Binocular Vision and Visual Processing Quiz

Binocular Vision Lecture Notes

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Development of Normal Binocular Vision

• Oculocentric Monocular Vision: Each point on the retina has its own visual direction, passing through the eye's nodal point.
• Principal Visual Direction: The principal visual direction is along the visual axis, where light passes through the nodal point to reach the fovea.
• Secondary Visual Direction: The secondary visual direction refers to the light passing through the nodal point to reach other points on the retina.

Visual Direction—Binocular Monocularly

• Egocentric Binocular Vision: Each eye has its own oculocentric frame of reference. Visual perception is imagined to originate from an imaginary cyclopean eye between the two real eyes.
• Two Foveae (f₁ and fR): These have identical visual directions, focusing on F—represented as a single cyclopean fovea (fc).

Finding the Cyclopean Eye

• Method: With both eyes open, point at a spot. Close the right eye, then close the left eye. The alignment of the fingers with the target suggests the dominant eye. If both are equally aligned, you have a true cyclopean eye—meaning both eyes give the same reading distance.

The Horopter

• Definition: For each focal point, all corresponding points are located on the horopter.
• Example: Point O lies on the horopter of point F.

Vieth-Muller Circle

• Definition (1818): Veith and Muller defined the circular shape of the horopter. It passes through the nodal points of each eye and the fixation point.
• Reality: The horopter is likely elliptical, not perfectly circular.

Corresponding Points

• Definition: Objects falling on the horopter have the same visual direction in both eyes, therefore at a corresponding point.
• Single Object Perception: These points are perceived as a single object.

Disparate Points

• Definition: Objects not falling on the horopter have different visual directions in each eye and are called disparate points.
• Double Vision (Diplopia): These points are seen as double. This is called physiological diplopia.

Panum's Fusial Area

• Tolerance: Each point on one retina corresponds to a larger area on the other eye.
• Single Vision: Single vision still occurs, as long as an object falls within Panum's area.
• Diplopia: Physiologic diplopia only occurs outside of this area.

Crossed Diplopia

• Location: The object lies in front of the horopter.
• Perception: The image from the right eye appears on the left side of fixation, and the image from the left eye on the right side of fixation.

Uncrossed Diplopia

• Location: The object lies behind the horopter.
• Perception: The right eye's image appears on the right side of fixation; the left eye's image on the left side of fixation.

Binocular Single Vision and Strabismus

• Adaption: Vision adaptation varies by the degree of strabismus. Students may experience diplopia/confusion, suppression, or abnormal retinal correspondence.

Right Exotropia

• Fixation: The left eye is fixated on the target.
• Strabismus: The right eye is exotropic—deviates outward from the target.
• Perception: The image falls on the fovea of the left eye, and the right eye's image is on the temporal retina, leading to the perception of the image on the left side of the right eye.

Sensory Adaption

• Three Possible Outcomes: Diplopia and confusion; suppression; abnormal retinal correspondence.

Diplopia and Confusion

• Image Fall: The retinal image of an object is on the fovea of one eye but on the peripheral retina of the other eye.
• Perception: Two images are perceived.
• Distress: Can be very distressing for patients.

Diplopia and Confusion (Confusion)

• Fovea Comparison: Different retinal images fall onto both foveas.
• Different Space Perception: These corresponding points fall into the same visual space, creating a conflict.
• Brain's Inability to Fuse: The brain cannot combine these distinct images.
• Distress: Also very distressing.

Exotropia

• Image Fall: Image falls on the fovea of the left eye. Temporal retina of right eye.
• Perception: The right eye sees the image on the left.
• Diplopia: This leads to crossed diplopia.

Esotropia

• Image Fall: Image falls on the fovea of the left eye; nasal retina of the right eye.
• Perception: The right eye sees the image on the right.
• Diplopia: This leads to uncrossed diplopia.

Hypertropia

• Image Fall: Image falls on the superior retina of the hypertropia eye.
• Perception: The eye sees the image below the fixating eye.

Hypotropia

• Image Fall: Image falls on the inferior retina of the hypotropia eye.
• Perception: The eye sees the image above the fixating eye.

What Can Be Done About Diplopia

• Adaptation: Some patients adapt by adjusting their head and neck posture.
• Prisms: Prisms can help, but aren't always effective.
• Surgery: Surgery may be necessary, but isn't necessarily helpful for restoring binocular vision. It is primarily for cosmetic benefit.
• Occlusion: If the issue is intractable, occlusion methods might be employed.

Occlusion Methods

• Occlusive Contact Lenses: A way to cover the eye to treat diplopia.
• Eyelid Occlusion: Covering the eye, often using an eye-patch or spectacle lens.
• Eye Patch: Used to cover strabismic eye.
• Botulinum Toxin: Administered to temporarily relax eye muscles.

Suppression

• Definition: Suppression occurs during binocular viewing when one eye does not get processed.
• Mechanism: The brain is suppressing the image from one eye to avoid the diplopia it causes.

Suppression of Binocular Visual Field

• Diplopia Correction: To avoid seeing double images, the brain suppresses images from one eye.
• Critical Period: This adaption occurs if the strabismus was present in childhood.

Critical Period

• Duration: Vision adaption occurs from birth to approximately age 8-10 years, a time critical for strabismus development.
• Neural Connections: Neural connections aren't fully formed or developed.
• Adaption Likelihood: Adaption is more probable within the critical period.
• Treatment Effectiveness: Treatment is more likely to be effective.

Full Suppression

• Binocular Vision: The brain ignores the non-fixating eye's image in binocular conditions.
• Depth Perception: This reduced depth perception is not ideal.

Suppression and Amblyopia

• Critical Period & Development: Proper sensory stimulation during the critical period is essential for proper visual development—including the function of both eyes.
• Suppression effect: During the critical period, suppression of one eye can lead to a reduction in visual function in that eye.
• Amblyopia Definition: Amblyopia is reduced vision owing to strabismus (misalignment of the eyes) that typically happens within the critical period of visual development.

Tests to Investigate Suppression

• Worth 4-Dot Test: Tests for binocular single vision, diplopia, or suppression using red and green glasses and dots.
• Stereopsis Tests: Tests for binocular vision or stereopsis.
• Mallet Unit: Used to observe fusion lock under various conditions.
• Bagolini Lenses: Assess stereopsis.
• Neutral Density Filters: Tests for the depth of suppression.

What to Know About Suppression

• Eye Identification: Which eye is affected?
• Area of Suppression: Where is the area of suppression?
• Pattern of Suppression: Is it constant or intermittent?

Assessment of Vision Considerations (Routine Assessment)

• Binocular Balancing: If suppression occurs, binocular balancing won't function as it should.
• Stereopsis absence: Stereopsis is not present due to the suppression and imbalance.
• Phoria Assessment: Subjective methods, like the Maddox rod, mallet unit, and Maddox wing, will not provide accurate phoria assessments if suppression is present.

Maddox Rod Testing

• Right Exotropia: In the case of right exotropia, the right eye will see the red line, while the left eye sees the spot of light.

Maddox Rod Suppression

• Suppression: The right eye, despite strabismus, will suppress the red line, only the left eye sees the spot of light on the rod.

Mallet Unit Testing

• Right Exotropia: If the patient had right exotropia and suppresses the right eye's visual field, the patient will see the two halves of the unit independently, one in each eye.

Mallet Unit Suppression Testing

• Suppression Impact: The right eye might suppress the information from the right side of the target if they have a right exotropia, causing the patient to see only the left on the Mallet unit.

Tests to Investigate Suppression (Specific Tests)

• Worth 4 dots: These are tests to investigate suppression using red/green glasses and target dots.
• Stereopsis Tests: These are tests to investigate how the eyes work together.
• Bagolini Lenses: These are corrective lenses, assessing depth perception in binocular vision.
• Neutral Density Filters: These filters are used to examine the intensity of an image, helping determine the depth of suppression.

Worth 4-Dot Test Considerations

• Setup: Red goggles over the right eye and green over the left eye.
• Test Distance: Testing is carried out at near (40cm) or more distant (6m) and under various lighting conditions.
• Patient Setup: The patient's gaze should be level; the light must be above the test target to see it without turning the head.

Worth 4-Dot Test Procedure

• Dots Visible: The displayed dots are used to check for binocular single vision in either one or both eyes.
• Number of Dots: The patient's visible dots will show the situation. Examples include:
• Four dots—normal binocular vision.
• Two dots—suppression of one eye.
• Three dots—suppression of the other eye.
• Five dots—the patient is seeing two sets of dots.

Suppression Depth

• Neutral Density Filters: To determine the depth of suppression, a neutral density filter is used to dim the light in the patient's good eye until the patient perceives only one dot in the good eye.
• Filter Application: A red filter is used near the good eye first. The visual experience with this will gradually change as the patient sees one spot, then a pink spot, then a white spot, when the intensity changes on the light source.

Stereoscopic Tests

• Total Suppression: No stereoscopic function will be detected if suppression is present.
• Partial Stereoscopic Function: If stereoscopic function is present, there is likely some degree of binocular vision.

Lang Stereotest

• Target: The target includes images of an elephant, a truck, or the moon, which are presented to the patient.
• Target Characteristics: These images have differing disparity values.

Frisby Stereotest

• Plates: These have graduated levels of disparity. The larger the distance, the more thickness and depth.
• Usefulness: The test is used to assess binocular vision and estimate the level of depth perception.

Titmus Stereotest

• Glasses: This is a test that assesses stereoscopic function and uses polarized lenses.
• Target: This test uses images projected on two screens or circles, designed to help calculate various degrees of disparity.
• Visual Cues: The test uses visual cues involving contour targets, creating a localised visual experience.

TNO Stereotest

• Pattern: Uses a random dot pattern.
• Goggles: Red and green glasses are necessary.
• Measurement: Measures disparity down to 15 seconds of arc.