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Najnin Sharmin, Brian Vohnsen; Relationship between the sign of defocus and accommodation of the human eye. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2947.
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The aim of this study is to examine whether the human eye can determine the sign of defocus and initiate accommodation before relying on other visual clues. This will determine if the retina itself can produce the signals needed for accommodation once subjected to defocused images and relatedly if this can be a driver for emmetropization of the developing eye.
A monocular vision system has been constructed using a current driven tuneable lens (Optotune) to make random changes of power every 10 seconds in a conjugated pupil plane. Subjects view with their right eye a bright fixation chart on a dark background at 1 m distance. The left eye is occluded by a patch. A fast (20 Hz) Hartmann- Shack wavefront sensor (Thorlabs) takes continuous readings of the ocular aberrations using a focused 980 nm laser as guide star on the retina. A total of 4 subjects, age 27 – 48 years, with normal vision have been analyzed using white, green, red and blue accommodation targets and their accommodative responses have been recorded. The results are compared to the predictions of a geometrical optics model for a schematic eye with and without defocus error.
Defocus and other Zernike aberration terms have been determined for the subjects as a function of time following rapid step changes in optical power of the tuneable lens. The eyes of all subjects adjusted quickly (100 – 400 ms) within their accommodative range to compensate changes in optical power of the tuneable lens. In all cases, there is no sign of accommodating in the wrong direction or overshooting of the response. Using white light, subjects accommodate within 50-200 ms. For green light, the range is 100-200 ms whereas for red and blue light the duration is within the range of 100-400 ms.
The experimental results show that accommodation of the human eye is fastest for white and green light where sensitivity is highest. Within the speed of detection, the accommodation always occurs in the correct direction that compensates defocus and therefore the eye is capable of determining the sign of defocus before other accommodative clues become apparent. This agrees well with the geometrical optics model predictions that takes account of the retinal thickness and thus it explains a plausible mechanism in the monocular accommodation of the human eye.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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