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12. Elliott DB.Clinical procedures in primary eye care E-book. 5th ed. Elsevier Health Sciences; 2020. p.205–75.

13. Borchert M, Sadun AA. Bright light stimuli as a mask of relative afferent pupillary defects. Am J Ophthalmol. 1988;106:98–9.

14. Johnson LN.The effect of light intensity on measurement of the relative afferent pupillary defect. Am J Ophthalmol. 1990;109:481–2.

15. Belliveau AP, Somani AN, Dossani RH. Pupillary light reex. 2022 Jul 25. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.

16. Levitan P.Pupillary escape in disease of the retina or optic nerve. Arch Ophthalmol. 1959;62:768–79.

17. Thompson HS, Corbett JJ, Cox TA. How to measure the relative afferent pupillary defect. Surv Ophthalmol. 1981;26:39–42.

18. Cox TA. Pupillary escape. Neurology. 1992;42:1271–3.

19. Enyedi LB, Dev S, Cox TA. A comparison of the Marcus Gunn and alternating light tests for afferent pupillary defects. Ophthalmology. 1998;105(5):871–3.

20. Liu GT, Volpe NJ, Galetta SL. Liu, Volpe, and Galetta’s neurophthalmology. 3rd ed. Elsevier; 2019. p.417–47.

21. Yotharak P, Aui-Aree N. Correlation between clinical grading and quantication by neutral density lter of relative afferent pupillary defect (RAPD). J Med Assoc Thail. 2012;95(Suppl 4):S92–5.

22. Bell RA, Waggoner PM, Boyd WM, et al. Clinical grading of relative afferent pupillary defects. Arch Ophthalmol. 1993;111(7):938–42.

23. Miki A, Iijima A, Takagi M, et al. Pupillography of automated swinging ashlight test in amblyopia. Clin Ophthalmol. 2008;2(4):781–6.

24. Gufoni M, Casani AP. The pupillary (Hippus) nystagmus: a possible clinical Hallmark to support the diagnosis of vestibular migraine. J Clin Med. 2023;12(5):1957.

25. Friedman NJ, Kaiser PK, Pineda R.Massachusetts eye and ear infirmary illustrated manual of ophthalmology. 5th ed. Elsevier Sanders; 2021. p.267–302.

26. Kawasaki A, Mayer C. Tadpole pupil. Neurology. 2012;79:949.

27. Onwochei BC, Simon JW, Bateman JB, etal. Ocular colobomata. Surv Ophthalmol. 2000;45:175–94.

28. Lee SM, Yu YS. Outcome of hyperplastic persistent pupillary membrane. J Pediatr Ophthalmol Strabismus. 2004;41(3):163–71.

29. Miller SD, Thompson SH. Edge-light pupil cycle time. Br J Ophthalmol. 1978;62:495–500.

30. Sood AK, Mithal S, Elhence A, Maini A.Pupil cycle time. Indian J Ophthalmol. 1985;33(1):41–3.

31. Thompson HS, Franceschetti AT, Thompson PM.Hippus. Semantic and historic considerations of the word. Am J Ophthalmol. 1971;71:1116–20.

32. Sakai T, Shikishima K, Mizobuchi T, etal. Bilateral tonic pupils associated with neurosyphilis. Jpn J Ophthalmol. 2003;47(4):368–71.

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Clinical Measurement

∗

∆

https://t.me/med1917

ofStereoacuity

MichelleBuckland andNickFogt

8.1 Introduction

Depth perception can be derived from both monocular and binocular cues. The term stereopsis, used throughout this chapter, refers to depth percepts from binocular vision. Stereopsis can be used to determine the relative depths of objects and is considered the highest form of binocular cooperation [1]. Stereoacuity is the smallest amount of disparity that results in accurate estimates of relative depth. In this chapter, we discuss many of the clinical tests of stereopsis and stereoacuity currently in use. The methodology of the tests, the normative data from these tests, and the clinical signicance of these tests will be described.

8.2 History

Corresponding retinal points are those that have perceptually identical visual directions [2]. For example, in an individual with normal binocular vision, the fovea in one eye corresponds to that in the other eye. Absolute binocular retinal disparity, the stimulus for fusional vergence, is created when an object is at a different distance than that

at which an individual is xating. Relative binocular disparity is created when objects at different distances from the observer project to different sets of retinal coordinates. The relative disparity can be used to recover the relative depths of objects and is relied upon in clinical tests of stereoscopic depth perception or stereoacuity.

While many investigators have contributed to our current understanding of stereoscopic vision, only a few will be mentioned here. For a complete history, see Howard and Rogers [2]. Alhazen described corresponding retinal points. Aguilonius named and provided an early conception of the horopter, now understood as the locus of points in space that projects to corresponding retinal points (Fig. 8.1). Points on the horopter have zero (absolute) disparity. Stereoscopic thresholds for two visual stimuli are increased when these stimuli are moved further from the horopter. Referring to Fig.8.1, an equation to cal- culate the angular disparity (n) of an object (in radians) relative to the xation point is as follows:

IPD

M. Buckland (*) · N. Fogt The Ohio State University College of Optometry, Columbus, OH, USA e-mail: buckland.14@osu.edu; fogt.4@osu.edu

where IPD is the interpupillary distance, Δd is the distance between the xation point (where the eyes are converged) and the object, and d is the distance from the eyes at which the eyes are converged [2].

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Fig. 8.1 The horopter represented as the Vieth-Müller circle. Point U has uncrossed (absolute) disparity, and point C has crossed (absolute) disparity relative to the F (xation point). IPD Interpupillary distance

Later, Vieth and Müller independently described the shape of the horizontal horopter as a circle (the Vieth-Müller circle), although empirically, it is known that the shape of the horopter deviates from this circle [2]. Shortly thereafter, Wheatstone developed the mirror stereoscope and demonstrated that binocular disparity results in depth perception. Finally, Bela Julesz developed the random dot stereogram, which eliminates or greatly reduces monocular cues that could otherwise be used to deduce the presence of a disparity. The random dot stereogram (RDS) has been extensively utilized, both, in research studies and in clinical stereopsis testing.

8.3 Types ofStereopsis Tests

Table 8.1 is a list of common commercially available stereoacuity tests. Note that this list is not exhaustive. Included in this table are the test distances and the mode of stereoacuity testing, namely global (cyclopean) and/or local (contour).

For clinical stereopsis testing, there are socalled real depth or physical depth tests that use targets that physically vary in their distance from

M. Buckland and N. Fogt

the observer (e.g., Howard-Dolman stereotest, Frisby stereotest) [3]. The Frisby plates are shown in Fig.8.2. On the other hand, a second group of commonly used printed tests utilize anaglyphic or polarized stimuli to create dichoptic viewing conditions (e.g., Randot stereotest, Titmus Stereo Fly stereotest, TNO stereotest). The Randot Stereoacuity test is shown in Fig.8.3. Observers wear red/green or polarized lenses for these tests. It has been suggested that anaglyphic stereotargets may underestimate stereopsis performance compared to polarized stereotargets [4]. The results of real-depth stereotests do not match those of other types of stereotests, and it has been suggested that real-depth tests measure “different aspects of stereoacuity” compared to polarized and anaglyphic tests [5]. Another method to achieve dichoptic viewing is to use (printed) lenticular technology (e.g., Bernell Evaluation of Stereopsis Test (BEST) and Lang I-R and Lang II-R stereotests). In this method, the observer is not required to wear lenses to see the depth in the single stereogram. Finally, a modern method to achieve dichoptic separation of disparate stimuli is to use 3D shutter glasses that alternately occlude the eyes [6].

With these dichoptic viewing techniques, clinical stereotests may assess local (contour) stereopsis, global (cyclopean) stereopsis, or both local and global stereopsis. Local stereopsis is assessed using dichoptic lines or contours displaced from one another to produce a disparity relative to a binocularly fused object near the disparity stimulus. These stimuli contain monocular cues that observers could potentially utilize to detect the relative offset of the monocular images in the disparate stimulus. For example, if an observer were to move their head, motion parallax between the disparate images could provide a cue to the presence of the disparate image [7]. In addition, perfect ocular vergence is not required for tests of local stereopsis. On the other hand, global stereopsis is assessed using random dot stimuli, in which the brain must correlate matching features of a disparate shape or gure embedded in an array of noncorresponding and fused dots or lines [8, 9]. Monocular features of the disparate stimulus are theoretically invisible in random dot

8 Clinical Measurement ofStereoacuity

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Table 8.1

Test name Company Unique feature Randot Preschool

Stereoacuity Test

Randot Stereotest (also, a variation including LEA symbols)

Stereo Fly Stereotest/The Fly-S (also, a variation including LEA symbols)

The Buttery-S Vision

Buttery Test (also, a variation including LEA symbols)

Random Dot-S (1, 2)

Random Dot 3-S Vision

Preschool Assessment of Stereopsis with a Smile (PASS Test)

Lang Stereotests (1-R & 2-R)

Bernell Evaluation of Stereopsis Test (BEST)

TNO Lameris

Frisby Near Stereotest 3 plates

Common commercially available stereoacuity tests

Stereo Optical

Vision Assessment Corporation (VAC) Stereo Optical

Assessment Corporation (VAC)

Stereo Optical

Vision Assessment Corporation (VAC)

Assessment Corporation (VAC)

Vision Assessment Corporation (VAC)

Lang Stereotest

Bernell Corporation

Ootech Frisby

Stereotests

3 booklets 40cm

5 versions with various test plates

Glasses free 40cm Lang I-R:

Glasses free 40cm Dinosaur: gross

Anaglyphic 40cm

Disparity dependent on test distance glasses free

Viewing distance Available disparities Target type Technology

800–40″

40cm

40cm House Fly: gross

40cm Buttery:

40cm

30–150cm

Shapes: 500–250″ Circles: 400–20″ Animals/Shapes: 400–100″

stereopsis Circles: 800–40/20″ Animals/Shapes: 400–100″

2000–600″ Circles: 400–20″ Shapes: 400–100″

2000–700″ Circles: 800–40″ Animals/Shapes: 400–100″

Shapes: 500–63″ Circles: 400–12.5″ Shapes children: 400–100″

Shapes: 900–400″ Circles: 160–12.5″ Shapes: 400–100″ (children)

480–30″ depending on the set

1200–400″, Lang II-R: 600–200″

stereopsis Animals: 400–40″

480–60″

600–5″

RDS Printed

RDS Contour

Contour Printed

RDS Contour

RDS Printed

RDS except Lang II: star viewable monocular (2D)

Contour Lenticular

RDS Printed

Physical Depth

Polarized Vectograph

Printed Polarized Vectograph

Polarized Vectograph

Printed Polarized Vectograph

Polarized Vectograph

Lenticular Screen

Screen

Anaglyph Plate

separation of target

(continued)

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Table 8.1 (continued)

Test name Company Unique feature The Frisby

Pocket Test

Random Dot E Stereotest

Distance Randot Stereotest

Random Dot Distance Test

Frisby-Davis 2 (FD2)

Bernell Stereo Test Book

Frisby Stereotests

Stereo Optical Vision Assessment Corporation (VAC)

Stereo Optical

Vision Assessment Corporation (VAC)

Frisby Stereotests

Bernell Corporation

Disparity dependent on test distance glasses free

Disparity dependent on test distance

2 targets at each disparity

Disparity dependent on test distance glasses free

Anaglyphic 40cm & 3m Near Contour:

M. Buckland and N. Fogt

Viewing distance Available disparities Target type Technology

40–60cm

50cm–16ft

3M

3–6m

340–150″

504″–52″

400–60″

400–63″

200–5″

1500–40″ Far Contour 180–60″ Random Dot: 800–25″

Physical depth

RDS Printed

Physical Depth

RDS Contour

Plate separation of target

Polarized Vectograph

Plate separation of target

Printed Anaglyph

Fig. 8.2 Frisby Near Stereotest

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Fig. 8.3 Randot Stereotest

stereograms, although there is some evidence that monocular cues may still be available [10,

11]. Tests of global stereopsis require very accu-

rate oculomotor (vergence) alignment (i.e., bifoveal xation) to detect the depth of the disparity stimulus [11].

8.4 Neurophysiology ofStereopsis

Many areas of the brain respond to correlated random dot stereograms and can therefore be considered to support stereoscopic depth perception [12]. Both dorsal and ventral portions of the brain are involved in stereopsis, and brain areas V3/V3a are important in processing random dot stereograms. The literature suggests that neural substrates for global and local stereopsis are different. For example, disparity detectors in the primary visual cortex are thought to be important in processing global stereopsis but may not be as signicant in processing local stereopsis [13].

8.5 Advantages ofStereopsis

Binocular viewing is advantageous for some tasks compared to monocular tasks. However, these advantages are not necessarily always correlated with stereopsis. Conversely, there is

emerging evidence that stereopsis provides specific advantages in tasks such as hand-eye coordination and adapting to terrain changes while walking [14]. O’Connor and colleagues found that individuals with no global stereoacuity performed worse on fine motor tasks compared to peers with normal stereoacuity [15]. Finally, reduced stereopsis may contribute to a greater risk of falls in elderly individuals [16].

Some occupations, such as surgeons, pilots, or architects, may require the applicants to demonstrate a particular level of stereoacuity for licensure. These requirements are supported to some extent by studies such as that of Burgess etal. who found that stereoacuity worse than 120″ was correlated with reduced surgical performance on an ophthalmic surgical simulator [17]. Al-Saud etal. demonstrated that dentists performed better when stereoscopic cues were available [18].

An emerging area of study is dynamic stereopsis. This is the ability of individuals to extract motion-in-depth information from changes in retinal disparity over time and changes in the retinal image velocity between the two eyes of an approaching object. Dynamic stereopsis could be of value, for example, in avoiding or intercepting objects, as in driving or sports. It has been shown that individuals who lack static stereopsis may be able to detect the motion of an object in depth [16].

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M. Buckland and N. Fogt

In summary, studies suggest that stereoscopic depth perception enhances an individual’s quality of life and that recovering stereopsis (in the case of stereoblind individuals) and improving stereopsis (in individuals with reduced stereopsis) are, therefore, worthwhile goals. Susan Barry describes a particularly compelling example of the impact of obtaining stereopsis in adulthood in her book entitled “Fixing My Gaze.” [12].

8.6 Clinical Usefulness

ofStereoacuity Measurements

Performance on clinical tests of stereopsis can be considered the “best indicator” [19] or the “gold standard” for binocular function, as stereoscopic depth discrimination generally requires good vision, good eye movement control, and properly functioning binocular neural processing [20]. Thus, while stereoscopic discrimination thresholds in individuals with a well-functioning visual system are very low (less than 10s of arc), these thresholds are very sensitive to and can be elevated by a variety of visual issues.

Saladin has summarized the ocular issues that can negatively inuence stereopsis [19]. Saladin’s list includes reduced contrast sensitivity or blur from uncorrected refractive error or amblyopia, uncompensated horizontal or vertical heterophoria resulting in xation disparity, suppression, aniseikonia, fatigue, and age.

Given this extensive list of conditions that could potentially impact stereoscopic thresholds, it seems that tests of stereopsis can be used to screen for several visual disorders. However, some studies doubt the usefulness of stereoacuity testing as a screening tool. For example, Ohlsson etal. concluded that commonly used clinical stereoacuity tests (Lang II, Frisby, Randot, Titmus Stereo Fly, TNO) are inadequate in screening for amblyopia and strabismus [21], and Richardson etal. reported that stereoacuity measures in preschool children were not useful in screening for unilateral vision impairment [22]. On the other hand, there is also evidence to support the efcacy of stereoscopic tests in screening for vision

disorders [23–25] when performed in combination with other tests [26–28]. Prior to discussing these studies, it should be noted that visual decits are more likely to negatively affect global stereopsis compared to local stereopsis [7]. Schmidt concluded that the Random Dot E distance stereotest held promise as a (perhaps singular) screening test for detecting vision problems (refractive error, ocular coordination, visual acuity) in preschool children [23]. Later, the Vision in Preschoolers (VIP) study group reported several ndings related to screening preschool children for visual abnormalities. In one study, this group concluded that in screening preschool children for targeted disorders, including amblyopia, strabismus, signicant refractive error, and unexplained reduced visual acuity, stereopsis tests (Random Dot E and Stereo Smile II) were less sensitive than refractive and acuity measures [27]. In the same study, a direct comparison of the Random Dot E and Stereo Smile II tests showed that the Stereo Smile II test was somewhat more sensitive in detecting vision anomalies. In 2014, the VIP study group demonstrated that preschoolers with no measurable stereoacuity as assessed with the Stereo Smile II stereoacuity test (now termed the PASS or Pediatric Assessment of Stereopsis with a Smile stereoacuity test; range of disparities 480–60s of arc) were 16.2 times more likely to have a (VIP) targeted vision disorder [26]. In that same study, preschool children who were detected to have a target disorder had a median stereoacuity of 120 s of arc compared to a median value of 60s of arc in children with no target disorder. In 2015, an expert panel published screening recommendations for children 36–72months of age [28]. These screening recommendations were directed at detecting amblyopia, strabismus, and signicant refractive error. The panel concluded that while tests of monocular visual acuity and instrumentbased measures of autorefraction are considered “best practice” screening methods, adding the Stereo Smile II stereotest (i.e., the PASS test) (Fig. 8.4) to these best practice methods may improve the detection of vision anomalies (particularly strabismus).

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Fig. 8.4 Preschool Assessment of Stereopsis with a Smile 2 (PASS 2 Stereotest)

Lastly, stereopsis is reduced to a greater extent in strabismus than in anisometropic amblyopia [14], and this agrees with some (but not all) experimental results demonstrating that screening for strabismus with stereoscopic tests is more effective than screening for anisometropic amblyopia with these tests [21, 25].

As alluded to previously, stereoacuities obtained by different stereotests generally do not match. These discrepancies can be at least partially attributed to differences in the type of disparity employed (global or local) [29], or differences in the method used to achieve dichoptic separation (anaglyphic versus polarized) [7], or differences in the method by which the stereoscopic thresholds are assessed (e.g., 2 or 4 alternative forced choice) [30]. Measures of stereopsis can be used to assess the outcome of clinical interventions such as refractive correction, orthoptic training [19, 31], and strabismus surgery [16], so it is important that practitioners use the same stereotest in the pretreatment and posttreatment phases. An additional reason for the differences in results between stereotests may relate to the test–retest variability of these tests [11, 12, 32–34]. Values for reliability assessed in this way can be rather low, making it difcult to assess changes in stereopsis brought about by ophthalmic treatment.

8.7 Technique

8.7.1 General Clinical Instructions forStereoacuity Testing withCommon Static Stereotests

The practitioner should follow the manufacturer’s instructions for each test. General instructions for practitioners in testing stereoacuity with typical static clinical tests include:

1. Provide proper visual correction for signi-

cant refractive error, as blur from uncorrected refractive error inuences the testing results. Clinically stereoacuity is often performed before refraction. If stereoacuity is absent or reduced initially and a large change in refractive error is subsequently measured, then the stereoacuity testing should be repeated with the best vision correction. This can often occur during the same appointment for older children and adults. For younger children, where inattentiveness could inuence the stereopsis test, stereoscopic testing may occur at another appointment.

2. Provide proper illumination by pointing a

stand lamp at the test plates for near stereoacuity tests. Ensure that there are no shadows or glare on the test plates.

3. Hold the test booklets directly before the

patient and ensure that the vergence posture is appropriate for the test distance.

4. Hold the test still and instruct the patient not

to move their head during testing. Holding the test booklet and the head still helps to avoid monocular parallax.

5. For patients with intermittent strabismus,

measure stereoacuity before any dissociating test, such as monocular visual acuities or cover test [35].

6. For patients with strabismus, perform stereo-

acuity testing at the distance at which the strabismus is manifest [36]. Distance stereopsis, for example, is a more useful measurement in cases of divergence excess exotropia than is near stereopsis [37]. For those with normal

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M. Buckland and N. Fogt

binocular vision, stereoacuity does not tend to vary at different testing distances [38].

7. Because most stereoacuity tests are crossed disparity tests, ask the patient to identify or match the target/gure that is “closer or popping out.”

8. For the Titmus Stereo Fly stereotest, to improve condence that patients are actually seeing the depth from the disparity, consider rotating the test booklet to reverse the direction of the disparity or compare the results with glasses that result in different images for the two eyes with the results with glasses that only allow for the same image in the two eyes [16].

8.7.2 Specic Aspects ofClinical

Stereoacuity Testing

Stereopsis develops in children as young as 3months and can reach adult-like levels between 5 [39] and 12years of age [40]. Normative stereoacuity data in children can vary according to the clinical stereotest and the patient’s age. A larger interpupillary distance may also be correlated with better stereoacuity [2]. For example, a group of age-related values has been reported by Birch etal. for the Randot Preschool Stereoacuity test (Fig.8.5).

The Randot Preschool Stereoacuity test has a high completion rate in preschool patients [41]. However, Read et al. have reported that about one-half of children with binocular issues could

pass the test. The test is no more sensitive in detecting amblyopia than detecting strabismus [42]. The means and lower tolerance limits (dened as the disparity value 2 standard deviations from the mean and above which 95% of the population “would be expected to perform” in a particular age-group), as reported by Birch etal. for the Randot Preschool Stereoacuity test (fourbook version), are as follows [43]:

• 3years:

• 4years:

• 5years:

• 7–8years:

• 11–18years:

100″ Lower limit 400″ 100″ Lower limit 200″ 60″ Lower limit 200″ 30″ Lower limit 60″ 30″ Lower limit 60″

These mean values are similar to the age normative values for stereoacuity summarized by Reading et al. who reported norms of approximately 150″ for ages 3–4 years, 70″ for ages 4–5years, and 40″ for ages 5–8years on various stereoacuity tests [3].

Of course, normative stereoacuity values that can be applied across different stereotests are difcult to establish. In addition to the aforementioned differences in test design, tests vary in the maximum and minimum disparities included. Further, differences between children also factor into the choice of stereotest administered to a particular patient. For example, a younger child may resist wearing stereoacuity spectacles; thus,

Fig. 8.5 Randot preschool stereoacuity test

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a glasses-free test such as the Lang Stereotest (Fig.8.6) may be preferred, although elimination of monocular cues with spectacle-free tests cannot be guaranteed [11].

For monitoring improvement in binocularity with interventions such as amblyopia treatment, refractive correction, or orthoptic therapy, ideally, a stereotest should contain adequate disparity levels to assess changes resulting from treatment [25]. However, assessing changes in stereopsis that result from interventions in young children can be difcult, at least partially because of issues around testability. Testability is broadly dened as a measure of how readily children can complete a test of stereopsis. In general, testability shows a large range of values (31–81% as calculated by O’Connor and Tidbury) until about 5years of age [16].

Assuming normal binocular function for older children and adults, stereoacuity can often be measured to near threshold values. Saladin suggests that achieving good scores on stereoacuity testing (15–20″) is indicative of a well- functioning accommodation and disparity vergence system and that threshold values >40″ are suggestive of visual dysfunction [19].

As a guide for stereoacuity in adults, Piano etal. published normative ranges and upper limits of normality for 16–40-year-olds on several stereoacuity tests [44]. These values were as follows:

• Preschool Randot

• Frisby

• Distance Randot

• Frisby-

Davis- 2

• TNO

Median 30″

Median 20″

Median 60″

Median 10″

Median 60″

Upper limit of normality 70″

Upper limit of normality 40″

Upper limit of normality 160″

Upper limit of normality 25″

Upper limit of normality 120″

While the list of potential stereoacuity tests is extensive (Table 8.1), a practitioner survey addressing those stereotests used in the British Isles and the USA and Canada found the Frisby stereotest is the most used test in the British Isles for both younger and older pediatric cohorts while the Titmus Stereo Fly and Randot circles/ animals were more commonly used in the USA and Canada [20]. The difference in preferred stereotests is likely secondary to disparate recommendations from professional organizations in different geographic areas. Global stereopsis indicates that a patient is bifoveally xating and that constant strabismus is unlikely [11, 45], and local stereoacuity thresholds can be more indicative of peripheral stereopsis or may be subject to measurement artifacts because of monocular cues [8, 11]. In summary, when choosing a stereoacuity test, the clinician must decide which test is appropriate for the situation with regard to patient demographics, the goal of the stereoacuity testing (presence or absence of stereopsis vs. threshold testing), and the distance of testing.

Fig. 8.6 Lang Stereotest I and II

8.8 Advancing Technology

Computerized stereo tests continue to be developed. The Senaptec Sensory Station Depth Perception (https://senaptec.com) and the M&S Smart System Tablet (https://www.mstech- eyes.

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