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D.S. Nassif, N.V. Piskun, D.L. Guyton, D.G. Hunter; Binocular vs. Monocular Accommodation to a Monochromatic Point Source as Detected by the Pediatric Vision Screener . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4837.
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Purpose: Limitations in instrumentation have made it difficult to measure objectively the accommodative state of both eyes simultaneously during near viewing. In this study, the Pediatric Vision Screener was used to quantify binocular focus during fixation on a monochromatic near target. Methods: Instrument Design: Focus and alignment of each eye simultaneously. To detect focus, a point fixation source of 830 nm light illuminates the eyes from 40 cm. Light reflected by the fundus of each eye is focused (by the optical system of the eye itself) onto a bull's eye photodetector (consisting of an annulus (A) and a center (C) of approximately equal active area). Data Acquisition: To generate focus curves, C/A was measured with a range of trial lenses in the light path. Subjects were studied both binocularly (lenses in front of both eyes) and monocularly (non-tested eye occluded). The device confirmed bilateral foveal fixation during binocular testing. The focus peaks obtained during monocular and binocular viewing were compared. Subjects: Four subjects (three controls and one with strabismus and amblyopia) ranged in age from 25 to 38. Results: For all subjects, the peak C/A occurred with an added lens of +1.0 to +2.0 diopters (D). The focus curve peaks were flatter during monocular viewing than during binocular viewing, (width 2.25 D at half maximum during binocular viewing.) Astigmatism produced an expected bimodal peak. In all control subjects, the left and right eye focus curve peaks coincided during binocular viewing; in contrast, the peaks were separated by 1 to 2 D during monocular viewing. In the strabismic subject, the focus curves were identical in the sound eye during binocular and monocular viewing, but the focus curve in the amblyopic eye shifted to the right during monocular viewing. Conclusions: Despite apparent accommodation in all subjects, a plus lens was required for best focus on the point source; this shift of the focus curve peak was most likely due to the chromatic aberration of the eye for near-IR light. The additional sharpening of the focus curve peak observed during binocular viewing indicates that convergence on the single point fixation target further restricted the subjects' ability to adjust accommodation without compromising convergence. The Pediatric Vision Screener provides a unique methodology for measuring accommodation in both eyes simultaneously during binocular viewing, however the monochromatic, near-IR point source may not be an adequate stimulus for accommodation in adult subjects.
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