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L. Chen, S. Somani; Proximal Accommodation During Measurement of Refraction. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4557.
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Clinical wavefront sensors and automatic refractors normally use a fogging fixation target to relax the patient’s accommodation during measurement. However, this kind of fixation target does not always work well because some eyes may over-accommodate when viewing a static fixation target. This study examined whether the proximal cue given by a moving fixation target could help relax the patient’s accommodation during refraction measurement.
Eight eyes from 4 subjects ranging in age from 21 to 29 were chosen for this study so that the experiment would be conducted on eyes with a tendency to accommodate while looking into the instrument. Each subject was aligned to a wavefront sensor while viewing the fixation target in white light, which was displayed on a color micro-display screen conjugate with the retinal plane of the eye. The fixation targets comprised five different movies made from sequences of images and played at 15 frames per second. Each movie featured a prominent object such as a baseball, a picture of a globe, etc. placed at the center of the screen, which subtended a six-degree visual angle. The micro-display was configured at optical infinity. As the movie progressed the object became progressively smaller. The demagnification rate was chosen to create a realistic perception of the object moving away from the viewer. The movies were paused at times to present a static target. Five different movies were played randomly on the micro-display. No pharmacological agent was used to dilate the patient’s pupil. Refractive error and higher order aberrations up to the 6th order in each eye were measured using a Shack-Hartmann aberrometer. Six measurements of each eye were performed with a static fixation target and 6 measurements of each eye were performed with a moving fixation target.
When the 8 eyes of the 4 subjects viewed a moving target, the standard deviation of the refractions in the multiple measurements was significantly smaller than when the eyes viewed a static target. The average standard deviation across 8 eyes was reduced by 57% when viewing the moving target. The mean sphere power of 6 of the 8 measured eyes moved towards positive when viewing a moving fixation target, which indicates improved relaxation of accommodation.
The proximal cues presented by the dynamic target helped to relax and stabilize the accommodation state of the eyes of this cohort. The perception of the target moving away from the observer appears to influence the neural system to command the ciliary and zonular muscles to relax accommodation to the farthest viewing point of the eye.
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