2) Alternatives to Fusion

What is binocular fusion?

Two eyes, two images, one single percept

What is motor fusion?

Fusion based on eye movements

What is sensory fusion?

Fusion performed by the visual cortex

What does sensory fusion require?

similarity b/w the two monocular images

What are the 4 alternatives to fusion?

1. Binocular confusion

2. Suppression

3. Rivalry

4. Luster

Define Binocular confusion.

Dissimilar images on corresponding retinal points appear at the same location → incorrect percept of different objects simultaneously occupying same location

Why does binocular confusion occur?

Grossly dissimilar images can’t be fused into one percept

Define Binocular suppression.

→ Ignoring all or part of one eye's image to avoid conflicting info

• arises from binocular interaction that suppresses some monocular info from binocular percept

What happens to sensitivity and reaction time in suppressed regions?

↓ Sensitivity = ↑ thresholds = ↑ rxn times

How long can suppression take to begin?

~75–150 ms (doesn’t instantly with diplopia and confusion)

What happens with similar vs. dissimilar target orientations?

Similar: fusion

Dissimilar: one eye may be suppressed

When does local suppression occur?

only under binocular viewing conditions

What is Physiological suppression?

→ Normal suppression that prevents diplopia

• e.g., suppressing objects beyond Panum's area during fixation

What is pathological suppression?

adaptation to prolonged diplopia

Which eye's image is more likely suppressed when inputs are unequal?

weaker or less salient image

Which image properties increase likelihood of suppression?

• Dim

• Low-contrast

• Blurred

• Stationary

• Peripheral images

Which image properties resist suppression?

• Bright

• High-contrast

• Sharply focused

• Moving

• Foveal images

Define Binocular rivalry (Alternating perception).

→ Alternating suppression when dissimilar images stimulate corresponding retinal areas

• likely when competing images have similar salience or clarity

• one eye's image is visible while the other's is suppressed, then they alternate

• Note: Simultaneous excitation of corresponding retinal areas by dissimilar objects leads to confusion → RETINAL rivalry

What is a requisite for re-establishment of binocular vision?

Return of retinal rivalry

Why do patients rarely notice normal rivalry?

occurs in peripheral vision

What visual features can undergo rivalry?

• Brightness

• Colour

• Contours

What is exclusive dominance?

→ One entire image dominates

• typical of small targets

What is Mosaic dominance?

→ Different local regions alternate dominance

• typical of large targets

What are spatial zones of binocular rivalry?

→ Independent local regions undergoing rivalry

• similar to cortical hypercolumn receptive fields

What does frequent rivalry of an attended object suggest?

Abnormal rivalry, possibly ocular misalignment

What is the clinical synonym for binocular rivalry?

Confusion

How long does each eye dominate with equally strong stimuli?

Approximately 50% of the time each

What happens when one rivalrous stimulus is stronger?

it dominates > 50% of the time

What is Da Vinci stereopsis?

→ Distant object partially occluded by nearer object

• near object and part of distant object fall on corresponding points

• lack of matching points in each eye’s view = relative depth cue

What is dichoptic masking?

target in one eye becomes less visible due to a mask in the other eye

• this is not the same as rivalry

When can monocular suppression occur?

During saccadic eye movements

Do rivalry and monocular suppression necessarily share mechanisms?

No - similarities don’t prove shared mechanisms

Is the exact neural location of binocular rivalry known?

No. It is thought to be higher-level, but its exact locus is unclear.

Define diplopia.

Seeing 2 images of 1 object in different visual directions (noncorresponding or disparate retinal points)

What distinguishes diplopia, confusion, and rivalry?

Diplopia: 1 object seen 2x

Confusion: 2 objects at 1 location

Rivalry: alternating suppression

What is the main determinant of fusion, suppression, or rivalry?

Similarity of the targets presented to each eye

What occurs when one eye sees a contour and the other a blank field?

zone of suppression forms around the contour

What happens to small vs. large rivalrous contours?

Small = often entirely suppressed

Large = undergo rivalry

What happens when orthogonal single contours are viewed binocularly?

An X appears with a suppressed zone near its crossing

How does stimulus strength affect suppression and rivalry?

Stronger stimuli = more complete suppression + longer dominance

How can a dominant contour affect the fellow eye's background?

it suppresses adjacent bg and replaces it with the dominant eye's bg

Define binocular luster.

→ shimmering, metallic percept from similar contours with different luminance, color, or contrast polarity

• specialized form of rivalry

What does luminance luster resemble?

A shimmering chrome or silvery reflective surface, not average gray.

Does binocular luster prevent stereopsis?

No - stereopsis can remain, even with opposite-contrast stereograms

How is binocular luster used clinically?

In vision therapy with red-green targets, luster suggests fusion

What is the Inhibitory-interaction theory of suppression?

→ Dominant-eye neurons inhibit nondominant-eye neurons

• in the LGN

What is the Feedback-loop theory of suppression?

Cortical feedback to the LGN gates attention and suppressed-eye info

What is the Lateral-connection theory of suppression?

Intracortical connections b/w cortical cells modulate suppression

What do tilt and spatial-frequency aftereffects suggest about rivalry?

Rivalry involves cortical mechanisms after orientation and spatial-frequency processing

What is Helmholtz's theory of rivalry?

Attention alternates between one eye and the other

What is Hering's theory of rivalry?

eye with more contours in its image dominates

What is the Reciprocal lateral inhibition model?

Monocular cortical cells inhibit each other before converging on binocular cells

What is the Comparator-neuron model?

comparator monitors output that alternately gates left- or right-eye input

How do brief presentations and blinks affect rivalry?

Brief presentations can prevent rivalry

Blinks can interrupt rivalry

What does the effect of blinks suggest about rivalry timing?

Rivalry is not instantaneous, it takes time to develop

Which retinal region tends to dominate: nasal or temporal?

Nasal retina

Why may esotropes develop amblyopia more often than exotropes?

Esotropes

• non-deviating eye's foveal image rivals an image forms on nasal retina of the turned eye

• since nasal retina is dominant → severe binocular confusion with the fellow fovea, forcing deep suppression → amblyopia

.

Exotropes

• image falls on the temporal retina → weaker rivalry/confusion with the non-deviating fovea → amblyopia less common

Why does monovision CL correction work?

→ sharp image suppresses the blurred image

• one eye is corrected for near

• dominant eye for distance

How can motion help break suppression?

Moving images are more salient and tend to dominate stationary images

List 6 clinical technique to measure suppression.

1. Worth four

2. Suppression scotoma (4 BO test)

3. Phoria Testing

4. Vergence Ranges

5. Vectographic VA slides

6. Bagolini lenses

What can Worth four-dot show during fusion?

Rivalry or luster

How does Worth four-dot help estimate suppression area?

Suppression at:

• 13 inches - suggests a large area

• only at distance - suggests a smaller area

What is the benefit and cost of suppression in strabismus?

(+) Avoids diplopia/confusion during navigation

(-) sacrifices binocular vision

Where does a suppression scotoma extend in esotropia?

From fovea to eccentric fixation point

How does the suppression scotoma compare in exotropia?

more extensive

What determines suppression scotoma size in strabismus?

proportional to angle of dev

What visual ability is related to suppression scotoma size?

Degree of stereopsis

What can deep, constant suppression lead to?

Amblyopia

Which prism test can reveal suppression?

4 prism BO test (suppresion scotoma)

What should occur during prism-dissociated phoria testing?

Diplopia = no suppression

No diplopia = suppression

What does normal binocular vision do during vergence testing?

Maintains a fused target through vergence movements

What happens during vergence testing if there is suppression?

→ A single image moves toward the prism apex

• Motion direction identifies the suppressing eye (eye seeing the motion = seeing eye)

How do vectographic visual acuity slides detect suppression?

→ Each eye sees different letters

• omitted letters identify the suppressed eye

What is the normal, strabismus and suppression Bagolini response?

Normal = 2 streaks crossing centrally as an X

Strabismus = streaks don’t cross at the central point

Suppression = 1 streak is seen

How can Bagolini lenses grade suppression depth?

Combine them with neutral-density filters

List 4 methods to break suppression.

1. Brief dominant-eye patching

2. ND filter

3. Blurred/frosted lens

4. Motion before the suppressing eye