Objective: The purpose of the current study was to investigate whether prisms that are selected by participants minimize binocular blur, increase visual comfort, and yield better stereoacuity. We also investigated whether fixation disparity (FD) and associated phoria are the only reason for the increased stereothreshold in individuals with binocular single vision.
Methods: Two groups of participants (14 males, 16 females; age 19-53 years old, mean 26.17±5.65 SD) without strabismus, amblyopia, or suppression were included in the study. Their imperfect stereoacuity (i.e., induced by sphere lenses) was measured without and with prisms. The prisms were obtained with associated phorometry (associated vergence) technique that takes into account the visual comfort and binocular blur (clarity) reported by the participants. In group A, stereoacuity of 13 participants with artificially-reduced stereoacuity (i.e., 16 seconds of arc or worse) were measured without and with prism corrections at near (40 cm) using the Randot 3 with Lea
Symbols® Stereoacuity Test (Randot 3 test). Similarly, in group B, stereoacuity of 16 participants with artificially-reduced stereoacuity were measured at near without prisms (known to the participant), with placebo correction (without prisms unknown to the participant), and with prisms, respectively, using Randot 3 test. FD was measured without and with prisms. 28 participants in group A and B were tested by a Saladin card, 22 of whom were also tested by a Sheedy disparometer. For far stereoacuity, degraded stereoacuity (36 seconds of arc or worse) was simulated in group B (17 participants) and their stereoacuity and associated phoria were measured at far without prisms using the Lea Symbols® Stereo test (Lea stereo test) and Nonius cross target test, respectively. Their far stereoacuity with prisms was measured with Lea stereo test.
Results: near stereoacuity with prism in group A improved significantly (Wilcoxon’s Z = -2.11, p = 0.035) when it was compared with baseline stereoacuity without prism. In group B, near stereoacuity with prism was significantly better than stereoacuity with placebo (Wilcoxon’s Z = -2.93, p = 0.003) and baseline stereoacuity without prism (Wilcoxon’s Z = -3.40, p = 0.001). The distribution of baseline stereoacuity between group A and B was not significant (Kolmogorov–Smirnov z= 0.682, p=0.74). The stereoacuity with prism in group A and stereoacuity with placebo in group B did not differ significantly (Mann -Whitney U= 81, p = 0.329). There was not a significant correlation between FD and stereoacuity at near. The results of FD (i.e., measured by Saladin card and Sheedy disparometer) showed that prisms reduced FD significantly (mean difference= -0.689 ± 1.46 SD, P=0.019) and (Mean difference = -2.36 ± 3.29 SD, P=0.003), respectively. For far stereoacuity, the improvement of stereoacuity with prism was significant (Wilcoxon’s Z = -2.36, p = 0.018) compared with baseline stereoacuity. Associated phoria and far stereoacuity were not significantly correlated (r=0.004, p=0.98). Moreover, the median of comfortable prisms (prism correction) was significantly greater than the median of associated phoria (Wilcoxon’s Z = -2.60, p = 0.009).
Conclusion: We suggest that comfortable prisms (i.e., selected by individuals) can improve near and far stereoacuities, although this was not supported when we compared group A with group B. It is possible that a practice effect (vergence adaptation) confounded our results. Moreover, the large differences in stereoacuity without prism that could not be controlled between both groups might explain the absence of significant differences between group A and B. Our findings also indicate that stereoacuity is not correlated with FD nor associated phoria. Thus, we propose that vergence variability or/and the conflict between vergence and accommodation systems are involved in stereoacuity. Further studies are needed to avoid our confounders and to confirm our suggestions.
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