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The LANG-STEREOTEST® is a popular natural stereo test that has been used since the 1980s. It is characterized by its ease of use and can be performed with very young children. The sensitive random-dot stereo test is available in two versions and was revised in 2020 to improve print quality and to reduce monocular clues. The aim of this prospective study was to validate the revised versions in a non-clinical sample.
Material and methods
Testing was carried out in randomized order using either an original test version (LANG I‑O or LANG II-O) or a revised test version (LANG I‑R or LANG II-R), first monocularly and then binocularly. The other test version (original vs. revised) was then offered and participants had to indicate whether this test version was easier, equally difficult, or harder to recognize. An orthoptic screening (including visual acuity, autorefraction, cover test, and Titmus® Stereotest) concluded the examination. The entire test procedure was carried out under standardized conditions.
Results
In total, 56 eye-healthy adults were included. For both LANG-STEREOTEST® I and II and the respective revised versions, we found no relevant differences in recognizability when tested binocularly. When tested monocularly, we found similar results for LANG I‑O and I‑R (100% vs. 93% negative) and for LANG II‑O and II‑R (both 93% negative). Overall, 61.5% of the participants found that LANG I‑O was harder to recognize than LANG I‑R, and 56% found LANG II‑O was easier to recognize than LANG II‑R.
Conclusion
The revised versions are harmonized and differ less in brightness than the original versions I and II. In a non-clinical adult sample, LANG I‑O and LANG I‑R, as well as LANG II‑O and LANG II‑R, were valid. The results of the original LANG-STEREOTEST® versions I and II are comparable to the revised versions. In addition, monocular clues are reduced, at least for LANG II‑R.
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Introduction
The LANG-STEREOTEST® (LANG-STEREOTEST AG, Küsnacht, Swizerland) is a well-established stereo test based on the cylinder grid method and the random-dot method [1]. It is particularly suitable for the examination of young children, as no glasses are required (natural stereotest; [2‐4]). In addition, a non-verbal examination is possible, because eye movements can be observed while the child is looking at the objects [5‐7]. These tests are frequently used in Europe [3, 4, 8], as they are highly specific and sensitive random-dot stereotests (for an overview, see, e.g., [9]).
In everyday use, it was particularly noted that original LANG II test version (LANG II‑O) was brighter and often easier to recognize than the original LANG I test version (LANG I‑O). In 2020, the LANG STEREOTESTs® were revised. According to the manufacturer, the revision improved the optical and printing quality of the tests and aimed to harmonize LANG I and LANG II. A second aim was to reduce monocular clues due to high print quality and to achieve almost freedom from monocular clues in the case of correct application. Further pleasant features for the examiner are printed on the backside of the test: a mirrored image of the position of the stereo objects, a schematic figure, and test instructions. The test objects of both LANG I and LANG II remained unchanged. The disparity of the stereo objects is also identical to the original test versions, with one exception: The car in the revised version I was reduced from 550″ to 400″ to increase the level of difficulty (Table 1; [4]).
Table 1
Disparities in the stereoscopic objects between the original and revised LANG STEREOTESTs® (in arc seconds, ″; [10])
Cat
Elephant
Star
Car
Moon
Lang I‑O
1200″
–
600″
550″
–
Lang I‑R
–
400″
–
Lang II-O/R
–
600″
–
400″
200″
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The aim of this prospective study was to validate the revised LANG-STEREOTESTs® I and II (LANG I‑R, LANG II-R). In particular, the comparability of the test results of the revised tests with the original test versions of LANG-STEREOTESTs® I and II (LANG I‑O, LANG II-O) was examined. In addition, the reduction in monocular clues and the equalization of the recognizability of the objects of LANG I‑R and LANG II-R—as intended by LANG-STEREOTEST® AG—was evaluated in a non-clinical sample. This is the first study validating LANG I‑R and LANG II‑R.
Methods
After a pre-test with 14 students of the bachelor’s degree program in orthoptics at the University of Applied Sciences in Salzburg (UAS), a non-clinical sample population was recruited. Participants were recruited from the student population and the staff of UAS. The examinations were conducted between 28 March and 27 June 2022.
Participants with manifest strabismus were excluded. To avoid confounding by low vision, participants with near visual acuity (monocular or binocular) of less than 0.1 were excluded from the study [11, 12].
Ethics approval and informed consent
Ethics approval was obtained from UAS Institutional Ethics Committee (approval number: R‑2021-OTK-001). Participants showed informed consent by participating in the study. All data were analyzed completely anonymously, with no personal information collected. Our study followed the principles of the Declaration of Helsinki.
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Procedure
The testing was carried out by orthoptics students (University of Applied Sciences, Bachelor of Orthoptics) as part of a master’s thesis (Danube University Krems) and under the supervision of experienced orthoptists. None of the examiners was also a participant.
At the beginning, participants were asked whether they currently had any complaints (blurred vision, diplopia, headaches, etc.) or whether they had any eye disease or history of strabismus. Further screening consisted of an autorefraction in miosis over the participants’ own correction using the Welch Allyn® Spot Screener (Welch Allyn GmbH, Hechingen, Germany) to determine residual refractive errors and to consider the influence of uncorrected refraction on stereopsis. The participants’ spectacles were measured with a manual lensmeter.
As rings 1–3 of the Titmus® Stereotest Fly (STEREO OPTICAL CO., INC., Chicago, Illinois, USA) have a similar disparity to the objects of the LANG-STEREOTESTs®, the Titmus® Stereotest Fly was selected as a “control test” (ring 1: 800″, ring 2: 400″, ring 3: 200″). For standardized testing conditions, all stereotests were offered at 40 cm.
After the Titmus® Stereotest, the LANG-STEREOTESTs® were performed in randomized order: Even sequence numbers started with LANG I‑O or LANG II‑O, followed by LANG I‑R or LANG II‑R. The odd-numbered runs began with the reversed versions, followed by original LANG-STEREOTEST® versions I or II (Fig. 1). Participants started testing with LANG I‑O (n = 28), LANG II‑O (n = 29), LANG I‑R (n = 29), or LANG II‑R (n = 28). Initially, the LANG-STEREOTESTs® were administered monocularly to test the validity. In random order, the participants’ right or left eye was covered with a masking film (double-layer Transpore®, 3M, St.Paul, Minnesota, USA), which was applied to their own glasses or to a spectacle frame without lenses. Strict care was taken to ensure that the participants did not move their head during the test.
Fig. 1
Study design and randomized test procedure in validating LANG-STEREOTESTs®. Participants in subgroups were asked to show and name the seen stereoscopic objects monocularly (step 1), binocularly (step 2), and to compare the recognizability between the original vs. revised test version I or II depending on subgroup. Note: number of participants before exclusion (n = 57). Positive*: all stereo objects shown and named; negative**: at least one stereo object not shown or not named correctly
The participants were asked to point out the objects seen and, if possible, to name them. Documentation in the examination form provided included the recording of positive, only localized, incorrectly named, or completely negative results. The Transpore® occlusive was then removed, and the procedure was repeated binocularly. For further analysis, the test was considered positive if all objects of the LANG-STEREOTEST® were correctly recognized and named. If one or more of the objects could not be localized and named correctly, the test was scored as negative. In the case of a positive result, the participants were asked to rate the recognizability of the test objects (easier, equally easy, or harder to recognize).
After that, a second examiner, who was blinded to the outcome of the stereoacuity tests, carried out a brief orthoptic examination to determine any influencing factors on stereoacuity results. This examination included visual acuity for near (monocular and binocular: LeaTM-symbols (GOOD-LITE, Elgin, Illinois, USA) crowded pocket test), cover test for near, prism cover test for near in cases of deviation, and determination of the near point of convergence.
All examinations were carried out in the same room, under standardized lighting conditions (average illuminance: 1701 lx) and with near correction, if necessary and available.
Statistical analysis
Data were analyzed using Microsoft Excel. Percentages were calculated for data that could not be analyzed with any statistical test and are presented descriptively.
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Results
A total of 57 adults participated in the study (75% female, 25% male; age range 18–71 years; average not known, as data only collected in age categories; Table 2). All participants completed the whole testing procedure. One participant did not meet the inclusion criteria because of manifest strabismus and was excluded.
Table 2
Participant characteristics
Participants (n)
Percentage (%)a
Gender
Male
14
25
Female
42
75
Age (years)
18–29
37
66
30–41
07
13
42–53
10
18
≥ 54
02
03
Cover test
Orthophoria
29
52
Exophoria
22
Mean: −7 pd
SD: 4.23
Range: −1 to −20 pd
39
Esophoria
04
Mean: +5 pd
SD: 4.99
Range: +1 to +12 pd
07
Vertical phoria
01 (4pd)
02
SD standard deviation
aPercentages have been rounded to the nearest single decimal place
Overall, 20 participants (36%) had no refractive corrections, 19 (34%) wore spectacles, seven (12%) had spectacles but did not wear them, and 10 (18%) wore contact lenses (Table 3).
Table 3
Refractive errors of participants
Participants, n (%)
Refractive correction
None
20 (36)
Glasses (worn)
19 (34)
Glasses (reported*)
07 (12)
Contact lenses
10 (18)
*Glasses reported: participants did not wear these glasses
Measurements with the Welch Allyn® Spot Screener showed residual refractive errors (measured “over their own correction”) for most participants. The spherical equivalent in both eyes ranged from +1.00 to −2.50 dpt. Two participants had astigmatism at ≥ 2.00 dpt.
Geometric mean near visual acuity was 1.17 binocular (GSD: 1.22; range: 0.63–2.0), 1.08 for the right eye (GSD: 1.22; range: 0.63–1.6) and the 1.12 for left eye (GSD: 1.23; range: 0.5–2.0; Table 4). Ten participants had at least one line difference between the right and left eye (max. three-line difference). However, their stereotest results did not differ.
Table 4
Binocular and monocular visual acuity reached
Visual acuity
Range
GM
GSD
Binocular
0.63–2.0
1.17
1.22
Right eye
0.63–1.6
1.08
1.22
Left eye
0.50–2.0
1.12
1.23
GM geometric mean, GSD geometric standard deviation
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The cover test showed that 29 participants had orthophoria (51.8%), 22 participants (39.3%) had exophoria (mean: −7 pd; SD: 4.23; range: −1 pd. to −20 pd), four (7.1%) had esophoria (mean: +5 pd, SD: 4.99, range: +1 pd. to +12 pd), and one (1.8%) had vertical phoria (4 pd; Table 2). The near point of convergence was less than 10 cm in all participants.
Most participants (87.5%) had no visual complaints. Seven participants (12.5%) experienced occasional complaints such as headaches, blurred vision, burning eyes, problems when changing focus, or eye fatigue.
Results of stereotests
All participants had a positive result for rings 1 and 2 of the Titmus® Stereotest, while 98% achieved a positive result for rings 1–3.
LANG I
In the monocular testing of LANG I‑O, one out of the 28 participants (3.6%) achieved a positive result, whereas the binocular testing was positive in 24 participants (85.7%). For LANG I‑R, two out of 28 participants (7.1%) were able to achieve a positive result monocularly, whereas 26 (92.9%) achieved a positive result binocularly (Fig. 2).
Fig. 2
Positive vs. negative results for LANG I‑O and LANG I‑R with a monocular and b binocular testing, respectively
Statistical analysis was not possible (data level, sample size, distribution).
Regarding the monocular recognizability of the objects in LANG I‑O, our results show that one participant (3.6%) recognized the cat, six (21.4%) the star, and four (14.3%) the car. In LANG I‑R, two participants (7.1%) recognized the cat, eight (28.6%) the star, and ten (35.7%) the car. In LANG I‑O, more participants were able to show single objects but were not able to name them: 13 participants (46.4%) localized the cat, ten (35.7%) the star, and 14 (50.0%) the car. In LANG I‑R, ten (35.7%) localized the cat, only one (3.6%) localized the star, and three (10.7%) the car.
Regarding the subjective judgment of recognizability of objects, ten participants (35.7%) found that the objects of LANG I‑O were easier to see than those of LANG I‑R, one (3.6%) found them equally easy to see, and 17 participants (60.7%) found that the objects of LANG I‑O were harder to see than those of LANG I‑R. In the other subgroup (comparing LANG I‑R with LANG I‑O), 18 participants (64.3%) found that the objects of LANG I‑R were easier to recognize than those of LANG I‑O, four (14.3%) found them equally easy to see, and six participants (21.4%) found that the objects of LANG I‑R were harder to recognize than those of LANG I‑O (Fig. 3).
Fig. 3
Subjective judgment of recognizability. Comparing LANG I‑O with LANG I‑R, LANG I‑R with LANG I‑O, and mean results
Summarizing the data of all 56 participants for ease of readability, 16 participants (28.6%) found that the objects of Lang I‑O were easier to recognize than those of Lang I‑R, five (8.9%) found them equal, and 35 participants (62.5%) found that the objects of Lang I‑O were harder to recognize than those of Lang I‑R (Fig. 3).
LANG II
In the monocular testing of LANG II‑O, two out of the 28 participants (7.1%) achieved a positive result, whereas the binocular testing was positive in 26 participants (92.9%). For LANG II‑R, no participant was able to achieve a positive result monocularly, whereas 25 (89.3%) achieved a positive result binocularly (Fig. 4). Statistical analysis was not possible (data level, sample size, distribution).
Fig. 4
Positive vs. negative results for LANG II‑O and LANG II‑R with a monocular and b binocular testing, respectively
Regarding the monocular recognizability of the objects in Lang II‑O, our results show that two participants (7.1%) recognized the elephant, seven (25.0%) the car, and eight (28.6%) the moon. In LANG II‑R, one participant (3.6%) recognized the elephant, three (10.7%) the car, and one (3.6%) the moon. More participants were able to show single objects in LANG II‑O but were not able to name them: 18 participants (64.3%) localized the elephant, ten (35.7%) the car, and two (7.1%) the moon. In LANG II‑R, six (21.4%) localized the elephant, three (10.7%) the car, and three (10.7%) the moon.
Regarding the subjective assessment of the recognizability of objects, 18 participants (64.3%) found that objects of LANG II‑O were easier to see than those of LANG II‑R, four (14.3%) found them equally easy to see, and six participants (21.4%) found that objects of LANG II‑O were harder to see than those of LANG II‑R. In the other subgroup (comparing LANG II‑R with LANG II-O), ten participants (35.7%) found that the objects of LANG II‑R were easier to recognize than those in LANG II‑O, five (17.9%) found them equally easy to see, and 13 participants (64.4%) found that the objects in LANG II‑R were harder to recognize than those in LANG II‑O (Fig. 5).
Fig. 5
Subjective judgment of recognizability. Comparing LANG I‑O with LANG I‑R, LANG I‑R with LANG I‑O, and mean results
Summarizing the data of all 56 participants for ease of readability, 31 participants (55.4%) found the objects in Lang II‑O were easier to recognize than those in Lang II‑R, nine (16.1%) found them equal, and 16 participants (28.6%) found that the objects in Lang II‑O were harder to recognize than those in Lang II‑R (Fig. 5).
All participants had positive Titmus® Stereotest results, and thus it was not possible to calculate a correlation between Titmus® Stereotest and LANG I‑O, LANG I‑R, LANG II‑O, and LANG II‑R.
Discussion
The LANG-STEREOTESTs® are very quick and easy tests for examining stereopsis [5‐7]. In 2020, LANG I‑O and II‑O were revised to improve the optical quality and homogeneity of random dots [4] There are currently no other studies addressing the validation of LANG I‑R and II‑R. The aim of this study was to validate LANG I‑R and LANG II‑R in a non-clinical adult sample. Although statistical analysis was not possible (data level, sample size, distribution), our data seem to confirm the validity of LANG I‑O and LANG I‑R as well as Lang II‑O and Lang II‑R (see also Figs. 2 and 4).
Monocular clues
Ancona et al. [15] reported that patients without stereopsis can use monocular clues and get a false-positive result. Lang [3, 4] emphasized the importance of using the LANG-STEREOTESTs® correctly to avoid monocular clues and false-positive results. This means that neither the test card nor the patient’s head should move during testing [3, 13‐15]. The revised versions of LANG I‑O and II‑O intend to avoid monocular clues [4].
Our results show that one participant (3.6%) was able to obtain a positive result for the LANG I‑O monocularly. Monocular clues can enable patients to identify the position of objects correctly and sometimes even to name them. Our study shows that 3.6% of the participants recognized the cat, 21.4% recognized the star, and 14.3% recognized the car in LANG I‑O (Table 5). Under monocular conditions, the star was recognized the easiest, although it is not the stereo object with the greatest disparity. Hahn et al. [16] reported similar results in a non-strabismus sample (13% recognized the star monocularly). This could be due to the object itself. Compared to the cat and the car, the star is a symmetrical object that leaves less room for interpretation (for more details see Table 5).
Table 5
Monocular clues in LANG I‑O and LANG I‑R (results in our study compared with the “binocular measured” results by Hahn et al. [16] with a strabismus sample)
Monocular recognizability of objects of LANG I
Positive (%)
Localized only (%)
Hahn et al. (2010)
Our results
Hahn et al. (2010)
Our results
Lang I‑O
Lang I‑O
Lang I‑R
Lang I‑O
Lang I‑O
Lang I‑R
Cat (1200″)
6
3.6
7.1
61
46.4
35.7
Star (600″; R: 400″)
13
21.4
28.6
39
35.7
3.6
Car (550″)
10
14.3
35.7
55
50.0
10.7
″ = seconds of arc
Concerning LANG II‑O, two participants (7.1%) obtained a positive result when tested monocularly, whereas none achieved a positive result for LANG II‑R. In Lang II‑R, the recognizability of single objects is less frequent than in LANG II‑O. In the case of LANG II, there is a non-explainable mismatch with the results of Hahn et al. ([16]; for details, see Table 6).
Table 6
Monocular clues in the LANG II‑O and the LANG II‑R (results in our study compared with the “binocular measured” results by Hahn et al. [16] with a strabismus sample)
Monocular recognizability of objects of the LANG II
Positive (%)
Localized only (%)
Hahn et al. (2010)
Our results
Hahn et al. (2010)
Our results
Lang II‑O
Lang II‑O
Lang II‑R
Lang II‑O
Lang II‑O
Lang II‑R
Elephant (600″)
21
7.1
3.6
51
64.3
21.4
Car (400″)
13
25.0
10.7
48
35.7
10.7
Moon (200″)
37
28.6
3.6
24
7.1
10.7
″ = seconds of arc
To sum up, our data show that the monocular recognizability increased in LANG I‑R and decreased in LANG II‑R.
Comparison of the LANG-STEREOTEST®
To test whether the results of LANG I‑R and LANG II‑R are more comparable than those of LANG I‑O and II‑O, participants were tested binocularly. Our results show that LANG II‑R is somewhat more difficult to recognize than LANG I‑R. This might also be explained by the lower disparities of the most difficult objects in LANG II‑R vs. LANG I‑R (200″ vs. 400″).
Our comparison of LANG I‑O and II‑O confirms a better binocular recognizability of LANG II-O—as seen in daily routine—which cannot be explained by the disparities. Other studies showed a lower sensitivity of LANG II‑O for amblyopia or manifest strabismus compared with LANG I‑O [15, 17].
In our non-clinical sample, LANG I‑R was slightly easier to recognize than LANG I‑O, whereas LANG II‑R was slightly more difficult to recognize than LANG II‑O. The subjective judgment of participants confirmed these findings (Figs. 3 and 5). To sum up, LANG I‑R and LANG II‑R are more comparable than the original LANG versions I and II, as intended by LANG-STEREOTEST® AG.
LANG-STEREOTEST® vs. Titmus® Stereotest
Moreover, it must be noted that the participants recognized rings 1–3 of the Titmus® Stereotest more often than the objects of the LANG II‑O (98% vs. 92.9%) and the objects of the LANG II‑R (98% vs 89.3%), although disparities for ring 3 of the Titmus® Stereotest and the moon of the LANG II‑O and II‑R are equal (200″). Furthermore ring 3 of the Titmus® Stereotest has a lower disparity and should be more difficult to recognize than the objects of the LANG I‑O and I‑R. This contradictory finding could be explained by the different types of stereopsis examined (local vs. global in LANG STEREOTEST® vs. Titmus® Stereotest). Local stereopsis is examined by displaying two identical objects horizontally shifted. This horizontal shift may enable monocular recognizability [18, 19].
Refractive errors and visual acuity
Two participants had a residual astigmatism of ≥ 2.00 dpt, but in these two cases astigmatism had no effect on visual acuity (not worse than 0.8). This could be caused by the overestimation of astigmatism by one third in the Welch Allyn® measurements [20]. Furthermore, the results of these two participants in LANG-STEREOTESTs® did not differ from the other participants.
Differences in visual acuity of up to three lines (n = 10 participants) showed no effects on the stereotest results.
Limitations of the study
We used the Titmus® Stereotest to compare results of the LANG-STEREOTESTs®. It might have been useful to use the TNO® stereotest, which also tests global stereopsis. Comparing the results of two random-dot stereotests would be more meaningful.
Another weakness of our study was the small sample size due to the COVID pandemic. Our sample was a non-clinical adult sample. Further research is necessary to determine validity in children and/or clinical samples with strabismus, amblyopia etc. Furthermore, we cannot exclude the possibility that our participants were already familiar with the LANG-STEREOTESTs®. This could have an influence on the monocular and binocular results.
Conclusion
Based on a non-clinical adult sample, we can conclude that LANG I‑O and LANG I‑R as well as LANG II‑O and LANG II‑R are valid and that the results of the original LANG-STEREOTEST® versions I and II are comparable to the revised versions. The intended alignment of LANG-STEREOTEST® versions I and II has been achieved with the revision. Since monocular clues cannot be ruled out, a comprehensive orthoptic examination is necessary to reliably rule out strabismus or amblyopia.
Declarations
Conflict of interest
S. Entacher, A. Brunnauer and R.E. Resch declare that they have no competing interests.
Ethical standards
Ethics approval was obtained from UAS Institutional Ethics Committee (approval number: R‑2021-OTK-001). Participants showed informed consent by participating in the study. All data were analyzed completely anonymously, and the participants were not required to provide any personal information. All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008.
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Roggenkämper P. Stereosehen bei vermindertem Visus. Klin Monatsbl Augenheilkde. 1983;183:105–9.CrossRef
12.
Vladimirova M. LANG®-I STEREOTEST. Study on the influence of visual acuity on the detectability of the test. Bachelorarbeit, Studiengang Orthoptik. Salzburg: Puch: University of applied sciences; 2023.
Scheiman M, Wick B. Clinical management of binocular vision: heterophoric, accommodative and eye movement disorders. 3rd ed. Philadelphia: Lippincott Williams and Wilkins; 2008.
19.
Daum K, McCormack G. Fusion and binocularity: local and global stereopsis. In: Benjamin, WJ and Borish, IM, Borish´s clinicla refraction, 2nd ed. Philadelphia: Buttermorth Heinemann Elsevier; 2006. pp. 145–91.CrossRef
20.
Harvey EM, Dobson V, Miller JM, et al. Accuracy of the Welch Allyn SureSight for measurement of magnitude of astigmatism in 3‑to 7‑year-old children. J Am Assoc Pediatr Ophthalmol Strab. 2009;13:466–71. https://doi.org/10.1016/j.jaapos.2009.08.013.CrossRef
