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TA Watson, NJ Coletta; Refractive Error and Interferometric Acuity . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2005.
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Purpose: The increased retinal image magnification that occurs in axial myopia should improve visual acuity, but the enlarged retinal image may be sampled by neural elements that have wider than normal spacing due to retinal stretching. In addition, myopic eyes may have greater optical aberrations that would tend to reduce acuity. The purpose of this study was to determine whether visual acuity varies with refractive error when optical effects are minimized, and whether any variation in neural acuity with refractive error is simply due to axial length magnification. Methods: Spatial acuity was measured in ten adult subjects with no signs of ocular pathology. The distance refractive errors ranged from -8.00 D to +2.25 D (spherical equivalent) and were primarily axial in nature. Laser interference fringes were formed directly on the retina and thus were unaffected by optical factors of the eye. The 543nm interference fringes were presented at the fovea, 4 and 10 degrees eccentricity. Acuity was estimated from psychometric functions of orientation discrimination, using a 2AFC method. The retinal magnification factor of each eye was estimated from corneal power and ultrasound measurements of anterior chamber depth, lens thickness and axial length. Interferometric acuity was then expressed in terms of conventional angular units (cycles/degree) or retinal spacing (cycles/mm). Results: Foveal interferometric acuity was higher in the myopes, even when acuity was converted to cycles/mm. This trend was similar at 4 degrees eccentricity, but there was little variation in acuity with refractive error at 10 degrees eccentricity. Conclusion: The myopes in this study had higher foveal acuity than emmetropes or hyperopes even when axial length magnification was taken into account. The results imply that the fovea of eyes with moderate myopia could have a higher neural sampling density than emmetropic eyes, but that retinal stretching reduces acuity in the parafovea of these myopes. Our results are consistent with a previous study of cone topography in the primate retina (Troilo, VSIA, 1998).
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