Stereopsis Testing

Key Points at a Glance

Stereopsis is the function by which the brain detects binocular disparity (the horizontal shift of images projected onto the left and right retinas) and converts it into depth perception. It is only established after fusion is achieved.
With normal stereopsis, depth perception is precise enough to discriminate an 8 cm depth difference at 10 meters.
The Titmus stereo test measures near stereopsis using polarized glasses, performed in the order: fly (3,000 seconds) → animals (400–100 seconds) → circles (800–40 seconds). Caution is needed for false positives in circles and animals.
The TNO test uses red-green glasses with random dots, has no monocular cues, and has few false positives (disparity 480–15 seconds).
The Lang Stereotest uses a cylindrical diffraction method without glasses and is suitable for infant screening.
Stereopsis development begins at 3–5 months of age and generally reaches 60 seconds of stereoacuity by around 4 years of age.
Stereoacuity remains normal until monocular visual acuity drops to 0.3, decreases at 0.2, and disappears at 0.1.

1. What is the Stereopsis Test?

Stereopsis is the function by which the brain detects binocular disparity (the horizontal shift of images projected onto the left and right retinas) and converts it into depth perception. It is only established after fusion is achieved. Fine stereopsis is enabled by foveal fusion, while coarse stereopsis is enabled by peripheral fusion.

With normal stereopsis, depth perception is precise enough to discriminate an 8 cm depth difference at 10 meters.

Clinical Significance of Stereopsis Testing

Stereopsis testing is performed for the following purposes.

Evaluation of strabismus: Assessment of binocular visual function before and after surgery and confirmation of treatment effect
Evaluation of amblyopia: Quantitative assessment of improvement in binocular visual function through amblyopia treatment
Pediatric ophthalmology screening: Evaluation of visual development and early detection of abnormalities
Postoperative evaluation: Confirmation of recovery of binocular visual function after strabismus surgery or cataract surgery
Employment and aptitude tests: Assessment of stereopsis required for precision work, driving, etc.

Units of Stereopsis

Stereopsis (stereoacuity) is expressed in arc seconds. The smaller the number, the finer the stereopsis.

Normal adults: approximately 40–60 arc seconds
High stereoacuity: 40 arc seconds or less
Coarse stereopsis: approximately 400–800 arc seconds
Practical stereopsis threshold: 200 arc seconds or less

Q Is it possible to have no stereopsis and still manage daily life?

Even without stereopsis, the world does not appear flat, and there is no major hindrance to daily activities. This is because monocular depth cues (perspective, shading, motion parallax, etc.) complement depth perception. However, it may be disadvantageous in situations requiring binocular visual function, such as precision work, sports, and driving. Since stereopsis can improve with treatment for strabismus or amblyopia, regular evaluation is useful.

2. Types and Procedures of Examination Methods

Random dot stereotest chart (FInD Stereo): Depth chart of ring and dip stimuli and psychometric function for threshold estimation

Wilkins L, et al. FInD stereo: a rapid, sensitive stereoacuity test. PLoS One. 2024;19(6):e0305036. Figure 1. PMCID: PMC11161055. License: CC BY 4.0.

A) Ring stimulus and B) dip stimulus depth charts of the FInD stereotest using random dot patterns (adaptively presenting disparity ranges from easy to difficult) and C) psychometric function for estimating stereoacuity from participant responses. This corresponds to the principle of stereoacuity measurement using the random dot method discussed in the section “TNO test (complementary color separation method).”

Simple tests: Two-pencil test and ring-on-string test

Two-pencil test is a very simple test in which the patient touches the tips of two pencils together. The patient holds a pencil and tries to touch its tip to the tip of a pencil held by the examiner at 33 cm in front of the patient. In both tests, if the patient can perform the task with both eyes but fails with one eye, practical binocular vision is considered present.

Ring-on-string test involves making a ring with a diameter of 2–3 cm, which the examiner holds, and the patient passes a key-shaped bent wire through it. Compared to the two-pencil test, this test has fewer monocular cues and requires greater perception of disparity.

Characteristics of both tests:

Natural tests that can be performed without separating the eyes
Can be passed with practice (not a strict stereoacuity measurement)
Simple and require no special equipment

Titmus stereotest (polarized glasses method)

This test measures near stereoacuity by presenting targets with disparity to the left and right eyes using polarized glasses. The target is placed at a distance of 40 cm from the eyes.

Test procedure:

Have the patient wear polarized glasses
First, present the fly with maximum disparity (3,000 seconds) and check if the patient can grasp the wings
If the fly is possible, test the three levels of animals (400–100 seconds)
Then proceed to 9 levels of circles (800 to 40 seconds of arc).

Disparity range of the Titmus Stereo Test:

Target	Disparity range
Fly	Approximately 3,000 seconds of arc
Animals	400 to 100 seconds of arc
Circles	800 to 40 seconds of arc

Note: Since monocular cues remain in the circles and animals, it is necessary to keep in mind that false positives may occur.

Q Why do false positives occur in the Titmus Stereo Test?

In the circles and animals of the Titmus Stereo Test, monocular cues (such as slight print irregularities, shadows, etc.) that are visible even without polarized glasses may remain. Therefore, false positives can occur even in patients who do not actually have stereopsis. The fly has a large disparity (approximately 3,000 seconds of arc) and fewer monocular cues, making false positives less likely. The TNO test, which uses random dot patterns and has no monocular cues, is used as a complementary test with fewer false positives.

TNO Test (Anaglyph Method)

This test uses red-green glasses to separate the two eyes and measures stereopsis with random dot patterns.

Features:

Binocular separation using red-green glasses (anaglyph method)
Random dot patterns completely eliminate monocular cues
Low false positive rate
Measurement range: 480 to 15 seconds of arc
Excellent for confirming the presence or absence of binocular vision

Frisby test (real-depth method)

This is the most natural method for measuring stereopsis using the front-back arrangement of real objects.

Features:

Does not use polarizing or complementary color filters
Measured without binocular separation
Uses real three-dimensional objects, allowing the most physiological stereopsis assessment
For near vision

Lang Stereotest (cylindrical diffraction method)

This method uses an array of cylindrical lenses to present different images to each eye, allowing assessment of stereopsis without glasses.

Features:

No need for glasses (polarized or anaglyph)
Mainly used for screening infants and children
Applicable even to young children with low cooperation
Suitable for confirming the presence or absence of stereopsis rather than quantitative measurement

Comparison of each test method:

Test method	Binocular dissociation method	Monocular cues	Measurement range	Primary target age
Two pencil test	None	Present	Not quantifiable	All ages
Lang Stereotest (Cylinder Diffraction Method)	None	Low	Not quantifiable	All ages
Titmus Stereo Test	Polarized glasses	Yes (circle/animal)	3,000 to 40 seconds	2 years and older
TNO Test	Red-green glasses	None	480 to 15 seconds	School age and older
Frisby test	None	Few	Near range	All
Lang Stereotest	Cylindrical diffraction	Few	Only rough quantification	Infants and up

3. Test selection by age and development of stereopsis

Developmental process of stereopsis

Studies using visual evoked potentials (VEP) show that binocular vision exists by 2 months of age, and fusion begins by 3 to 5 months of age. Stereopsis is also detected in the same age range (3 to 5 months). By 20 weeks of age at the latest, stereopsis is detected in more than 75% of children.

Studies using random dot cards have reported that stereopsis reaches 60 seconds of arc by 1 year and 5 months of age, but it is generally considered that stereopsis of 60 seconds of arc is achieved around 4 years of age.

Key periods in the development of stereopsis:

2 months after birth: Establishment of binocular vision
3 to 5 months after birth: Onset of fusion and stereopsis
20 weeks (about 5 months) after birth: Stereopsis detectable in over 75% of children
Around 4 years old: Stereopsis reaches about 60 seconds of arc

Recommended Tests by Age

Age	Recommended Test
Infants (up to 2 years)	Lang Stereotest, behavioral observation
Toddlers (2–4 years)	Titmus fly, Lang Stereotest
School-age children and older	Titmus full test, TNO test, Frisby test
Adults	All tests (select according to purpose)

In infancy, cooperation is low, so the Lang Stereotest without glasses and behavioral observation are easy to perform. In early childhood, the Titmus fly’s “grasping the wings” action can be done as a game. From school age onward, precise measurements like the TNO test become possible.

Precautions for Testing

Points to consider when selecting tests according to age are shown below.

For children under 3 years old, prioritize confirming the presence or absence of stereopsis over quantitative testing.
Polarized glasses (Titmus) may be difficult for young children to wear cooperatively.
The Lang Stereotest is suitable for children because the test is completed in a short time.
For adults as well, select the test according to the purpose (screening, detailed evaluation, aptitude testing).

4. Relationship between Stereopsis and Visual Acuity

Relationship between Visual Acuity and Stereoacuity

There is some relationship between visual acuity and stereoacuity, but many patients with amblyopia still show fairly good stereoacuity. When visual acuity drops below a certain level, stereoacuity also decreases sharply.

Experimental data with monocular visual acuity reduced by ND filters:

Visual acuity down to 0.3: stereoacuity remains within normal range.
Visual acuity 0.2: stereoacuity decreases.
Visual acuity 0.1: stereopsis may disappear.

From this relationship, even with moderate amblyopia in one eye (visual acuity around 0.2 to 0.3), stereopsis may be preserved. However, high-resolution stereoacuity (40 to 60 seconds of arc) is easily impaired, and only coarse stereopsis (several hundred seconds of arc) may be retained.

Relationship between amblyopia and stereopsis

In amblyopia, stereopsis is often impaired, but amblyopia treatment (e.g., patching of the healthy eye, optical correction) may improve stereopsis.

Types of amblyopia prone to stereopsis impairment:

Strabismic amblyopia: Due to misalignment of the eyes, corresponding points are not established, and stereopsis is severely impaired.
Anisometropic amblyopia: Stereopsis decreases due to reduced vision in one eye, but may recover with improvement of vision in the amblyopic eye.
Form deprivation amblyopia: When vision is blocked for a long period due to congenital cataract, etc., recovery of stereopsis may be difficult.

Q Is stereopsis preserved even with amblyopia?

It depends on the type and severity of amblyopia. Stereopsis is often maintained within normal range when monocular visual acuity is up to about 0.3. In strabismic amblyopia, stereopsis is markedly impaired due to ocular misalignment. In anisometropic amblyopia, stereopsis may recover when visual acuity in the amblyopic eye improves to 0.3 or better. Evaluation of stereopsis after amblyopia treatment (patching, prisms, surgery, etc.) is an important indicator of treatment efficacy.

5. Comparison of test methods and selection guidelines

Degree of binocular dissociation and monocular cues

In stereopsis testing, eliminating monocular cues is important to prevent false positives. Comparing tests by the strength of binocular dissociation and the scarcity of monocular cues, they can be organized as follows.

Direction of more monocular cues (higher risk of false positives):

Two pencil test → Frisby test → Lang Stereotest → Titmus stereo test (circle/animal) → TNO test (fewest)

Clinical selection guidelines:

For screening purposes (infants): Lang Stereotest, behavioral observation
Qualitative assessment (presence of stereopsis): Titmus fly, Lang Stereotest
Quantitative assessment (precise measurement of stereoacuity): TNO test, Titmus circles
To avoid false positives: TNO test (random dot method)
When wearing glasses is difficult: Lang Stereotest, Frisby test

Testing distance and distance vs. near stereopsis

Most clinical stereopsis tests are performed at near (30–40 cm). This is due to the convenience of handling the test instruments, but it does not fully reflect stereopsis in daily life (including distance vision).

Titmus Stereo Test: near (40 cm)
TNO test: near (approximately 40 cm)
Frisby test: near to intermediate distance (adjustable)
Two-pencil test: near (33 cm)

When assessment of distance stereopsis is required (e.g., occupational aptitude tests), use a distance-specific stereopsis test (e.g., Howard-Dolman apparatus).

6. Neural mechanisms of stereopsis and clinical applications

Neural mechanisms of stereopsis

The processing of stereopsis begins in the visual cortex (V1) and involves a broad visual processing system extending to V2, V3, V4, and MT areas.

Main processing pathways:

Detection of binocular disparity: In the primary visual cortex (V1), inputs from the left and right eyes are integrated, and binocular neurons that respond according to disparity function.
Conversion to depth: Through processing in V2, V4, and MT (V5), disparity information becomes conscious as depth perception.
Ventral stream (What pathway): Involved in recognition of object shape and color.
Dorsal stream (Where pathway): Involved in processing object location and motion; stereopsis is mainly processed in this pathway.

Relationship between fusion and stereopsis

Stereopsis presupposes fusion (the ability to align the images from both eyes into one). There are two types of fusion:

Foveal fusion: Precise fusion near the fovea. Basis for fine stereopsis (tens of seconds of arc).
Peripheral fusion: Fusion in the peripheral retina. Maintained even in coarse stereopsis (hundreds of seconds of arc).

In strabismus and amblyopia, fusion is impaired, so stereopsis is also impaired. After strabismus surgery, improvement in fusion function may lead to recovery of stereopsis.

Sensitivity and limitations of stereopsis tests

Stereopsis tests are useful for screening amblyopia and strabismus, but attention should be paid to the following limitations:

A single stereopsis test alone cannot definitively diagnose amblyopia or strabismus.
Even with good stereopsis, microstrabismus (monofixation syndrome) may be present.
Even with poor stereopsis, visual acuity and eye position may be normal (cases where only stereopsis is selectively impaired).
In infants, understanding and cooperation for the test are difficult to obtain, and false negatives are likely to occur.

The results of stereopsis tests should be comprehensively evaluated in combination with visual acuity tests, ocular alignment tests, and eye movement tests.

7. References

Held R, Birch E, Gwiazda J. Stereoacuity of human infants. Proc Natl Acad Sci U S A. 1980;77(9):5572-5574.
Simons K. A comparison of the Frisby, Random-Dot E, TNO, and Randot circles stereotests in screening and office use. Arch Ophthalmol. 1981;99(3):446-452.
Westheimer G, McKee SP. Stereoscopic acuity for moving retinal images. J Opt Soc Am. 1978;68(4):450-455.
Heron G, Dholakia S, Collins DE, McLaughlan H. Stereoscopic threshold in children and adults. Am J Optom Physiol Opt. 1985;62(8):505-515.
Julesz B. Foundations of Cyclopean Perception. University of Chicago Press; 1971.

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