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Questions and Answers
What is stereopsis primarily based on?
What is stereopsis primarily based on?
Which monocular cue is NOT mentioned in the context of depth perception?
Which monocular cue is NOT mentioned in the context of depth perception?
What does a larger disparity indicate in terms of depth perception?
What does a larger disparity indicate in terms of depth perception?
Which condition could cause reduced stereopsis?
Which condition could cause reduced stereopsis?
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How is stereoacuity typically measured?
How is stereoacuity typically measured?
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What visual factor has more impact on stereoacuity?
What visual factor has more impact on stereoacuity?
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What is the primary difference between stereopsis and stereoacuity?
What is the primary difference between stereopsis and stereoacuity?
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What indicates a failure of sensory fusion in stereopsis testing?
What indicates a failure of sensory fusion in stereopsis testing?
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Which of the following is NOT a reason to assess stereopsis?
Which of the following is NOT a reason to assess stereopsis?
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What is one effect of anisometropia on stereopsis?
What is one effect of anisometropia on stereopsis?
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What is the primary purpose of stereopsis tests?
What is the primary purpose of stereopsis tests?
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Which test is primarily used to assess gross stereopsis?
Which test is primarily used to assess gross stereopsis?
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What distinguishes crossed disparity from uncrossed disparity in stereopsis tests?
What distinguishes crossed disparity from uncrossed disparity in stereopsis tests?
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Which method is NOT used to create disparity in stereopsis tests?
Which method is NOT used to create disparity in stereopsis tests?
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How does the Frisby Stereotest measure stereopsis?
How does the Frisby Stereotest measure stereopsis?
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What is classified as 'normal stereopsis' in clinical terms?
What is classified as 'normal stereopsis' in clinical terms?
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Which statement is true about the Lang I and II tests?
Which statement is true about the Lang I and II tests?
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When using the Frisby Stereotest, how is disparity affected if the distance is doubled?
When using the Frisby Stereotest, how is disparity affected if the distance is doubled?
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What type of tests are suitable for preschoolers without requiring filters?
What type of tests are suitable for preschoolers without requiring filters?
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What is one of the challenges in accurately determining stereoacuity?
What is one of the challenges in accurately determining stereoacuity?
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What factor can affect the detection of strabismus and amblyopia in stereo tests?
What factor can affect the detection of strabismus and amblyopia in stereo tests?
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Which of the following tests is specifically designed with polarization techniques?
Which of the following tests is specifically designed with polarization techniques?
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What happens when filters are reversed in a stereopsis test?
What happens when filters are reversed in a stereopsis test?
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What is the purpose of the PASS test?
What is the purpose of the PASS test?
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Which method does the Mallett Unit utilize?
Which method does the Mallett Unit utilize?
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What is the main limitation of distance stereopsis tests mentioned?
What is the main limitation of distance stereopsis tests mentioned?
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At what viewing distance does the Titmus (Wirt) Stereotest take place?
At what viewing distance does the Titmus (Wirt) Stereotest take place?
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What distinguishes the Randot test from other stereopsis tests?
What distinguishes the Randot test from other stereopsis tests?
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How does the TNO test achieve dissociation?
How does the TNO test achieve dissociation?
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Which test is designed specifically for older children and adults?
Which test is designed specifically for older children and adults?
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Which of the following best describes 'testability' in the context of stereo tests?
Which of the following best describes 'testability' in the context of stereo tests?
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What factor is generally considered to contribute to the decline of stereopsis with age?
What factor is generally considered to contribute to the decline of stereopsis with age?
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What does the Randot distance test primarily assess?
What does the Randot distance test primarily assess?
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Which test requires the use of specific angled polarising filters for its assessment?
Which test requires the use of specific angled polarising filters for its assessment?
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What characterizes the TNO test plates used for screening?
What characterizes the TNO test plates used for screening?
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What is the significance of the 'lower limit of normal' in stereopsis tests?
What is the significance of the 'lower limit of normal' in stereopsis tests?
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Study Notes
Depth Perception
- Depth can be perceived through binocular disparity (stereopsis) and monocular depth cues
- Monocular depth cues include:
- Relative size of objects
- Texture gradient
- Motion parallax
- Aerial perspective
- Linear perspective
- Accommodation for near objects
- Overlapping contours
Stereoacuity
- Stereoacuity is the angular measurement of the minimal resolvable binocular disparity needed for depth appreciation.
- It is measured in seconds of arc (").
- Smaller numbers indicate better stereoacuity.
- Greater disparity results in a greater depth effect.
- Better stereoacuity allows for detection of smaller amounts of disparity, leading to smaller depth effects.
Stereopsis vs. Stereoacuity
- Stereoacuity is a measurement of the limits of disparity detection.
- It is not frequently assessed in clinical settings.
- Stereopsis is a clinical assessment of disparity detection.
- It is usually assessed in larger increments, not at precise thresholds.
Reasons to Assess Stereopsis
-
Children:
- To demonstrate the presence of binocular single vision (BSV) as an indicator of normal development.
- As a screening test to detect strabismus, anisometropia, and amblyopia.
- Reduced stereopsis may indicate:
- Failure of motor fusion (strabismus)
- Failure of sensory fusion or simultaneous perception (anisometropia and amblyopia)
- To monitor improvements due to treatment, including strabismus surgery and occlusion therapy.
-
Adults:
- In diagnosis of decompensating latent deviations, suggesting the visual system is struggling to maintain motor and sensory fusion.
- In management of decompensating latent deviations, showing whether management strategies are effective.
- Normal adult levels should be at least 40".
Factors Affecting Stereoacuity
- Monocular blur is more detrimental to stereoacuity than binocular blur, reducing stereopsis in cases of anisometropia and amblyopia.
- Stereoacuity should be assessed with the patient wearing their spectacles to ensure optimal correction at the test distance.
- Stereopsis will be reduced or absent in individuals with failure of motor fusion or sensory fusion.
Stereopsis Theory
- Stereopsis involves perceiving relative object depth based on binocular disparity.
- Inputs from the right and left retinas are combined and compared.
- Images at corresponding points (or within Panum's areas) are compared.
- Due to the interpupillary distance, there's always a slight difference in the images from each eye, as each eye sees the same object from a slightly different angle.
- The brain compares the inputs from each eye and matches details it recognizes as being the same.
- The disparity between the matched details is then used to judge depth.
Local vs. Global Stereopsis
-
Local Stereopsis:
- Contour-based.
- Contours in each image are matched, identifying disparity.
- Requires less visual perception.
- Tests include Titmus, Frisby, and Randot (in parts).
-
Global Stereopsis:
- Disparity of clusters within a relatively large stereoscopic pattern (random dot).
- Requires more visual perception.
- Tests include TNO, Lang, and Randot (in parts).
- More susceptible to strabismus.
Stereopsis Testing
- Stereopsis is typically tested clinically at near.
- Assessments can be qualitative, demonstrating evidence of stereopsis, or quantitative, providing a measure of stereoacuity.
- Tests are calibrated for a specific viewing distance assuming an average interpupillary distance (IPD).
How Stereopsis Tests Work
- Disparate images are presented to each eye, with a slight displacement of the target seen by each eye.
- If the brain detects this disparity, requiring sensory fusion of the two images, the targets will be seen in depth.
- Most tests use horizontal disparity to simulate depth.
- Some tests create disparity through different viewing distances in free space.
Stereopsis Test Results
- In clinical settings, placing the patient in a bracket is sufficient, rather than finding a precise endpoint.
- When assessing strabismus and anisometropic amblyopia, the test should have good sensitivity and specificity.
- Test results are categorized as:
- No stereopsis.
- Gross stereopsis > 300".
- Reduced, but present stereopsis: 80-300".
- Normal stereopsis: 60" or better.
Dissociation in Stereopsis Tests
- Dissociation is the separation of the images presented to each eye in a test.
- Tests with no dissociation (real depth) include Frisby, Lang two-pencil, and Howard Dolman.
- Partial dissociation methods include:
- Polarization: Titmus (Wirt), Randot, Mallett Unit.
- Red & green filters: TNO, Computerized tests.
- Optical: Lang.
Crossed and Uncrossed Disparity
- Tests can be designed with either crossed or uncrossed horizontal disparity.
- Most tests are crossed when used correctly with the appropriate filters.
- Crossed disparity generally produces slightly higher stereo values in normal and exotropic patients.
- Holding the test upside down or reversing the filters switches the disparity from crossed to uncrossed.
Tests Suitable for Preschoolers
- Tests that do not require filters:
- Frisby.
- Lang.
- Tests that require filters:
- Stereo Smile.
Lang Two-Pencil Test
- This is a qualitative test for gross stereopsis.
- The patient attempts to place a pencil on top of a pencil held by the examiner.
- Patients with stereopsis will find it easier to perform the task binocularly than monocularly.
- Those with reduced stereo will find both tasks difficult.
Frisby Stereotest
- Features a range of disparities adjustable through variable distances.
- Uses random shapes on transparent perspex sheets of different thicknesses (6 mm, 3 mm, 1.5 mm) to create different depth values.
- Shapes forming a square on one side and a circle on the other simulate depth in one of the four squares.
- Measures stereopsis from 340" to 55".
- Good for young children as it does not require filters.
- Allows for observation of eye movements.
- Sounds can enhance the test.
- Monocular cues (parallax) can demonstrate depth without stereopsis if plates are held flat against a surface.
- Standard test distance is 40 cm.
- Doubling the distance quarters the disparity.
Lang
- Uses panography dissociation with high-powered cylinders to separate images into strips.
- Does not require filters.
- Includes:
- Random dot: Utilizes Julesz principle.
- Lang I (cat, car, star): At 40 cm, measures from 1200" to 550".
- Lang II (elephant, car, star, moon): At 40 cm, measures from 600" to 200".
Stereo Smile Test (PASS Test)
- A useful screening test for young children.
- Stereo Smile II includes one demo card, one blank card, and four test cards, used at 40 cm.
- Can be used as a preferential looking test for infants and toddlers (6-24 months) by observing the direction of their gaze.
- Older children can point to the smiley face.
- Measures 480, 240, 120, and 60".
Tests Suitable for School-Age Children and Adults
- Tests that require filters:
- Local stereopsis:
- Titmus (Wirt).
- Randot (part).
- Mallett Unit.
- Global stereopsis:
- TNO
- Randot (part).
- Local stereopsis:
Mallett Unit
- Uses polarized dissociation with two rows of symbols.
- A 3Δ BO prism is used to shift the image from one eye.
- Requires motor fusion to fuse the two rows of shapes.
- Symbols vary in disparity from 10' (600") to 30" of arc.
- Viewing distance is 36 cm.
Titmus (Wirt, 1971)
- Employs partial dissociation with crossed polarizing filters and vectographs polarized to oblique meridians (45 degrees for LE and 135 degrees for RE).
- Provides both qualitative, assessing relative depth, and quantitative measures.
- Qualitative assessment (fly): ~ 3000 seconds of arc.
- Quantitative assessment (animals): 400-100 seconds of arc.
- Quantitative assessment (circles): 800-40 seconds of arc.
- The patient is asked which circle or animal from a group seems to stand out.
- A suppression check at the bottom of the 'fly' determines if either the right or left eye is suppressing, indicated by a disappearance of the target in the suppressed eye.
- Viewing distance: 16" (approximately 40 cm).
Randot
- Uses partial dissociation with polarized vectographs.
- Features both contour stimuli (some monocular cues) and random dot stimuli (no monocular cues).
- Simple geometric shapes are used.
- Provides quantitative assessments from 500" to 20" of arc.
- Viewing distance is 16" (approximately 40 cm).
TNO
- Uses partial dissociation with red and green filters (red before the left eye).
- Uses the random dot principle, eliminating monocular cues.
- Includes three screening plates (1980") and three quantitative plates (4 presentations on each plate, 2 presentations at each level, 480-15 seconds of arc).
- Viewing distance is 40 cm.
- 'Pie' plates are used for grading.
Distance Stereopsis Tests
- Most tests are designed for near.
- They are not useful for binocular vision (BV) problems at distance.
- DV tests for assessing distance stereopsis are available:
- FD2 (Frisby).
- Randot distance.
- Versions on computerized test charts.
Testability, Sensitivity, and Specificity
- The effectiveness of a stereo test can be judged by:
- Mean stereopsis values: These can vary between tests due to test design.
- Testability: The percentage of people tested who can successfully perform the test.
- Sensitivity: The measurement of false negative rates (people with a condition who pass the test).
- Specificity: The measurement of false positive rates (people who are normal but fail the test).
Normal Values (Randot)
- 3 years:
- Mean: 100".
- Lower limit of normal: 400".
- 5 years:
- Mean: 60".
- Lower limit of normal: 200".
- 7 years:
- Mean: 40".
- Lower limit of normal: 60".
- Seven-year-olds generally demonstrate adult levels of stereopsis.
- The lower limit of normal is helpful in making referral decisions.
Stereopsis and Aging
- Stereopsis generally declines with age.
- This decline might be associated with reduced visual acuity and contrast sensitivity, as well as cognitive decline.
- Elderly individuals typically perform worse on more complex stereo tasks.
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Description
Test your knowledge on depth perception, exploring concepts such as binocular disparity and monocular depth cues. Understand the differences between stereopsis and stereoacuity and how they relate to detecting depth. This quiz covers key principles vital for appreciating visual depth.
