   + 35 degrees on both sides: monocular field
4. Stereopsis

Question 4

Discuss the timeline of development of binocularity from birth to infancy.

Answer 4

1st 2 years of life: SENSITIVE period for binocular vision development

Timeline:

Birth: poor fixation reflex; (+) random non-conjugate aimless ocular movements

< 1 month: follows light with 1 eye; (+) optokinetic movements

1 - 6 months: follows light with 2 eyes, (+) convergence, (+) retinal disparity/stereoacuity

6 months: convergence well-developed, (+) accommodation (delayed due to late development of ciliary muscles)

> 6 months: (+) stereoacuity development
- start: 4 months
- peak: 2 years
- well-developed: 4 years
- decline & cessation of development: 9 years

Question 5

What is Cortical Fusion?

Answer 5

Unification of images from both eyes in the visual cortex by simultaneous stimulation of CORRESPONDING retinal areas.

Question 6

What is Motor Fusion?

Answer 6

• Ability for BOTH eyes to move TOGETHER in a COORDINATED and PRECISE manner in ALL gaze directions (and convergence)
• Involves:
  1. Conjugate gazes (VERSIONS)
  2. Disconjugate gazes (CONVERGENCE)

Goal: for the image to fall in two CORRESPONDING retinal points to achieve SENSORY FUSION

Pathologies:
1. EOM pathologies: Congenital Fibrosis of EOM, Duane Syndrome

2. CN pathologies: in the nerve trunks or nuclei
3. Craniofacial malformations: Crouzon syndrome (shallow orbit)

Question 7

Give an example of conjugate gaze pathway.

Answer 7

E.g. Right gaze

1. LEFT cerebral cortex frontal eye fields (Area 8) sends signals the RIGHT paramedian pontine reticular formation (PPRF)
2. RIGHT PPRF sends signals to the RIGHT Abducens nucleus.

Right side:
RIGHT Abducens nucleus, through Cranial Nerve 6 or Abducens nerve, causes contraction of the RLR

Left side:
RIGHT Abducens nucleus, through the Medial Longitudinal Fasciculus (MLF), send signals to the Oculomotor Nucleus and passes this on via the Cranial Nerve 3 or Oculomotor nerve to contract the LMR.

Question 8

What happens during accommodation?

Answer 8

1. Visual cortex sends signals to Edinger-Westphal nucleus on BOTH sides located in the Oculomotor nucleus.
2. Signals travel via the Oculomotor nerve into three locations:

• Ciliary muscles: contraction to relax the lens zonules thereby increasing the AP diameter of the lens and forward displacement
• Both medial rectus: causing adduction of both eyes or convergence
• Iris sphincter: contraction to decrease pupillary size

Question 9

What is Sensory Fusion?

Answer 9

Ability to perceive two SIMILAR images and fuse them into ONE image

Prerequisite:
1. Retinal Correspondence: images must fall on corresponding retinal areas
2. Same size, brightness and sharpness

Pathologies:
1. Internal: media opacities, errors of refraction, retina and ON lesions

2. External: ptosis, anisometropia, astigmatism

Question 10

What is Retinal Correspondence?

Answer 10

• retinal elements of the two eyes that share a COMMON subjective visual direction
• BOTH fovea should have one common visual direction
• object seen by the two eyes should form an image in corresponding areas of the two retinas for it to be interpreted as one
  + retinal points in the NASAL area of R eye corresponds to retinal points in the TEMPORAL area of the L eye

Non-Corresponding/Disparate Points
- all other points that are not conjugate retinal points

Question 11

What is a Cyclopean eye?

Answer 11

An abstract eye with a SINGLE AXIS of perceived direction representing the combined visual axes of TWO EYES

Question 12

What is a Horopter?

Answer 12

• “Horos” meaning boundary + “Opter” meaning eye = “boundary of the eye”
• space containing all object points that can be projected on corresponding retinal points at a given fixation distance
• locus of points in space seen as a SINGLE IMAGE by the two eyes

3 Interpretations:
1. Vieth-Muller Circle/Theoretical Horopter
2. Empirical Curve/Empirical Horopter
3. Panum’s Fusional Area

Question 13

What is the Vieth-Muller Circle or Theoretical Horopter?

Answer 13

• a circle passing through the center of rotation of the eyes (nodal points) and the fixation point
• formed by tracing corresponding points that have a regular horizontal distance from the retina
• circle becomes SMALLER as the point of fixation becomes NEARER.

Question 14

What is the Empirical Curve or Empirical Horopter?

Answer 14

• flatter than the Vieth-Muller Circle with greater radius of curvature
• passes through the fixation point and gradually extends further away on both sides of the VMC upto the halfway between the fixation point and nodal points
• all points OUTSIDE the curve are DISPARATE or non-corresponding and will be seen as DOUBLE

Hering-Hillebrand Deviation
- area in between the VMC and the Empirical Curve representing a deviation in the horopter mapping in the visual cortex
- attributed to neural and optical factors where the NASAL hemiretina at any given eccentricity contains MORE PHOTORECEPTORS per unit area than the temporal hemiretina

Question 15

What is the Panum’s Fusional Area?

Answer 15

• narrow band around the Empirical Curve with the fixation point at its center
• objects stimulate DISPARATE retinal points BUT produce SINGLE vision
• Zone of Stereopsis

Center/Fixation Point
- starts narrow
- portion with smallest horizontal extent
- image falls on both FOVEA
- 6 - 10 arc-minutes

Periphery
- gradually broadens with increasing horizontal extent at a rate of 1 - 2 arc-minutes per degree of visual field eccentricity
- 12 degrees from fovea: 30 - 40 arc-minutes
- related to anatomic and physiologic differences known to exist between foveal cone system and rod WITH cone system of the periphery

Question 16

What is Physiologic Diplopia?

Answer 16

• diplopia produced by object points OFF the empirical curve BUT WITHIN the Panum’s area
• all points NOT LYING in the empirical curve are imaged by DISPARATE retinal elements and is seen as DOUBLE
• however, even if the points are not on the EC but still within the Panum’s area, diplopia may result BUT can still be fused

2 Types:

1. Crossed/Temporal/Heteronymous
   - due to objects NEARER than the horopter
2. Uncrossed/Nasal/Homonymous
   - due to objects FARTHER than the horopter

Question 17

What is stereopsis?

Answer 17

• ability of the eyes to fuse images that simultaneously fall on DISPARATE retinal areas BUT within the Panum’s area, resulting in 3D perception
• major contributor to depth perception obtained through binocular vision
• due to horizontal/binocular disparities: each eye receives a different image due to slightly different positions on one’s head
• perceived depth INCREASES with INCREASING disparity but TOO MUCH disparity will eventually cause loss in stereopsis leading to DIPLOPIA
• good stereopsis indicates brain’s ability to fuse images and recognize depth

Question 18

What is the Correspondence and Disparity Theory?

Answer 18

• when both retina is stimulated simultaneously at CORRESPONDING (within EC) areas by ONE object point or TWO object points with SIMILAR characteristics:
  + SINGLE fused image
  + but NO depth/stereopsis
• when both retina stimulated simultaneously at DISPARATE (outside EC) areas by ONE object point: binocular DISPARITY
  + disparity still falls WITHIN Panum’s area: physiologic diplopia BUT can still be fused –> (+) depth perception or STEREOPSIS
  + disparity falls OUTSIDE Panum’s area: (+) unfused DIPLOPIA or double vision
• when both retina stimulated simultaneously at CORRESPONDING areas by TWO object points with DISSIMILAR characteristics: binocular RIVALRY –> (+) visual confusion

Assumption: 1:1 retinocortical relationship between two eyes

Question 19

What is the Neurophysiologic Theory?

Answer 19

Binocularly-driven cells
- 25%: EQUAL stimulation from each eye
- 75%: GRADED stimulation from either eye
- nearly EQUAL-sized receptive fields and in CORRESPONDING positions
- assumed to have a precise and orderly arrangement of connections along the retinogeniculocortical pathway
- upto 80% of neurons in the visual cortex can be activated by either eye in response to a stimulus

• Supports the theory that the decussation of the ON at the CHIASM is essential for the development of binocular vision:
• visual information from corresponding retinal areas of each eye runs via PARALLEL, ADJACENT but SEPARATE circuits to the LGN and optic tract to the visual cortex

Question 20

What comprises the RetinoGeniculoCortical Pathway?

Answer 20

1. Retinal Ganglion Cells
2. Lateral Geniculate Nucleus
   - principal thalamic visual nucleus linking the retina and cortex
   - contains 6 lamina sheets
   - layer 1, 4, 6: nasal retina of contralateral side
   - layer 2, 3, 5: temporal retina of ipsilateral side
   - terminates in V1 of striate cortex or may proceed to other areas

Parts:
A. Magnocellular Pathway
- large soma and axons
- rarely connected with GCs in the fovea
- SENSITIVE to MOVING stimuli
- INSENSITIVE to COLOR
- 10% of retinal input
- innermost 2 layers

B. Parvocellular Pathway
- small soma and axons
- connects with GCs in the fovea
- SENSITIVE to object’s borders, color contrast and shape
- 90% of retinal output
- outermost 4 layers

C. Koniocellular Pathway: largely unknown

3. Primary Visual Cortex

Question 21

What are examples of sensory adaptations to fusional pathologies?

Answer 21

• adaptation mechanisms in IMMATURE visual systems that develop when fusional pathologies are present during the sensitive period of binocular vision development (1st 2 years of age)

1. Anomalous Retinal Correspondence
2. Suppression

Question 22

What is Anomalous Retinal Correspondence (ARC)?

Answer 22

• an ADAPTATION mechanism in IMMATURE visual systems to prevent diplopia
• adaptation of the brain to a small-angle strabismus wherein the brain uses a pseudofovea on one eye superimposed with the true fovea on the other eye to fuse an image
• eye’s attempt to regain binocularity and stereopsis
• can result in fusion inspite of a manifest misalignment
• ONLY present in binocular conditions: when the fixating eye is occluded (monocular condition), the deviated eye would shift back to using its true anatomic fovea

Question 23

What happens during normal retinal correspondence vs manifest deviation without ARC vs manifest deviation with ARC?

Answer 23

Normal RC
- images which fall roughly on the same retinal areas relative to the fovea (CORRESPONDING AREAS) are perceived as lying in the same visual direction

Manifest deviation WITHOUT ARC
- DIFFERENT object points fall on corresponding areas of the retina
- leads to non-fusion, confusion and double vision (diplopia)

Manifest deviation WITH ARC
- adaptation mechanism in an IMMATURE visual system
- meaning neural changes can still be made
- retinal corresponding points are REMAPPED to account for the deviation and prevent diplopia
- object points falling at very different retinal areas much further than the normal range of fusible disparity will now be perceived as lying in the same visual direction

Question 24

What is suppression?

Answer 24

• an ADAPTATION mechanism in IMMATURE visual systems to prevent diplopia
• active inhibition of a retinal input when:
  1. CORRESPONDING retinal areas are stimulated by DISSIMILAR stimuli
  2. DISPARATE retinal areas are stimulated by SIMILAR stimuli
• one of the other input is temporarily suppressed
  + foveal/central suppression: prevent visual confusion
  + extrafoveal/peripheral suppression: prevent diplopia

2 Types:
1. Facultative Suppression
- happens in binocular conditions ONLY
- VA normal in monocular conditions
- NO monocular scotomas

2. Obligatory Suppression
   - happens even in monocular conditions
   - VA abnormal even in monocular conditions
   - effect: AMBLYOPIA

Question 25

What are the two types of abnormalities of binocular vision?

Answer 25

Diplopia
- due to retinal DISPARITY
- ONE object point falling simultaneously on two DISPARATE retinal areas or different visual directions
- two images of the SAME object (double vision)

Confusion
- due to retinal RIVALRY
- TWO DISSIMILAR object points falling on CORRESPONDING retinal areas or the same visual direction
- see BOTH objects due to inability to fuse images because of different characteristics

Question 26

What is the Red Glass Filter Test?

Answer 26

• tests for suppression and anomalous retinal correspondence
• detects diplopia
• measure amount of deviation using prisms prior to test
• red glass is placed over the fixating (non-deviated) eye

Results:
1. One pink light: NORMAL/ORTHOPHORIC

2. One red or white light: SUPPRESSION
3. Uncrossed diplopia: ESOTROPIA
   - white light DOES NOT CROSS OVER the side of the tested (non-deviated) eye
   - because eye is ADDUCTED, object point falls on the NASAL hemiretina
   - this is interpreted by the brain as coming from the TEMPORAL visual field thus image does not cross over
4. Crossed diplopia: EXOTROPIA
   - white light CROSSES OVER the side of the tested (non-deviated) eye
   - because eye is ABDUCTED, object point falls on the TEMPORAL hemiretina
   - this is interpreted by the brain as coming from the NASAL visual field thus image crosses over

Question 27

What is the Bagolini Striated Glasses Test?

Answer 27

• done in COOPERATIVE, OLDER patients because they need to properly describe what they see
• tests for retinal correspondence and to identify suppression
• test most likely to allow the demonstration of fusion in patients who fuse intermittently
• (+) ARC: Binocular Single Vision (BSV) BUT with a strabismus on cover test

Set-up:
1. Patient is asked to wear glasses with angulated striations:
+ Left eye: 45 degree striations (line: )
+ Right eye: 135 degree striations (line: /)
2. Testing distance: 6 meters
3. Lights ON

Questions:
Ask how many LIGHT POINTS are seen:
- TWO: Diplopia (Crossed vs Uncrossed)
- ONE: BSV or Total Suppression

–> Ask how many LINES are seen:
- ONE: TOTAL Suppression
- TWO: BSV

–> Ask if lines are COMPLETE:
- Complete: No suppression/scotoma
- Incomplete: Central (Monofixation) or Peripheral (Scotoma)

–> Do Cover Test if BSV:
- if (-) shift: Orthophoric + NRC
- if (+) shift: Strabismus + ARC

Results:
1. Complete X: BSV
- see a cross with ONE light point in the middle
- means fusion is intact
- do cover test to differentiate NRC from ARC

2. Incomplete X
   - BSV/fusion intact since (+) X but with a gap
   - the line with the gap corresponds to the eye with the suppression or scotoma
   - remember to always confirm if the lines seen are complete or with gaps

A. Monofixation Syndrome
- central suppression + intact peripheral fusion
- gap is centrally located
- due to small deviations (< 8 PD)

B. Peripheral Scotoma
- gap is peripherally located
- due to larger deviations (> 10 PD)
- larger shift = larger scotoma = larger gap

3. Entire line missing: (+) Total Suppression
   - /: Left suppression
   - : Right suppression
4. CROSSED diplopia: EXOTROPIA
   - see TWO light points
   - light point of L crosses to the R while that of R crosses to the L
   - because eye is ABDUCTED, object point falls on the TEMPORAL hemiretina
   - this is interpreted by the brain as coming from the NASAL visual field thus line appears to move more NASALLY
5. UNCROSSED diplopia: ESOTROPIA
   - see TWO light points
   - light point of L remains to the L while that of R remains to the R
   - because eye is ADDUCTED, object point falls on the NASAL hemiretina
   - this is interpreted by the brain as coming from the TEMPORAL visual field thus line appears to move more TEMPORALLY
6. Hypotropia:
   - object point falls on INFERIOR retina responsible for SUPERIOR visual field
   - LEFT: \ higher than /
   - RIGHT: / higher than \
7. Hypertropia:
   - object point falls on SUPERIOR retina responsible for INFERIOR visual field
   - LEFT: \ lower than /
   - RIGHT: / lower than \

Question 28

What is the Worth 4-Dot Test?

Answer 28

• done in COOPERATIVE patients (even young children) to test for motor fusion and identify suppression
• patient is shown 4 dots:
  + Upper dot: Red
  + Lower dot: White
  + Horizontal dots: Green

Set-up:
1. Left eye: Green glass
- blocks red color
- sees the two green dots and one white dot
2. Right eye: Red glass
- blocks green color
- sees the one red dot and one white dot
3. Testing distance
- NEAR: 1 feet (for peripheral fusion)
- DISTANCE: 6 meters (for central fusion)

Results:
1. 4 Dots: NRC (Orthophoric) or ARC (Strabismus)
- white dot can be green, red or pink

2. 5 Dots: (+) Diplopia
3. 2 Red Dots (upper and lower dot)
   - (+) Left suppression
4. 3 Green Dots (horizontal and lower dot)
   - (+) Right suppression

Special circumstance: Monofixation Syndrome
- (+) CENTRAL suppression
- intact PERIPHERAL fusion
- manifests only at FAR testing distances (6 m)

FAR testing distance (6 m)
- testing angle is WITHIN the angle of the suppression scotoma
- 2 Red Dots: Left suppression
- 3 Green Dots: Right suppression

NEAR testing distances (33 cm)
- testing angle is OUTSIDE the angle of the suppression scotoma
- 4 Dots (1 Red, 2 Green, 1 Any)

Question 29

What is an Amblyoscope or Haploscope?

Answer 29

• definitive test for ARC
• each eye looks through a tube containing two different images
• has 2 arms that can be adjusted in all directions (even rotationally) with the goal of superimposing the images from both tubes to measure the SUBJECTIVE angle

Terms:
1. Objective Angle
- measures the position of the TRUE foveas
- angle by which the target’s image on the true foveas produce NO movement with alternate target presentations
- measures the angle of deviation, if present, by alternate prism cover test
- obtained in monocular conditions

2. Subjective Angle
   - measures the position of the PSEUDOfoveas IF present
   - amount in degrees the examiner/patient must move the amblyoscope arms to see both images superimposed
   - measured in binocular conditions
3. Angle of Anomaly
   - measurement of the distance of the pseudofovea (if present) from the true fovea
   - Objective Angle MINUS Subjective angle

Results:
1. NORMAL Retinal Correspondence
- Angle of Anomaly = ZERO
- Objective angle = Subjective angle
- image superimposition happens at the TRUE foveas [O = S] thus NO PSEUDOfovea [AoA = 0]

2. ANOMALOUS Retinal Correspondence
   - Angle of Anomaly NOT ZERO
   - Objective angle NOT = Subjective angle
   - image superimposition DOES NOT happen at the true foveas [O not = S] thus (+) PSEUDOfovea [AoA not = 0]
   - patient then asked to superimpose the targets:

HARMONIOUS ARC
- superimposition occurs with Subjective angle at ZERO
- Angle of Anomaly = Objective angle
- distance of the pseudofovea from the true fovea EQUAL angle of deviation
- images are still fusible even if the eye is deviated

UNHARMONIOUS ARC
- superimposition occurs with SUBJECTIVE angle BETWEEN zero and objective angle
- Angle of anomaly NOT = Objective angle
- distance of the pseudofovea from the true fovea NOT EQUAL angle of deviation
- (+) diplopia or (+) suppression