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068 - Le gain d’acuité visuelle binoculaire est-il lié à une modification de la dioptrique de l’œil mesurée en vision monoculaire chez le sujet myope ?

H. Kobashi et al. have demonstrated that subjective spherical refraction in monocular vision was significantly more myopic than in binocular vision. Their study was based on subjective visual acuity (VA) measurements and on refractive errors of the right eye measured using a monocular Shack-Hartmann device. The aim of the present study is to bring additional outcomes to the Kobashi’s study through the comparison of the objective refractive error measured simultaneously for the two eyes, i.e. in binocular vision, with the one measured successively for each eye in monocular vision, in myopic subjects. This study, carried out using for the first time a binocular automatic refractometer equipped with a double Shack-Hartmann, seeks to ascertain whether the enhancement of the subjective VA measured in binocular vision condition, is linked to some objective changes in the optical power of the eye measured in monocular vision.

Retrospective data from 136 myopic eyes (68 subjects) over 0.25 diopter, without further ophthalmologic pathology, were analysed. Refractive measurements were carried out using an automated refractometer, the "Eye-Refract". The measurement protocol includes an objective measure of the spherical ametropia and the astigmatism simultaneously for both eyes, followed by the same measure in monocular vision. Measurements are made twice at 10 minutes interval. The decimal VA is noted after the optical compensation is automatically applied by the Eye-Refract device. Student’s T-tests, Wilcoxon’s tests, and Spearman’s tests have been applied to evaluate the repeatability of the measurements, and to compare the values of sphere, astigmatism and VA obtained under conditions of both binocular and monocular vision.

For all comparisons between measurements, the Student’s and the non-parametric Wilcoxon's tests demonstrate the good repeatability of the two series of measurements. The high correlation coefficients, >0.98 for the sphere and >0.91 for the cylinder, show that the measurements obtained in binocular vision and in monocular vision are very similar. In termes of VAs assessents, the results show a difference between the binocular and monocular vision, with a better VA of about one line (in the VA chart) observed in the binocular vision.

The Eye-Refract device provides refractive measurements with a repeatability better than that obtained with conventional automatic refractors. The results show that there is no significant difference between the refractive errors (sphere and cylinder) in binocular and in monocular vision. But there is a significant difference between the corresponding VAs. This is in agreement with many studies and is related to the binocular summation effect. This study shows that the gain of binocular VA is not explained by a factor related to sphere and astigmatism corresponding to low order aberrations of the eye.

The results of this study are in agreement with those of H. Kobashi et al. with regard to the increase of the binocular visual acuity compared to the monocular vision. Moreover, they show that this gain of subjective VA, observed between binocular and monocular vision, is not linked to variations in the spherical and cylindrical refractive error for patients with myopia. This difference is rather due to optical factors, such as pupil diameter, luminance level, high level aberrations and to a possible effect of neurophysiological cortical summation.