4 Fusion and Correspondence

What is the Hierarchic Arrangement of BV System? Mention its components.

→ artificial concept, matches the neurologic arrangement of the visual system

1. Sensory

2. Integrative

3. Motor

What is the most fundamental vs least fundamental part of the Hierarchic Arrangement of BV System?

Most fundamental = Sensory

Least fundamental = Motor

Define Sensory process.

anatomic, physiologic and psychologic activities involved in the collection and transmission of visual info to the cortex

Define Integrative process.

activities involved in the fusion of the 2 cortical images to form a single binocular percept of visual space

Why should anomalies of the Integrative process be manipulated with caution?

b/c eliminating them can’t be easily reversed

List and define 4 main Anomalies of Integrative Processes.

1. Suppression – lack or inability of perception of normally visible objects in all or part of the FOV in one eye, due to cortical inhibition

2. Anomalous retinal correspondence – fovea of two eyes not aligned for a common visual direction

3. Horror fusionalis– can’t obtain fusion or superimposition of haploscopically presented targets

4. Aniseikonia – relative difference in image size between both eyes

Define Motor process.

activities necessary to properly align eyes at various distances and directions of gaze

How are motor anomalies approached?

→ treated last (usually after treating sensory and integrative issues)

• common and have a significant effect on visual comfort and performance

List and describe the 4 Anomalies of Motor Processes.

1. Vergence dysfunction – e.g. esophoria, exophoria, vertical phoria, convergence insufficiency

2. Strabismus – crossing of the eyes arising congenitally or due to trauma, surgery, tumor

3. Nystagmus – rhythmic oscillation of the eyes, beyond normal fixational or endgaze movements

What is the most commonly diagnosed binocular anomality?

Vergence dysfunction 

What does deficits in the sensory process cause?

deficiencies in either or both of the remaining system

List and define the 5 Anomalies of Sensory Processes.

1. Ametropia – refractive condition where far point is not at infinity (Myopia, Hyperopia, Astig)

2. Amblyopia – ↓ VA not correctable with best refraction

3. Eccentric fixation – amblyope doesn’t use central foveal area for fixation under monocular conditions

4. Accommodative dysfunction – poor amplitude, ↓ facility, fatigue, spasm

5. Disease (e.g., ptosis, keratoconus, media opacities, retinal or visual pathway disease)

Define Binocular Fusion.

→ process by which 2 images, one from each eye, give rise to a unified percept of one single object

• separating 2 eyes doesn’t tell us about the process or limits to fusing 2 images

How is Binocular Fusion classified?

All 3 degrees of fusion are needed to fully benefit from binocular vision

• Grade 1: Simultaneous perception

• Grade 2: Flat fusion

• Grade 3: Fusion with stereopsis. (highest level of BV function)

Draw out the table for Degrees of Binocular Fusion.

What is Simultaneous Perception?

→ Simultaneous view of 2 disparate, dichoptically-viewed DISSIMILAR images

• Dichoptic: viewing a separate and independent field by each eye

  • e.g., A = OS, B = OD

• No context for Fusion

• Diplopia and/or Confusion

• tested with Major Amblyoscope

What is Super Imposition?

→ Superimposition of 2 disparate, dichoptically-viewed DISSIMILAR images (e.g. A = OS, B = OD), shared context

• ❌ TRUE Fusion

• ❌ diplopia or confusion

• ❌ frame or object to serve as fusion lock

What is Flat Fusion?

→ Represents 2 dichoptically-viewed images with SIMILAR + DISIMILAR features combined into a single percept

• involves binocular summation, binocular correspondence and Fusion without depth (2D)

• ✅ True fusion (fusion lock)

• ❌ Stereopsis (no depth)

What is Stereopsis?

→ Binocular 3D depth perception derived from SIMILAR, dichoptically-viewed images to OD and OS

• Stimulation of non-corresponding points that are fairly close together → small retinal disparity gives rise to stereopsis

  

What is the highest level of binocular visual function?

Stereopsis

Define Fusion.

→ process of cohesively merging the 2 images (one from each eye)

• unifying visual excitations from the corresponding retinal images into a single visual percept

Where does fusion occur?

always a central process (occurs in visual cortex)

Define Motor Fusion.

→ uses Vergence eye movements (reflex) to position the eyes so that corresponding points are super imposed

• physical ability to align the eyes in such a manner that sensory fusion can be maintained

• Stimulus: retinal disparity outside Panum’s area, causing the eyes to move in opposite directions (vergence)

• e.g., Convergence or divergence

What is motor fusion exclusively the function of?

extrafoveal retinal periphery

Why are strabismic pt not able to achieve motor fusion?

due to to their inability to place the images of the object on the fovea of each eye

What is impairments of BV in strabismic pt caused by?

• Visual Stress

• Pathology

• Trauma

• Drugs

• Fatigue

Define Sensory Fusion.

→ neurophysiological and psychological process by which the visual cortex –”combines the superimposed views from each eye into single percept”

• ability to appreciate 2 similar images and interpret them as one

  • requires strict similarity between the 2 monocular images

• can be present without convergence or divergence (motor fusion)

  • e.g., slit lamp with parallel optics

For sensory fusion to occur, the images must be… (List 3).

1. Located on the corresponding retinal areas

2. Presented simultaneously

3. Similar in Size, Brightness & Sharpness

What can disrupt sensory fusion?

→ Dissimilarities b/w the 2 retinal images can disrupt sensory fusion

• Anisometropia

• Visual differences caused by amblyopia

• Absence of motor fusion

When there’s disruption to the sensory fusion, what does the visual system resort to?

Suppression or anomalous correspondence to form the percept of unified world

Define Suppresion.

❌ sensory fusion

• e.g,  Strabismic patient who suppresses has inadequate motor and sensory fusion

Define Anomalous correspondence.

✅ sensory fusion

poor motor fusion

• e.g., Strabismic patient who compensates by anomalous correspondence

What does the Worth-4-dot test assess?

Fusion + Suppression + Diplopia

What does the Bagolini Striated Lens Tes assess?

Fusion + Suppression

• Normal - see an X

• Strabismus– streaks don’t cross at the central points

• Suppression– only 1 streak is seen

What is the Stereopsis Test (Local & Global?

3^rd degree of Fusion

List the 2 theories on how sensory fusion is possibly accomplished.

1. Alternation or suppression theory

2. Fusion theory – we can attend to similar images in both eyes at the same time

Define Alternation or suppression theory.

→ only one of the monocular images reaches consciousness at a time

• assumes “fusion” occurs due to rapid alternation of Right and Left eye views

• dissimilar images seen independently by right and left eyes (dichoptic view)

What is Binocular rivalry?

Perception alternates b/w different images presented to each eye

What is the Alternation or Suppression Theory proven to be?

False

• e.g., rxn time experiment -where a flash of light superimposed on only one or other eye’s image (sine wave grating) → short rxn time irrespective of the eye stimulated

Under what circumstances does Alternation or Suppression not occur vs occur?

Does not occur - under natural viewing conditions with similar images in both eyes (as we can experience stereopsis)

Can occur (True )- under Artificial viewing conditions (e.g., experiment) → Binocular Rivalry, but takes a long time to switch attention from one eye’s image to other

What is the Suppression Theory?

→ suggests higher-level access to information from only one eye at a time

• irregular pattern suggests local rivalry*

What is the Fusion theory?

→ “Similar images in the right and left eye can be processed simultaneously”

• processing is not successive, as in the alternating theory

• occurs under natural viewing conditions

  • but not under ALL viewing conditions

What is the Fusion Theory proven to be?

True

What are the 2 minor exceptions of Fusion Theory?

1. Binocular rivalry does occur with dissimilar images to each eye

   • Fusion = rule with similar monocular images

   • Rivalry = rule with dissimilar images

2. Even though we combine the 2 eyes info, the visual system still has some access to uncombined monocular info so that it can detect the small differences b/w the 2 eyes’ images (that gives us binocular disparity/stereopsis)

What is Panum’s Fusional Area (PFA)?

→ region in visual space over which we perceive single vision

• range of horizontal retinal disparities around the horopter within which a stimulus is perceived as single

  • narrow band around the horopter

• represents an area on one retina that can fuse with a corresponding region (not a single point) on the other retina

Define Horopter.

line or locus of points in space whose images fall on corresponding retinal areas of OD/OS

What does Panum’s area demonstrate about fusion and visual direction?

→ Fusion is not synonymous with identical visual direction

• Images can be fused and seen as single despite having slightly different visual directions in the two eyes

How does Panum’s area help maintain single vision during fixation?

→ allows some imprecision or drift in eye movements without causing diplopia

• e.g., can accommodate the uncorrelated microdrifts and tremors of the 2 eyes during fixation

How does Panum’s area reduce the adverse effects of fixation disparity?

allows fusion and single vision despite a small monocular misalignment of the eyes (fixation disparity)

What is required for an object to be perceived as single?

only need to fall within Panum’s fusional area to be fused and perceived as single (don’t need identical visual directions)

What can differences in visual direction within Panum’s area produce?

depth perception (stereopsis)

What is Panum’s Limiting Case (PLC)?

→ minimum conditions to perceive stereopsis made up of 3 lines (1 line for 1 eye, 2 lines for other eye)

• called “Limiting Case” b/c 3 targets is the min number needed to see stereopsis and removing any one abolishes depth

• can be tested vertically and horizontally

What happens when a target reaches the border of Panum’s fusional area (PFA)?

target becomes diplopicabout 50% of the time at the border of the PFA

How does the shape of Panum’s fusional area differ horizontally vs. vertically?

• 3–6× larger horizontally than vertically

  • is elliptical, partly b/c eye movements are more limited vertically

What is the size of Panum’s fusional area at the fovea?

5–20 arcmin

How does Panum’s fusional area change with eccentricity?

PFA increases with increasing eccentricity from the fovea

• beyond 5° from the fovea, it’s 6–7% of the angle of eccentricity

What effect does target exposure duration have on Panum’s fusional area?

duration of target exposure alters PFA size

Why is the effect of target orientation on PFA difficult to quantify?

due to cyclorotation of eyes + unequal retinal and horizontal disparity along the target length

How do changes in luminance or contrast affect Panum’s fusional area?

no effect

How does tolerance for aniseikonia differ between the central and peripheral retina?

More aniseikonia is tolerated peripherally than centrally

What happens when aniseikonia is present centrally?

similar or smaller image-size difference centrally → loss of binocular fusion, making central suppression more likely than peripheral suppression (e.g., in strabismus)

How are targets used during VT to train fusion on PFA?

Larger, low-spatial-frequency targets are used in the periphery first b/c the PFA is largest there

• As fusional abilities improve, smaller foveal targets are introduced

List 3 factors that can affect the size of PFA.

1. Strabismus

2. Small-angle strabismus (cause large PFA)

3. ARC (cause abnormally large PFA)

What’s unique about your dominant eye?

• activates large portion of striate cortex than the non-dominant eye

• differ at distance and near (e.g., OD at near, OS at distance)

Define Utrocular discrimination.

ability to identify the stimulated eye under binocular condition

Can people with normal binocular vision perform utrocular discrimination?

No - even when cues such as target shift and contrast differences are controlled, people with normal binocular vision are unable to determine which eye was stimulated

So, once combined into a single percept, Is each eye’s monocular information lost?

 Monocular info lost to conscious perception

• may be used internally by the visual system without our awareness

Define Dichoptic (bi-ocular). What’s a benefit to this?

→ similar or dissimilar info presented independently to each eye, while still achieving a single, unified percept

• (+) help identify which eye is suppressing info and allow clinicians to adjust stimulus strength b/w the eyes to treat amblyopia under binocular viewing conditions.

What does heterophoria represent?

fusional vergence demand

What type of fusional vergence compensates for esophoria vs exophoria?

Esophoria - NFV

Exophoria - PFV

What is heterophoria?

latent ocular misalignment that’s successfully compensated by fusional vergence, allowing single binocular vision

What happens when heterophoria can’t be compensated?

strabismus - b/c they eyes can’t achieve adequate motor fusion

What is Fixation Disparity?

→ small error in vergence (purposeful) prevents image of fixation target from falling on corresponding retinal points

• FD of more than few minutes of arc → indicator of BV problems

  • larger FD = more symptomatic the pt

• can be used to help vergence movement system compensate

  for heterophoria

How is Fixation Disparity measured?

→ measured clinically at distance or at nearpoint with simple targets (2 parts)

1. Some binocularly visible details that serve as a binocular fusion lock

2. 2 monocularly seen nonius lines (via polaroid filters)

Define Nonius lines.

→ used for precision measurements (similar to fine lines of vernier caliper)

• offset of lines is measured when binocular fusion is induced

How can you measure Fixation Disparity?

1. Wesson disparometer

   • fixation disparity card that’s tested with polarized glasses/lenses

2. Sheedy disparometer

What is the Forced Vergence Fixation Disparity Curve (FVFDC) used for?

→ uses prism-induced changes in fixation disparity to assess the relationship b/w vergence demand and fixation disparity

• predict how a pt may respond to prism or other vergence stresses, inducing PFV or NFV

What is the Type 1 FVFDC curve?

• Curve obtained by 60% (near) and 70% (distance) of people

• Symmetric

• Gradual change in fixation disparity (with addition of prism) except at extremes

• steeper central slope of the FD curve = pt will more likely be symptomatic of BV problems

What is the Type 2 FVFDC curve?

• 25% people at distance and near

• Represent intolerance of forced divergence

• Corresponds with esophoria (intolerance to divergence)

• Flat on BO side

What is the Type 3 FVFDC curve?

• 10% of people at near and 0% at distance

• assoc with high exophoria (intolerance to convergence)

• Flat on BI side

What is the Type 4 FVFDC curve?

• 5% people at distance and near

• Flat at BO & BI

  • Rare-exhibits little change in Fixation disparity

• assoc with aniseikonia and sensory fusion problems

What type of FVFDC is this graph?

Type 1

What type of FVFDC is this graph?

Type 2

What type of FVFDC is this graph?

What type of FVFDC is this graph?

Type 3

What type of FVFDC is this graph?

Type 4

What type of phenomenon is correspondence?

cortical

What is ARC?

→ adaptation of the visual system to ocular misalignment

• allows binocular vision as an alternative to suppression or binocular confusion

• cortical phenomenon assessed as a retinal misalignment

What test can be used to assess ARC?

Hering-Bielschowsky After Image Test

Define Harmonious ARC (HARC).

shift in corresponding retinal points MATCHES angle of strabismic deviation

Define Unharmonious (paradoxical) ARC.

imprecise shifting of corresponding retinal points

Define Eccentric fixation.

off-foveal point in the retina of the deviating strabismic eye is used for fixation under monocular AND binocular conditions

List 2 subjective angle tests to measure Strabismus.

1. Maddox Rod test

2. Hess-Lancaster Test

3. Amblyoscope?

When OD = Maddox rod, OS = None. What does it mean when the line is to the left vs right?

Line is to left =  Exophoria or tropia

Line is to right = Esophoria or tropia

What is the Hess-Lancaster Test consist of?

→ Red-green anaglyphic glasses for binocular dissociation

• pt superimposes red and green annuli → orthophoric pt

List 1 Objective angle tests to measure Strabismus.

1. Cover test

Define Angle of anomaly.

Difference between the subjective and objective strabismus angle measurement

List the 3 theories of ARC.

1. Sensory theory

2. Motor theory

3. Abnormal disparity vergence stimulus detection