What is suppression in binocular vision?
An active cortical inhibition where part or all of one eye’s visual field is ignored under binocular conditions
It is involuntary.
What is the purpose of suppression?
To eliminate conflicting visual input (e.g. diplopia)
At the cost of fusion and stereopsis.
What is physiological suppression and how is it normal?
- Occurs in normal vision
- Lateral inhibition
- Suppression of physiological diplopia
- Binocular rivalry
Balanced between eyes → no visual problems.
How does strabismic suppression differ from physiological suppression?
Unbalanced between eyes
One eye is suppressed more than the other, leading to loss of binocular function.
What happens in unilateral strabismus suppression?
Fixing eye suppresses the deviating eye
Deviating eye cannot suppress the fixing eye.
What happens in alternating strabismus suppression?
Suppression switches between eyes depending on fixation
Each deviating eye develops deep suppression when not fixing.
What causes strabismic suppression?
Develops during the critical period due to:
* Strabismus
* Anisometropia
* Aniseikonia
* Form deprivation
These factors contribute to the development of suppression.
Why does suppression develop in strabismus?
To eliminate diplopia and visual confusion caused by misaligned images
This is a compensatory mechanism.
What are the clinical signs of suppression?
- Manifest strabismus without diplopia
- Functionally monocular vision despite both eyes open
- Closing one eye in bright light
- Reduced stereopsis
These signs indicate the presence of suppression.
What is central suppression and when is it seen?
Central vision suppressed, peripheral vision fused
Leads to central diplopia with peripheral fusion; common in anisometropic amblyopia.
What is a suppression scotoma?
Area of visual field suppressed only under binocular conditions
Not present in monocular viewing.
Can suppression be overcome?
Yes — if the stimulus to the suppressed eye is strong enough
Used in clinical testing and therapy.
Why can peripheral fusion occur in strabismus?
Due to cortical magnification:
* Peripheral receptive fields are large → tolerate small misalignments
* Foveal receptive fields are very small → require precise alignment
Fusion may occur peripherally but not centrally.
What is the key requirement for sensory fusion?
Images must fall on corresponding retinal points
Misalignment must be within receptive field size.
What determines whether fusion or diplopia occurs?
If misalignment < Panum’s fusional area → fusion
If misalignment > Panum’s area → diplopia
This is based on the degree of misalignment.
Why is central fusion more difficult than peripheral fusion?
Foveal receptive fields are small and precise
Requires stable bifoveal fixation; even small deviations cause diplopia centrally.
How can suppression be tested qualitatively (binary)?
- Detects presence/absence of suppression
- Worth 4 Dot
- Mallet unit
- Stereo tests
These methods help identify suppression.
How can suppression be tested quantitatively?
- Measures depth or size of suppression scotoma
- Synoptophore (amblyoscope)
- Bagolini lenses
- Dichoptic psychophysical tests
These tests provide a quantitative measure of suppression.
Why can suppression affect Von Graefe testing?
Von Graefe requires simultaneous perception
If suppression present → no simultaneous perception → cannot measure angle S.
How does central suppression affect clinical testing?
Small targets may fall داخل suppression scotoma → not seen
Use larger targets → stimulate peripheral retina → allow measurement.
What are treatment options for suppression?
- Anti-suppression (vision therapy)
- Strabismus surgery
- Amblyopia therapy (if bifoveal potential exists)
More effective at younger ages.
What happens if a patient has no motor or sensory fusion?
Suppression may re-develop to eliminate diplopia
Or patient develops intractable diplopia.
What is fixation switch diplopia?
Occurs when:
* Previously dominant eye loses vision
* Previously suppressed eye becomes fixing
Patient can no longer suppress → diplopia appears.
What are signs of fixation switch diplopia?
- Reduced VA in previously dominant eye
- Change in fixation (deviating eye now fixing)
- New-onset diplopia in long-standing strabismus patient
These signs indicate a shift in fixation and the onset of diplopia.
