Purpose: Night myopia, the myopic shift occurring at low luminance, has been studied during decades. In most cases, the experiments were performed monocularly with scarce data available under natural binocular viewing conditions. We used a new open-view binocular sensor to study binocular accommodation responses under natural viewing conditions to evaluate the effect of binocularity on night myopia.
Methods: The instrument is a Hartmann-Shack based sensor to measure refraction, aberrations, pupil diameter and the relative position of both eyes simultaneously in real time (25Hz) with an invisible infrared (1050nm) beam. Subjects unobtrusively observe a 1.3 degree high-contrast black Snellen E letter on white background displayed on an LCD monitor at 2.75 meters. Seven young subjects participated in the study: age 28.7±2.69 years; spherical refractive error was -0.11±0.32 D and -0.24±0.33 D, and astigmatism was -0.46±0.19 D and -0.31±0.14 D for the left and right eye respectively. Accommodation error, high-order aberrations, inter-pupillary distance and pupil size were measured simultaneously in both eyes at 7 different luminance levels ranging from 180 to 0.0005 cd/m2. Measurements were performed both monocularly (with the fellow eye covered) and binocularly.
Results: The average values of myopic shift when measured monocularly were -0.51±0.57 D and -0.47±0.55 D for each eye. In binocular vision, these values were reduced to -0.27±0.32 D and -0.26±0.32 D for the left and right eyes respectively. Both the magnitude and variability of the defocus shift were larger in the monocular cases. The average pupil size was smaller in the binocular case for most luminances. A shorter relative inter-pupillary distance in monocular viewing revealed the presence of dark-vergence that is correlated to the myopic shifts.
Conclusions: Myopic shifts occurring at low luminance are smaller under binocular conditions than when measured monocularly. Dark-vergence occurred under monocular conditions and may have a larger impact on vision. Natural binocular viewing reduces myopic shifts at low luminance and can mitigate the visual impact of night myopia.