Driving accommodation using simulated higher-order aberrations

Abstract

The purpose of accommodation is to minimize blur. Defocus blur is the major source of blurring in the retinal image, but defocus blur itself cannot tell the eye if it is focused too near or too far. When the human eye is shown real blur, accommodation always changes in the correct direction without searching, so the visual system can somehow determine the sign of defocus. Potential signals for the sign include temporal fluctuations of accommodation (e.g., microfluctuations), chromatic aberration, and higher-order aberrations (HOAs). We investigated whether simulated HOAs—specifically, astigmatism and spherical aberration—provide the needed sign information to drive accommodation in the right direction. Measurable astigmatism occurs in most people; its magnitude and axis varies across individuals. The point-spread function (PSF) of a defocused astigmatic eye is elliptical with the major axis in one direction when the object is farther than current focus and that axis rotated by 90deg when the object is nearer. Spherical aberration is also present in nearly all people. It generally causes marginal rays through the pupil to focus in front of the retina when central rays are focused on the retina. Because of this marginal-central difference, spherical aberration creates different PSFs for objects at different depths relative to the eye's current focus. Therefore, astigmatism and spherical aberration can in principle provide sign information to guide accommodation in the correct direction. We investigated whether they really do aid accommodation. We measured observers' aberrations with a Shack-Hartmann wavefront sensor and generated rendered stimuli that simulated either their native astigmatism or spherical aberration for various simulated distances. We found that simulated astigmatism and spherical aberration can indeed drive accommodation in the right direction, but they are less effective than real changes in focal distance. We conclude that these natural aberrations provide useful sign information for accommodation. Meeting abstract presented at VSS 2018