Most primary visual sensory responses are monocular
When an observer moves, different retinal receptors are stimulated when viewing an object and yet the object does not appear to move

This is due to ‘image stabilisation’ at the cortical level

Normal retinal correspondence: required for BSV

Retinal elements that share a common subjective visual direction
Equal distance from the two foveal centres
Connected to the same areas of the cortex

Horopter: imaginary line in space connecting corresponding retinal points with no binocular disparity.

Panum’s area: region bordering a horopter.{' '} Images in this area are seen as single despite falling on disparate retinal elements.

Without Panum’s fusional area, we experience physiological diplopia.
Takes the shape of an ellipse: greater disparities can be fused in the horizontal compared to the vertical
Panum’s area is narrowest at fixation and broader in the periphery

Fusional amplitudes

The degrees of retinal disparities outside of Panum’s area that can be overcome by eye movements (motor fusion)

Visual direction: stimulus falls on corresponding retinal units and is perceived as being in the same direction regardless of which eye sees it

Requires functioning extraocular muscles
The visual direction is perceived relative to the fovea
When the visual axis of the foveas intersects, binocular fixation is achieved

Worth’s classification of binocular vision (in order of increasing quality)

Grade I: simultaneous macular perception. Visual cortex can perceive separate stimuli from the eyes simultaneously.

Often absent in amblyopia

Grade II: sensory fusion. The two images are fused with some capacity for superimposing the images despite obstacles eg. eye movements/object movements (so-called motor fusion).

Required for BSV
Occurs in the visual cortex

Grade III: stereopsis. Images from each eye are blended to achieve higher perceptual synthesis.

Achieves appreciation of depth
Images fall on disparate retinal areas but within Panum’s fusional area, otherwise diplopia occurs

Dichoptic presentation: different images in the same retinal area helps development of binocular rivalry

Different contours presented to corresponding retinal elements makes fusion impossible and causes confusion
To remove the confusion, the image from one eye is suppressed leading to dominance of the other eye.

Retinal rivalry is necessary for binocular vision

Suppression: this is a neuro-physiological mechanism to prevent confusion or diplopia (due to simultaneous stimulation of non-corresponding retinal elements).

Foveal suppression prevents confusion
Extrafoveal suppression prevents diplopia
Obligatory suppression continues even under monocular conditions resulting in reduced VA (amblyopia)

Driving standard field of BSV: 20 degrees above and below horizontal and 60 degrees either side of the vertical
Normal BSV: bifoveal
Anomalous BSV: images are projected from the fovea of one eye but an extrafoveal area of the other eye.

The visual direction of the retinal elements has changed (manifest strabismus) and there is abnormal retinal correspondence (ARC)
This is an attempt to regain binocularity: the one fovea and the extrafoveal point share a common subjective visual direction

When the normal eye is closed, the fovea of the other eye retains control over the extrafoveal element and in this monocular condition, central fixation is retained by the fovea. This is the principle underlying the cover test.

Confusion vs diplopia

Confusion: different images stimulating corresponding points (images on top of each other) eg. different image on the two foveas
Diplopia: same image stimulating non-corresponding points eg. same image on fovea of one eye and extrafoveal region of other eye.
Central suppression: almost always present. Prevents confusion by suppressing one foveal image
Peripheral suppression: occurs in undeveloped visual systems. Prevents diplopia by suppressing the extrafoveal image.

Adults with new strabismus complain of diplopia rather than confusion

Normal fusional reserves

Normal fusional reserves
	Near	Far
Convergence	25 PD	15-20 PD
Divergence	9-12 PD	6-10 PD
Vertical	2-3 PD
Torsional	2-3 degrees

## Stereopsis

The perception of distance/depth in the third dimension of space using binocular disparities.

Note: not required for binocular single vision.
Occurs in the cortex and requires fusion of images within Panum’s area.

Poor beyond 20 degrees from the fovea (Panum’s area is narrowest here)
Objects on the horopter are seen as flat (stereoacuity of zero) because the image projects to corresponding retinal regions with no horizontal disparity.

Stereoacuity increases approaching the horopter but is zero on the horopter itself

Varies with object size: disparities will be more easily detected in larger objects

Requirements:

Images have some corresponding points on both retinal
Some proportion of points are non-corresponding (“binocular disparities”).

These points also need to be fused

Convergence contributes to stereopsis

Stereoscopic acuity: the smallest binocular horizontal disparity that can be detected as depth

Normal stereoacuity is about 60 arcseconds or better
Maximum stereoacuity is within the macula
A stereoacuity of better than 250 arcseconds is thought to exclude significant amblyopia

## Adaptive mechanisms

Suppression: remove one image causing either confusion (central) or diplopia (peripheral)

Leads to amblyopia if not treated

Abnormal retinal correspondence: cortical process to remap non-corresponding retinal points to produce single perception.

Allows BSV despite a manifest deviation

Abnormal head posture: behavioural mechanism to move image into a more useful position

Microtropia

Demonstrates adaptive measures
A small manifest deviation with BSV achieved through abnormal retinal correspondence, eccentric fixation and a central suppression scotoma
May be no movement on cover test (see Part 2 package for further exploration of microtropia)

## Monocular depth cues

Some perception of depth is possible without BSV
Illumination
Relative position of objects situated at different distances

## Diplopia

Sensation caused by stimulating two points outside Panum’s area
Physiological vs non-physiological (heterotropias)
Heterotropias: manifest deviations

May be horizontal, vertical or torsional

Heterophorias: latent deviations

## Tests of stereopsis

Frisby:

Three clear plates of different thicknesses
Four squares on each plate with one containing a hidden circle printed on the back surface
The test is three-dimensional
Viewed at 40cm generally
Measures a range from 15-600 arcseconds by altering the viewing distance

Random dot stereograms

Lang stereotest

Fine vertical lines viewed through cylindrical lens elements
Displacement of the random dot images creates disparity between 550-1200 arcseconds

TNO stereotest: computer generated random dot anaglyphs

Anaglyphs: stereogram in which disparate views are printed in red and green respectively on a white background
Viewed through red-green spectacles
The eye behind the red lens sees the green picture as black and vice versa
The two views can be fused
Range from 15-450 arcseconds

Linear polarisation stereotests: require polaroid filter glasses so that each eye views a disparate image emitting vertical or horizontal light

Titmus test

Vectographs: two superimposed views presented so that the light is polarised. The light from each image travels at right angle to that of the other
Viewed through a polarising visor or spectacles at 40 cm
Wirt fly tests for gross stereopsis: if the wings of the fly are perceived standing out from the body
Range from 40 to 3000 arcseconds with different plates

Synoptophore:

Measures all aspects of binocular single vision

Simultaneous perception
Fusion
Stereopsis, but cannot quantify it
Can measure the degree of any misalignments
Detects suppression and abnormal retinal correspondence

If the patient can see both images: simultaneous perception
“Put lion in cage”: patient can adjust angle of one eyepiece to create a subjective measure of angle of deviation
Examiner can also measure objectively by corneal reflections or alternate cover test
The angle of anomaly (AOA) is the objective angle (OA) minus the subjective angle (SA)

Normal retinal correspondence: SA = OA and AOA is 0
Unharmonious ARC: SA < OA and SA > 0
Harmonious ARC: SA = 0 so that the AOA = OA

## Tests of suppression

Worth’s four dot test: subjective test of retinal correspondence

Four circular lights typically mounted in the Snellen light box
Patient views them through red-green goggles creating partial dissociation

Red goggle in front of the right eye

Two green, one red and one white light
Right eye sees the red light and left eye sees the green
Four lights seen

Normal retinal correspondence, or;
Abnormal retinal correspondence (in presence of a heterotropia)

Two red lights seen (red and white): left suppression
Three green lights seen (two greens and white): right suppression
Five lights seen (two red and three green): diplopia with no BSV.

Two red lights to the left of the three greens: exotropia
Two red lights to the right of the three greens: esotropia

Synoptophore
Friend test
Amsler grid
Bagolini’s striated glasses
4PD base out prism test

When placed in front of the suppressed eye, no movement is detected (eg. in microtropia)
When placed in front of the normal fellow eye, it moves towards the apex (normal response)

The suppression eye also then moves as a conjugate movement
But there is no re-fixation movement of the affected eye