Does dynamic accommodation respond to the shape of the blurred retinal image without changes in physical vergence?

Abstract

Purpose : Although some eyes use cues from astigmatism and high-order aberrations (HOAs) to detect the sign of defocus of moving monochromatic stimuli, evidence has been found that eyes can also accommodate when all aberrations except defocus are corrected. But, is dynamic accommodation a response to the shape of the blurred image itself or to the physical change in vergence? To tackle this question, simulations of a sinusoidally moving stimulus with and without aberrations were generated and presented to observers.

Methods : Fourteen subjects were recruited with ages between 20 and 38. Subjects had no ocular pathology and astigmatism below 1 D. Subjects were first presented a control condition with a Maltese cross in a monochromatic microdisplay (550 ± 5 nm) through a 4-mm artificial pupil. The stimulus vergence was changed sinusoidally at 0.2 Hz during 25 seconds between 1 D and 3 D from the subject's far point. Aberrations were measured with a Shack-Hartmann sensor at 15 Hz and accommodative response calculated. From these measurements, simulations were generated of a Maltese cross with varying sinusoidal defocus for a 4-mm pupil and (1) no aberrations, (2) subject’s natural aberrations, (3) unbalanced spherical aberration of 0.2 µm, and (4) astigmatism, c₂^–2, of 0.1 µm. The Figure shows snapshots of the simulations generated at different defocus levels. Simulations were presented in random order during 25 seconds through a 0.75-mm pinhole, effectively eliminating feedback from changes in accommodation. The retinal luminance was kept constant at 100 Td in all control and simulation trials. Each condition was repeated 6 times.

Results : Nine out of 14 subjects accommodated for the control condition when physical vergence changed sinusoidally (average gain was greater than 20 %). The average gains for the 4 simulation conditions over these 9 subjects were very similar at 5−7 %. All pairwise differences in gain between simulation conditions were lower than 2 %. None of them were statistically significant.

Conclusions : No evidence was found that changes in shape of the retinal image of the aberrated eye alone are used as cues to detect the sign of defocus when accommodation feedback is eliminated.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Simulations of the Maltese cross at −0.5 D and 0.5 D of no aberrations (upper panel) and with a subject’s own aberrations (lower panel).