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

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 rep- resent nonlinear computer- 320 fit models for changes

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