Role of ocular aberrations in dynamic accommodation control

Abstract

Background: Accommodation control is mediated by a number of cues, including blur, chromatic aberration and target proximity. Data from wavefront measurements have shown clear shifts in ocular aberrations during increasing accommodative demand, most notably a negative shift in spherical aberration. Work in adaptive optics, where aberrations have been corrected, has suggested a role for aberrations in the control of accommodation for some individuals. This study aimed to determine the relative effects of aberration correction and inversion on closed-loop stepwise accommodation responses to small increases and decreases in stimulus vergence. Methods: An adaptive optics system was used to modify ocular aberrations, while five participants viewed a high contrast target stepping 0.5 D in an inward or outward direction. Aberrations were variously unchanged, corrected or inverted following the step change in stimulus vergence. A Shack-Hartmann wavefront sensor was used to record accommodative responses to the stepwise changes in stimulus vergence. Accommodative response data were analysed using a curve fitting method to calculate the gain, latency and response time. Results: Correction of aberrations failed to have a significant effect on dynamic accommodative responses. Inversion of even-order aberration terms produced a significant reduction in accommodative response gain for outward steps in stimulus vergence. Additionally, an increase in the number of accommodative responses in the wrong direction was seen following aberration inversion. Inward steps were not altered significantly by aberration inversion. Conclusion: Accommodation in humans appears to derive a cue from the even-order aberrations of the eye to help guide the initial direction of responses to reductions in stimulus vergence (that is, disaccommodation). With all aberrations and even-order aberrations inverted, the number of incorrect directional responses to outward moving stimuli increases, suggesting that aberrations are important in determining the initial path of stepwise accommodative responses.

Accommodation Responds to Optical Vergence and Not Defocus Blur Alone

“…In Marín-Franch et al (IOVS 2016;57:ARVO E-Abstract 3952), the study with greater number of subjects, 5 out of 14 subjects (35%) could not accommodate in monochromatic light. In Chin et al, 12 the number of subjects who could not accommodate in monochromatic light was 1 out of 5 (20%), and in Chen et al, 15 1 out of 6 (17%). It is not possible to extrapolate the results of this study to the 15% to 35% of subjects who do not accommodate in monochromatic light.…”

“…It has long been thought that correct accommodation is achieved by using a trial-and-error strategy. [18][19][20][21] Nevertheless, the correct and fast response of the eye to variations in the accommodative demand over time 11,12,15 cannot be explained 4 by such a simple trial-and-error strategy. A provocative alternate explanation proposed by Fincham 22 is that the visual system is able to detect or infer the sign of defocus directly from wavefront spherical curvature, or optical vergence.…”

“…9 Among optical cues, chromatic aberration has been shown to provide a strong reliable directional cue for accommodation. 10 Monochromatic aberrations, [11][12][13][14][15] microfluctuations of accommodation, 3,4,16 and the Stiles-Crawford effect 17 may also provide cues for accommodation.…”

The Human Eye and Adaptive Optics

“…Furthermore, the improvement in these measures seems to be only found with larger pupil sizes (Elliott & Chapman, 2009) which do not occur in natural viewing. It also seems that the presence of aberrations is utilized as a cue by the visual system for accommodation (Kruger et al, 1993;Chen et al, 2006;Chin et al, 2009b) and perhaps for control of other oculomotor functions of the eye. Therefore, correcting these aberrations may negatively interfere with these visual functions.…”

Dynamics of the near response under natural viewing conditions with an open-view sensor

“…Several approaches [12][13][14][15] have been proposed to characterize accommodation gain, peak velocity, temporal response and latency. However the dynamic interaction of convergence response, accommodation and pupil constriction have not been fully illustrated under natural viewing conditions.…”