Positional Correlation Between Visual Axis and Stile-Crawford Effect Peak in Normal Eyes

Abstract

Purpose : One of the major challenges in cataract/refractive surgery is to determine the optimal center position for the optical correction. The entrance pupil center (EPC) and the 1^st Purkinje image are common centration references. The visual (foveal achromatic) axis along which no transverse chromatic aberration may be another candidate. The purpose of this study was to locate the visual axis with respect to EPC and evaluate the extent of its correlation to the peak of the Stiles-Crawford effect (SCE).

Methods : Six young typical subjects (12 eyes) were enrolled in the study. An optical system was developed to locate the visual axis and measure SCE. A translatable pinhole (diameter: 600 μm) was placed in the pupil conjugate and its location was imaged with a pupil camera. To find the visual axis, two small laser spots at 450 and 680 nm were co-aligned and delivered to the retina. Subjects were asked to move the translatable pinhole until these spots are perceived to be overlapped each other, i.e. zero transverse chromatic aberration. With the same system, the SCE was assessed by a conventional two-channel (reference and test) Maxwellian view system. Three repeated measurements at 19 pupil entrance points distributed within the central 4 mm pupil diameter were conducted. The peak positions were estimated by a two-dimensional Gaussian fitting function and correlated with the visual axis positions.

Results : The visual axis was, on average, located inferonasal to the EPC [x: 0.22 ± 0.40 mm, y: -0.18 ± 0.42 mm]. The SCE peak was also located inferonasal to the EPC [x: 0.26 ± 0.39 mm, y: -0.16 ± 0.38 mm]. The SCE peak positions were highly correlated in both horizontal and vertical meridians to the visual axes (R² = 0.97 for x, 0.96 for y coordinates). Five of the six subjects demonstrated mirror-symmetry in the coordinates of the visual axis and the SCE peak of both eyes (R² = 0.92 for the visual axis, 0.90 for the SCE peak).

Conclusions : The visual axis and the SCE peak locations were found to be variable between the subjects but highly correlated with each other within each individual. These findings may suggest a potential feedback mechanism between the ocular optics and the cone photoreceptor alignment.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.