Accommodation Responds to Optical Vergence and Not Defocus Blur Alone

Abstract

Citation: DelÁguila-Carrasco AJ, Marín-Franch I, Bernal-Molina P, et al. Accommodation responds to optical vergence and not defocus blur alone. Invest Ophthalmol Vis Sci. 2017;58:175858: -176358: . DOI:10.1167 PURPOSE. To determine whether changes in wavefront spherical curvature (optical vergence) are a directional cue for accommodation.METHODS. Nine subjects participated in this experiment. The accommodation response to a monochromatic target was measured continuously with a custom-made adaptive optics system while astigmatism and higher-order aberrations were corrected in real time. There were two experimental open-loop conditions: vergence-driven condition, where the deformable mirror provided sinusoidal changes in defocus at the retina between À1 and þ1 diopters (D) at 0.2 Hz; and blur-driven condition, in which the level of defocus at the retina was always 0 D, but a sinusoidal defocus blur between À1 and þ1 D at 0.2 Hz was simulated in the target. Right before the beginning of each trial, the target was moved to an accommodative demand of 2 D.RESULTS. Eight out of nine subjects showed sinusoidal responses for the vergence-driven condition but not for the blur-driven condition. Their average (6SD) gain for the vergencedriven condition was 0.50 (60.28). For the blur-driven condition, average gain was much smaller at 0.07 (60.03). The ninth subject showed little to no response for both conditions, with average gain <0.08. Vergence-driven condition gain was significantly different from blurdriven condition gain (P ¼ 0.004).CONCLUSIONS. Accommodation responds to optical vergence, even without feedback, and not to changes in defocus blur alone. These results suggest the presence of a retinal mechanism that provides a directional cue for accommodation from optical vergence.

Aberrations and accommodation

“…The experimental system should include the possibility of changing the vergence of the target (by changing the distance between the eye and the target, or by adding lenses), to stimulate the subject's accommodation. There are several commercially available devices that can measure aberrations while stimulating accommodation (for example, irx3, COAS‐HD, WASCA, iTrace) as well as custom‐built systems …”

“…Knowledge of how aberrations vary with static accommodation provides information about the shape of the surface of the lens as well as information about its internal structure . Dynamic accommodation studies usually shed light on fundamental questions such as which cues trigger the accommodation system to accurately change the power of the lens and accommodate in the right direction, which is of particular interest concerning myopia development . From an applied science perspective, knowledge of how aberrations change with accommodation can lead to improved designs of multifocal and accommodative intraocular lenses, which imitate the profile of ocular aberrations during accommodation.…”

Dynamic accommodation without feedback does not respond to isolated blur cues

“…The average accommodation gain for the experimental conditions was almost a factor of five smaller than the gain for the control natural condition, which emphasizes the importance of real dioptric demand on accommodation responses (Del Águila-Carrasco et al, 2017). No significant differences were observed across the four experimental conditions, either in average gain or in phase lag, so the results appear to be robust to the statistical analysis.…”

“…Dynamic accommodation responses gain worsened considerably when real changes in target vergence were removed, even when the observed images with simulated blur contained information on defocus and information on the subject's own eye HOAs. This drastic decrease in accommodation gain cannot solely be attributed to the absence of feedback from changes in accommodation, because previous open-loop experiments (Del Águila-Carrasco et al, 2017, Kruger et al, 1997, Kruger et al, 1995, Lee et al, 1999) have shown that accommodation responds strongly even without feedback, especially when the stimulus includes reliable directional cues to the sign of defocus.…”

“…As part of the inclusion criteria, subjects had to be able to accommodate with a gain greater than 0.2 to a monochromatic target (550 ± 5 nm) that moved sinusoidally towards and away from the eye, changing its dioptric demand between 1.0 D and 3.0 D. The targets were presented under normal closed-loop conditions with feedback from accommodation, no pinhole, and viewed through a 4-mm pupil. This control experiment was used to ensure that subjects were able to accommodate under the monochromatic light used in our experiment (Del Águila-Carrasco et al, 2017). The results of these control trials are reported in the Results section.…”