Effects of Aniseikonia, Anisometropia, Accommodation, Retinal Illuminance, and Pupil Size on Stereopsis

John Vincent Lovasik* and Mory Szymkiw

The sensitivity of clinical measures of stereoacuity in the detection of interocular differences in retinal images was examined in 50 adults with normal binocularity. Interocular differences in retinal image size (aniseikonia), clarity (anisometropia) and brightness, as well as differences in absolute and relative pupil size (anisocoria) were created in small steps over a large range to determine their effect on threshold levels of stereopsis. Their effect on stereoacuity was measured in both contour (Titmus test) and random dot (Randot test) stereograms. Stereoacuity measured by both types of stereograms decreased in a curvilinear manner for aniseikonic and anisometropic test conditions. Monocular blur caused a more rapid decrease in stereoacuity than induced aniseikonia. Stereoacuity measured by the contour stereogram decreased about 1.8 times faster than that measured by the random dot stereogram during induced aniseikonia and anisometropia. This differential sensitivity suggests that the Titmus test would detect small interocular differences in retinal images more effectively than the Randot test in clinical screening procedures for vision abnormalities. However, both tests can miss clinically significant amounts of aniseikonia and anisometropia, and fail to differentiate the cause of reduced stereopsis. Interocular differences in retinal image brightness and pupil size within a normal physiologic range did not reduce stereopsis to clinically unacceptable levels. Invest Ophthalmol Vis Sci 26:741-750, 1985

Stereoacuity is the smallest amount of horizontal flicting reports on the effect of monocular blur on retinal image disparity (measured in arcsec) giving stereopsis. Some reports suggest that good visual rise to perception of relative depth or stereopsis.1'2 acuity is not required for stereopsis,12 while others Since depth perception relies upon accurate binocular report that relatively low amounts of monocular blur alignment by sensory and sensory-motor processes, a severely reduce or eliminate stereopsis.1314 Still others disruption of fusional mechanisms generally decreases report the retention of stereoacuity in clinically sig- stereoacuity or eliminates stereopsis altogether.34 As nificant anisometropias.7 a result, tests of stereoacuity are often administered Part of the apparent contradiction and confusion as screening procedures to detect amblyopia, aniso- arises from differences in methods, limited numbers metropia or aniseikonia, all of which can disrupt of subjects in experimental groups, or narrow ranges normal binocularity and consequently stereopsis.5"7 of test conditions simulating ocular conditions com- However, the value of stereoacuity tests for detecting promising stereopsis. The present study was under- optical or neural factors degrading depth perception taken to systematically examine factors that have is uncertain. Vision literature contains reports indi- been reported to affect stereoacuity and their influence cating both a loss of stereopsis with low amounts of on clinical measurements of stereopsis. We examined aniseikonia8 as well as its retention in the presence the effects of induced aniseikonia, anisometropia, of large order aniseikonia.9"1' There are similar con- accommodation, retinal illuminance, and pupil size on stereoacuity measures by two commonly employed clinical tests of stereoacuity, the Titmus stereo test (a From the University of Waterloo, School of Optometry, Waterloo, contour stereogram), and the Randot test (a random Ontario, Canada. dot stereogram).15"18 * Supported by grants from Natural Sciences and Engineering Research Council of Canada and Canadian Optometric Education Trust Fund. Materials and Methods Submitted for publication: June 11, 1984. Reprint requests: Dr. John Vincent Lovasik, Associate Professor, Fifty experienced observers (29 men, 21 women) University of Waterloo, School of Optometry, Waterloo, Ontario, ranging in age from 20 to 32 yr, served as subjects. Canada N2L 3G1. All subjects were informed about the nature of the

741

Downloaded from iovs.arvojournals.org on 09/29/2021 742 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / May 1985 Vol. 26

study prior to experimentation, and agreed to partic- Test Condition Number 2 ipate in the study. Each volunteer had healthy eyes Induced anisometropia: The effects of anisometropia with 6/6 or better visual acuity at 6 m and 0.4 m in each eye, normal fusional vergence amplitudes, mon- on the predetermined stereo threshold was examined ocular accommodation amplitudes of 8 D or more, by placing plus lenses in 0.5 D steps before the minimal anisometropia, and threshold stereoacuity dominant eye in random sequence. Testing continued of 40 arcsec or better. The subjects wore their most until one of the endpoint criteria was reached. updated glasses or contact lenses with crossed pola- roids overtop during stereoacuity measurements. Test Test Condition Number 3 plates were held parallel to the facial plane at a Stabilization of accommodation: In order to ex- distance of 40 cm according to the recommendations amine the possibility that accommodative fluctuations of the manufacturers. The subjects were allowed to resulting from optically induced anisometropia could tilt the plates slightly to eliminate surface reflections. affect measures of stereoacuity, the accommodative Ambient room lighting was maintained at 215 Lux, demand for the stereo test plates was eliminated by while an incandescent light source provided an illu- adding +2.50 D lenses before each eye. A new stereo- minance of 915 Lux on the test plates. acuity threshold was established and then stereopsis Subjects were given as much time as they needed was determined for various levels of anisometropia for stereopsis to develop for the test patterns in either as in Test Number 2. The same endpoint criteria stereo test. A test threshold response was taken as the were used in this test condition as in the previous last correct response before two incorrect or negative two test conditions. responses. A special procedure minimized the effects of memorization of correct responses. Once a stereo Test Condition Number 4 test threshold was determined, subjects were asked to Retinal illuminance: To examine the effect of retinal "remember" the amount of depth that was present. illuminance on stereopsis, 10 randomly selected sub- In subsequent testing protocols, subjects were asked jects from the subject pool had both pupils dilated to indicate which target appeared to have the same with ophthalmic Neo-Synephrine (10% epinephrine amount of depth as their predetermined test threshold HC1, Winthrop Laboratories, Division of Sterling value. Threshold testing was repeated whenever sub- Drug Ltd; Aurora, Ontario, Canada) and changes in jects had difficulty remembering their threshold depth stereopsis measured as neutral density filters were value. This method of testing, although more difficult added in 0.1 ND steps to the dominant eye. All and time consuming than simple identification of the measurements were made with the subject viewing component in a test pattern having depth, did not the stereo targets through 3.0-mm apertures. Neutral pose any serious difficulties to the observers. If a density filter values were increased until stereoacuity subject could attain a stereoacuity of 40 arcsec in the was lost. Titmus test, the smallest on the nine plates, the viewing distance was increased to 80 cm to determine if 20 arcsec was attainable. When this was required, Test Condition Number 5 the local light source was moved to maintain the Pupil size: The influence of anisocoria on stereopsis same illuminance on the test plates. Stereoacuity was examined in the same 10 subjects prepared for thresholds were determined for each of five test Test Number 4. The pupil of the nondominant eye conditions. was fixed at 3.0 mm, while that of the dominant eye was changed from 1 mm to 8 mm in 0.5 mm steps. Test Condition Number 1 Stereoacuity was determined for each pupil size. The influence of binocular pupil size on stereopsis Induced aniseikonia: Aniseikonia between 1.2% was examined by changing the pupil aperture for and 32.3% was induced in 26 steps by placing afocal each eye from 1 mm to 8 mm in 0.5 mm steps. magnifiers before the dominant eye in random order. Stereo threshold was determined in the regular manner Presentation of the Titmus and Randot Tests was for each pupil size. regularly alternated for each level of induced aniseikonia. For each degree of aniseikonia subjects were Results asked to identify the test target that had the same Test Number 1 depth as did their threshold target. Testing was continued until one of the following endpoint criteria The results for test condition Number 1 (induced occurred: diplopia, monocular suppression, or stereo- aniseikonia) are shown in Figure 1. Group averaged acuity reduced to a level not measurable by either stereoacuity measurements are presented as a function stereo test. of induced aniseikonia for the Randot and Titmus

Downloaded from iovs.arvojournals.org on 09/29/2021 No. 5 OCULAR FACTORS AFFECTING STEREOPSIS / Lovosih and Szymkiw 740

560- 560

520- 520

480 480

440- 440 Fig. 1. Stereoacuity shown as a function of induced an- 400- 400 iseikonia for Randot and Titmus tests. The curves RANDOT TITMUS 360 through the data points represent nonlinear computer- 320 fit models for changes in 230 stereoacuity with aniseikonia. Vertical lines through 240 data points represent ±1 SEM. These group-averaged 200 data indicate a more rapid loss of stereoacuity with in- 160 creasing aniseikonia when measured by the Titmus test. 120

0 20 24 28 32 0 4 8

INDUCED ANISEIKONIA (°/o)

tests. Each point represents averaged data for 10 or tests varied considerably between individuals. Figure more subjects. As larger amounts of aniseikonia were 2 shows the group averaged amounts of induced created fewer subjects were able to pass either stereo- aniseikonia (filled circles and squares) that decreased acuity tests; all subjects demonstrated stereopsis up stereoacuity to the graded steps provided in each test. to 13.3% aniseikonia for both tests, while an average The horizontal lines through each data point indicate of 82% showed stereopsis for aniseikonia between the range of induced aniseikonia over which the 13.3% and 22.3%, and an average of only 34% were various stereoacuity test levels were possible. The able to respond for aniseikonia above 22.3%. For curves through the data points represent nonlinear progressively higher amounts of aniseikonia, the de- computer fit models relating the average amounts of creasing number of subjects able to show stereopsis, induced aniseikonia to the nominal levels of stereo- and the increasing response uncertainty account for acuity presented in each stereoacuity test. The pre- the increasing value of ±1 SEM bars through each dictive models and data points show a very high data point. The level of stereoacuity decreased with correlation coefficient (Randot r = +0.99, Titmus r aniseikonia in a curvilinear manner for both Randot = +0.98) and are defined by:

and Titmus tests. The solid lines through the data +0.64 Aniseik points for each test represent nonlinear computer fit Randot stereoacuity = 1 - 0.67e models, each with a correlation coefficient of +0.99. Titmus stereoacuity = 1 - 0.78e-0.72 Aniseik The equations describing stereoacuity as a function of induced aniseikonia are: Both models predict an exponential loss of stereoacuity with increased amounts of aniseikonia. Randot stereoacuity = 26.5 + 0.40 aniseik + 0.33 aniseik2 Test Number 2 Titmus stereoacuity 2 = 36.7 + 0.83 aniseik + 0.58 aniseik . The effect of monocular blur on stereoacuity for These equations predict a more rapid loss of stereo- the subject population of this study is illustrated in acuity with aniseikonia when measured by the Titmus Figure 3 by the curves drawn through the filled circles test relative to the loss measured by the Randot test. (Randot test) and squares (Titmus test). In this test The magnitude of induced aniseikonia required to condition the subjects wore their habitual prescription reduce stereopsis to the progressively coarser stereo- (Rx), and the level of accommodation was determined acuity levels contained in the Randot and Titmus by the dioptric distance of the stereo plates. The

Downloaded from iovs.arvojournals.org on 09/29/2021 744 INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / May 1985 Vol. 26

400 400

360 RANDOT TITMUS 360

320- 320 Fig. 2. Data showing the average amount of aniseikonia found to decrease stereoacuity to successive levels contained in the Ran- dot and Titmus tests. The horizontal lines through each data point represent the 120 range of aniseikonia values associated with each level of stereoacuity.

24 28 32 0 4 8 INDUCED ANISEIKONIA (•/.)

induced anisometropia caused a nonlinear loss of expressed in the following nonlinear computer fit stereoacuity as measured by both tests. The Titmus models for the data, each with a correlation coefficient test, however, showed a larger and more rapid dete- greater than +0.98: rioration of stereopsis than did the Randot test for the same degree of monocular blur. The different Randot stereoacuity rates of stereoacuity loss shown by these two tests are = +22.75 + 3.06 blur + 11.21 blur2

440

400

360 RANDOT TITMUS 360

O u 320 320 0) u CO 280 280

24O

200

160

120 120

80 80

0 •1.00 .2.00 .3.00 .4.00 .5.00 0 .1.00 •2.00 .5.00 MONOCULAR BLUR ( D) Fig. 3. Group-averaged data showing stereoacuity as a function of monocular blur for Randot and Titmus tests. The curves through the data points represent computer-fit models for the deterioration of stereoacuity with monocular blur. The vertical lines through data points represent ±1 SEM. This figure illustrates the effect of monocular blur on stereoacuity with normal accommodation (Rx), and accommodation relaxed by plus lenses (Rx +2.50 D). In both test conditions, the Titmus test shows a more rapid loss of stereoacuity with monocular blur than the Randot test.

Downloaded from iovs.arvojournals.org on 09/29/2021 No. 5 OCULAR FACTORS AFFECTING STEREOPSIS / Lovosik and Szymkiw 745

Titmus stereoacuity 400 2 = +28.02 + 13.02 blur + 18.04 blur . 360-

320- RANDOT Test Number 3 280 Relaxing the accommodative effort for the viewing 240 distance by the addition of +2.50 D to each eye over the habitual Rx altered the basic relationship between 200 stereoacuity and monocular blur as describd above 160-

only minimally. The level of stereoacuity as a function 120 of monocular blur with the accommodative demand for the dioptric distance of the test target (+2.50 D) eliminated is shown in Figure 3 by the curves drawn through the open circles (Randot test) and squares (Titmus test). The addition of +2.50 D bilaterally 3 400- did, however, reduce the overall level of stereoacuity O 360 measured by each test for identical levels of monocular W Q: UJ blur. This is seen as an upward displacement of the u> 32° stereoacuity-monocular blur functions. For the Ran- 280 dot test, the +2.50 lenses also caused a slight steep- ening of the stereoacuity-monocular blur function. 240 The nonlinear computer fit models predicting stereo- 200 acuity as a function of monocular blur show a high correlation coefficient for the data points for both Randot (r = +0.98) and Titmus (r = +0.96) stereo tests. These predictive models are defined as follows: Randot (+2.50) stereoacuity

2 •200 .3.00 .4.00 = +32.86 + 24.96 blur + 11.66 blur MONOCULAR BLUR(D)

Titmus (+2.50) stereoacuity Fig. 4. Data showing the average amount of monocular blur 2 found to decrease stereoacuity to the nominal levels contained in = +35.01 + 59.54 blur + 12.70 blur . the Randot test. The data are presented for test conditions in which accommodation is active (Rx), and relaxed with plus lenses (Rx As was the case for test condition number 1, the +2.50 D). The horizontal lines through each data point represent magnitude of monocular blur required to reduce the range of monocular blur values with each level of stereopsis. stereopsis to the various stereoacuity levels contained in the Randot and Titmus stereo tests varied between individuals. The average monocular dioptric blur data show a correlation coefficient greater than +0.97 values that reduced stereoacuity in step-fashion for and are defined as follows: the Randot test are shown as filled circles in Figure 49 Blur 4 for normal viewing conditions, and open circles for Titmus stereoacuity = 1 - 0.82e~°- the same test conditions but accommodation relaxed Titmus (+2.50) stereoacuity = 1 - 0.89e"079 Blur. bilaterally by +2.50 D lenses. The horizontal lines through each data point represent the range of mon- Test Number 4 ocular blur values associated with each nominal level of stereopsis. The solid lines through the data points Figure 6 presents the effects of a monocular reduc- for each test condition are nonlinear regression models tion of retinal illuminance on the level of Randot showing a correlation coefficient for data points greater and Titmus stereoacuity while the subjects with bi- than +0.98 and defined by the following equations: laterally dilated pupils viewed the stereo targets through 3.0-mm apertures. The solid lines through +0.84 Blur Randot stereoacuity = 1 — 0.87e the data points were positioned by eye. Analysis of Randot (+2.50) stereoacuity = 1 - 0.80e+0.77 Blur these data by the Neuman-Keuls test indicated that the level of stereoacuity did not decrease significantly Figure 5 presents similar data for the Titmus test. until monocular retinal illuminance was reduced by The nonlinear regression lines through each set of 1.6 ND and 1.7 ND for the Randot and Titmus tests,

Downloaded from iovs.arvojournals.org on 09/29/2021 746 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / Moy 1985 Vol. 26

400 pupil size less than 1.5 mm is required before a significant reduction in stereoacuity is observed. 360 For each stereoacuity test no significant differences 320 TITMUS in the stereoacuity-pupil size functions were noted

280 for pupil size changes induced monocularly or bin- ocularly. A comparison of monocular and binocular 240 pupil size data between each test shows similar stereo- 200 acuity-pupil size functions, with the Titmus data 160 displaced upwards along the ordinate because of the coarser stereoacuity steps in that test.

Discussion Our study indicates a curvilinear loss of stereoacuity with increasing amounts of aniseikonia. Earlier studies by Ogle,2'8 and Reading and Tanlamai" indicated that the threshold of stereopsis increased in a linear fashion for induced aniseikonia in the range of 0- 8%. Our data for this low level aniseikonia supports these earlier observations. However, for higher aniseikonia levels, the degradation of stereopsis proceeds at a much more rapid rate than that indicated in the Reading and Tanlamai study. Their study indicated

T RANSMIT TANC E (%)

31.6 10

14O

.1.00 -2.00 -3.00 -4.00 .5.00 .6.00 120 RANDOT MONOCULAR BLUR (D) 100- V Fig. 5. Data for the Titmus test showing the average amount of monocular blur degrading stereoacuity to successive stereoacuity levels presented in the Titmus test. Data are presented for normal viewing conditions (Rx), and accommodation relaxed by plus lenses (Rx +2.50 D). The horizontal lines through data points represent 40 the range of monocular blur values associated with each level of I O o stereoacuity.

respectively. This indicated that a very large decrease in monocular retinal illumination (approximately 98%) was required before stereopsis was significantly TITMUS impaired.

Test Number 5 The effect of anisocoria and bilateral pupil size on stereoacuity measurements by Randot and Titmus tests is shown in Figure 7. The curves through the data points for each test condition show that neither anisocoria nor binocular pupil size significantly influ- 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 enced stereoacuity until one or both pupils were very ABSORBANCE (N.D.) small. Analysis of the Randot data by the Newman- Fig. 6. Figure showing the effect of decreased monocular retinal Keuls test indicates that a significant drop in stereo- illuminance by neutral density niters on stereoacuity levels measured acuity occurs only when monocular or binocular by Randot and Titmus tests. Vertical bars through each data point represent ±1 SEM. A significant decrease in stereoacuity is only pupil sizes are less than 2.5 mm. Similar analysis of seen when retinal illuminance is reduced by 1.6 ND for the Randot the Titmus data shows that a monocular or binocular test, and 1.7 ND for the Titmus test.

Downloaded from iovs.arvojournals.org on 09/29/2021 No. 5 OCULAR FACTORS AFFECTING STEREOPSIS / Lovosik ond Szymkiw 747

100 RANDOT TITMUS 100

80 80

60 60 Monocular Monocular

40 -e—• g—D o—B—B—D a 40

100

8O •80

60 60 Binocular Binocular 40 40

20 20

1.0 2.0 3.0 4.0 5.0 , 6.0 7.0 8.0 9.0 0 1.0 2.0 3.0 4.0 5.0 6.0 70 8.0 9.0 PUPIL DIAMETER (mm)

Fig. 7. Figure showing the effect of induced anisocona and changes in bilateral pupil size on stereoacuity measurements by Randot and Titmus tests. Vertical bars through data points represent ±1 SEM. No significant differences in the stereoacuity-pupil diameter functions are seen in either test for monocular or binocular viewing conditions. Significant changes in stereoacuity on the Randot test are only seen when monocular or binocular pupil sizes are less than 2.5 mm. In the Titmus test, a significant reduction in stereoacuity is seen for pupil sizes smaller than 1.5 mm.

that a stereoacuity of 40 arcsec was possible with 20% aniseikonia between 0 and 16% on both the Randot aniseikonia, and 50-100 arcsec with 30% aniseikonia. and Titmus tests. However, the average value of Figure 1 shows that a stereoacuity level of 40 arcsec induced aniseikonia associated with that level of was possible only for aniseikonia levels less than 6% stereoacuity was only 6%. on the Randot test, and 2% on the Titmus test. Our data on the effects of monocular blur on Furthermore, only one of the 50 subjects experienced stereoacuity (Fig. 3) indicate that a clinically acceptable stereopsis for an aniseikonic level of 32.2%. The level of 40 arcsec can be maintained with a 1.0 D discrepancies in these findings are likely due to several blur on the Randot test and 0.5 D on the Titmus factors. Reading and Tanlamai used the Howard- test. Most of our subjects were able to maintain Dolman stereo test, in which the stimulus conditions moderate levels of stereopsis with a 2.00 D monocular are inherently different from those experienced in the blur while approximately 20% of subjects maintained Randot or Titmus test. Furthermore, they examined gross stereopsis with even 4.00 D of monocular blur. only two subjects, five levels of induced aniseikonia, These results differ radically from those of Peters14 and they often provided their subjects with relieving who reported that 80% of his subjects lost stereopsis prisms whenever they experienced diplopia during with a 1.00 D monocular blur. However, our results testing. In our study, prisms were not utilized, and are in agreement with those of Levy and Glick,13 who diplopia was considered an endpoint for testing. In reported a stereoacuity level of 50 arcsec on the view of the larger number of subjects and levels of Titmus test for subjects with a 2-line interocular induced aniseikonia used in the present study, we difference in Snellen visual acuity. feel our data are likely to be more typical of the The synkinetic relationship between accommoda- relationship between stereoacuity and aniseikonia. It tion and accommodative convergence suggests that should also be noted that there are considerable unstable focussing may have a detrimental effect on interindividual differences in the magnitude of anis- stereoacuity due to unstable ocular alignment. We eikonia causing a reduction in stereoacuity. This therefore investigated the possibility that creating point is emphasized in Figure 2, which illustrates that anisometropia by monocular plus lenses might lead a stereoacuity level of 40 arcsec was associated with to unstable accommodation and binocularity and

Downloaded from iovs.arvojournals.org on 09/29/2021 748 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / May 1985 Vol. 26

24 24 • 400 22 TITMUS 22

O • /140 18

R+2.50D. J /#100 16

Q 10 LJ U D Q Z

4.00 0 •1.00 •2.00 •3.00 • 4.00 MONOCULAR BLUR (D ) Fig. 8. Figure illustrating the relationship between induced aniseikonia and monocular blur for Randot and Titmus tests. Data points for this composite graph were drawn from the results of test conditions no. 1 and no. 2, illustrated in Figures 2, 4, and 5. The solid lines through data points are linear regression lines denned as: Randot (Rx) Aniseik = +7.12 blur -3.11, r = +0.99; Titmus (Rx) Aniseik = +6.41 blur -1.74, r = +0.98. The small bold numbers beside data points indicate the stereoacuity levels represented in each stereo test. The dashed lines indicate how aniseikonia values can be equated with monocular blur values.

consequently reduce stereoacuity. Attempting to sta- This suggest that neutralizing the accommodative bilize accommodation in the nonblurred eye by the demand for the fixation distance used in stereoacuity addition of +2.50 D for the observation distance did measures would not be clinically useful in enhancing not significantly affect the stereoacuity-monocular the sensitivity of either the Randot or Titmus test. blur relationship established under normal viewing A theoretic consideration of the effects of anisei- conditions where the accommodative demand was konia and monocular blur on stereoacuity predicts set by the viewing distance, but did reduce the overall that monocular blur induced by plus lenses would level of stereo sensitivity in both Randot and Titmus have the greater effect on stereopsis since plus lenses tests. It is concluded that any accommodative fluc- over the patient's prescription not only blur the tuations in optically induced anisometropia did not retinal image but also cause some interocular mag- account for the relationship determined between ste- nification differences, ie, aniseikonia. By using a reoacuity and monocular blur. This conclusion is spectacle magnification formula it can be shown that consistent with that of Westheimer and McKee,16 a +4.00 D lens creates an aniseikonia of approximately who reported that accommodative instability has an 11% in addition to a myopic blur. The differential insignificant effect on stereopsis. However, the greater effect of aniseikonia and monocular blur on stereo- scatter of data points seen for each value of monocular acuity is illustrated in Figure 8. Data points in this blur in Figure 3 leads one to speculate that a sudden figure represent the average aniseikonia and monoc- decrease in accommodative convergence associated ular blur values at Randot and Titmus stereoacuity with the 40-cm test distance likely placed a stress on levels extrapolated from the graphical representation fusional vergence mechanisms, resulting in decreased of the results for test numbers 1 and 2 (Figs. 2, 4, 5). stereopsis for each level of induced monocular blur. The linear regression lines through the data points

Downloaded from iovs.arvojournals.org on 09/29/2021 No. 5 OCULAR FACTORS AFFECTING STEREOPSIS / Lovosik ond Szymkiw 749

have a correlation coefficient greater than +0.97. turbances of binocularity resulting from conditions Regression lines for normal viewing conditions (Rx) such as strabismus, unilateral ocular pathology, and and accommodation relaxed by plus lenses (Rx +2.50 anisometropia greater than 1.50 D. For these cate- D) for either stereoacuity test are not statistically gories of patients, partial or complete suppression different at the 5% level for slope or Y-intercept may occur because of the relatively severe nature of values. binocular imbalance and, consequently, the three- The data presented in Figure 8 indicate that the dimensional images contained in the Randot test are cause of reduced stereoacuity can be undetected likely to disappear. With the Titmus test, however, aniseikonia or anisometropia. For example, a +1.5 complete suppression may still be associated with a D monocular blur has the same effect on stereoacuity false-positive response due to monocular cues to as an aniseikonia close to 8% (see dashed lines in Fig. "depth perception" such as lateral displacement of 8). Since the calculated spectacle magnification for a monocular targets.21 By comparison, the Titmus test + 1.5 D lens is only about 4%, the additional decrease appears more appropriate for populations not dem- in stereoacuity is attributed to retinal blur. Therefore, onstrating overt binocular anomalies. Our data suggest clinical screening procedures employing either Randot that the Titmus test would be more effective for the or Titmus tests may be effective in detecting reduced detection of conditions such as low-grade aniseikonia, stereopsis but will not be able to specify its cause refractive amblyopia, and anisometropia, all of which (aniseikonia vs anisometropia) without additional may cause an interocular difference in retinal image testing. size. Consequently, it would be the test of choice for A quantitative comparison of present data on detecting binocular abnormalities in children because stereoacuity as a function of induced aniseikonia or of its sensitivity to subtle binocular disorders, and monocular blur indicates that stereoacuity decreases since technical limitations restrict random dot stereo- approximately 1.84 times faster by the Titmus test grams to somewhat abstract symbols often too difficult than by the Randot test for identical levels of anisei- for young children to identify. The importance of konia or blur. These results imply that the Titmus early detection of binocular disorders for prevention test would be more useful than the Randot test for of irreversible visual damage is well documented. In detecting ocular abnormalities compromising binoc- adults, the Randot test is relatively easy to perform, ular function. This conclusion appears to be at odds but appears to have inferior sensitivity for the detec- with some previous studies, which have indicated tion of aniseikonia or anisometropia as indicated by that random dot stereograms are superior to contour the results of our study. Our results also indicate that stereograms for screening purposes.17"20 This discrep- variations in absolute and relative pupil size within ancy may be accounted for by differences in the the normal physiologic range, as well as interocular subject population of the present study and those of differences in retinal image brightness are not of earlier studies. Most studies concerned with deter- practical concern in the clinical measurement of 16 mining the relative effectiveness of random dot and stereopsis as previously suggested in earlier studies. contour stereograms for screening purposes have in- Key words: stereopsis, aniseikonia, anisometropia, accom- cluded subjects with strabismus and significant levels modation, retinal illuminance, anisocoria of anisometropia, aniseikonia, and amblyopia. Pre- sumably, these subjects would have inferior binocu- Acknowledgments larity and stereopsis possibly since birth, and, therefore, their performance on any tests of stereoacuity The authors thank Professor Arnulf Remole for providing the afocal magnifiers, the subjects for their kind cooperation, may differ from a population of normals. All our and Mrs. Gwen Smith and Joan MacLean for their patient subjects had normal binocularity and none of the typing of the manuscript. ocular factors known to degrade stereopsis. Further- more, all subjects were experienced observers capable References of fine stereoscopic discrimination and avoidance of 1. Romano PE, Romano JE, and Puklin JE: Stereoacuity devel- false positive responses on the Titmus test resulting opment in children with normal binocular single vision. Optician from monocular cues to depth perception. 170:18, 1975. The effectiveness of screening procedures to detect 2. Ogle KN: Researches into Binocular Vision. New York, Hafner, binocular anomalies by stereoacuity tests would likely 1964, pp. 133-140. be maximized if both Randot and Titmus tests were 3. Parks MM: Stereoacuity as an indicator of bifixation, Arruga employed for selected segments of the target popula- A, editor. International Strabismus Symposium, University of Giessen, 1966. Basel, Karger, 1968, pp. 258-260. tion. Based on the results of our study it appears that 4. Campos EC and Enoch JM: Amount of aniseikonia compatible the Randot test would be more useful for detecting with fine binocular vision: some old and new concepts. J binocular abnormalities related to more severe dis- Pediatr Ophthalmol Strabismus 17:44, 1980.

Downloaded from iovs.arvojournals.org on 09/29/2021 750 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Moy 1985 Vol. 26

5. Reinecke RD and Simons K: A new stereoscopic test for 14. Peters HB: The influence of anisometropia on stereosensitivity. amblyopia screening. Am J Ophthalmol 78(4):714, 1974. Am J Optom Arch Am Acad Optom 46:120, 1969. 6. Walraven J: Amblyopia screening with random-dot stereograms. 15. Cooper J: Clinical stereopsis testing: contour and random dot Am J Ophthalmol 80:893,1975. stereograms. J Am Optom Assoc 50:41, 1979. 7. Cooper J and Feldman J: Random-dot-stereogram performance 16. Westheimer G and McKee SP: Stereoscopic acuity with defo- by strabismic, amblyopic and ocular-pathology patients in an cused and spatially filtered retinal images. J Opt Soc Am 70: operant-discri mi nation task. Am J Optom Physiol Opt 55:599, 772, 1980. 1978. 17. Marsh WR, Rawlings SC, and Mumma JV: Evaluation of 8. Ogle KN: Precision and validity of stereoscopic depth perception clinical stereoacuity tests. Ophthalmology 87:1265, 1980. from double images. J Opt Soc Am 43:906, 1953. 18. Cooper J, Feldman J, and Medlin D: Comparing stereoscopic 9. Julesz B: Binocular depth perception without familiarity cues. performance of children using the Titmus, TNO and Randot Science 145:356, 1964. stereo tests. J Am Optom Assoc 50:821, 1979. 10. Lubkin V, Stollerman H, and Linksz A: Stereopsis in monocular aphakia with spectacle correction. Am J Ophthalmol 61:273, 19. Frisby JP, Mein J, Saye A, and Stanworth A: Use of random- 1966. dot stereogramns in the clinical assessment of strabismic patients. 11. Reading RW and Tanlamai T: The threshold of stereopsis in Br J Ophthalmol 59:545, 1975. the presence of differences in magnification of the ocular 20. Mitchell DE, Howell ER, and Keith CG: The effect of minimal images. J Am Optom Assoc 51:593, 1980. occlusion therapy on binocular visual functions in amblyopia. 12. Julesz B: Foundations of Cyclopean Perception. Chicago, Uni- Invest Ophthalmol Vis Sci 24:778, 1983. versity of Chicago Press, 1971. 21. Cooper J and Warshowsky J: Lateral displacement as a response 13. Levy NS and Glick EB: Stereoscopic perception and Snellen cue in the Titmus stereo test. Am J Optom Physiol Opt 54: visual acuity. Am J Ophthalmol 78:722, 1974. 537, 1977.

Downloaded from iovs.arvojournals.org on 09/29/2021