Binocular Vision

• 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

• 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

• 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. 

• 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

• 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

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

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

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