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N. J. Coletta, A. Raghuram, T. Nguyen, M. Rondon; Neural Contribution to the Mesopic Acuity Deficit in Myopia. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1434.
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© ARVO (1962-2015); The Authors (2016-present)
Myopes experience a greater loss in acuity with decreasing luminance than emmetropes (Coletta et al., ARVO, 2007). Optical and neural factors may contribute to this effect, since myopic eyes are reported to have lower optical quality than emmetropic eyes and the axial elongation of myopic eyes is associated with retinal stretching that may increase the spacing of retinal neurons. Retinal illumination should also be decreased in eyes with increased axial lengths. We measured acuity at a low mesopic light level with interference fringes that minimize the effect of the eye's optical quality, in order to better estimate the effects of retinal illumination and possible neural contributions to mesopic acuity in myopia.
Measurements were made on twenty-three subjects who had 20/25 or better spectacle-corrected acuity for a photopic high contrast acuity chart. Spherical equivalent refractions ranged from plano to -8.25 D. The retinal magnification factor (RMF; mm/deg) for each eye was estimated from corneal topography and IOLMaster biometry of anterior chamber depth and axial length, and the RMF was used to convert acuity in angular units of cyc/deg to retinal units of cyc/mm. Interferences fringes were 543nm and had a mean retinal illuminance of 0.1 troland. After 15 minutes of dark adaptation, a two-alternative forced choice vertical-horizontal discrimination task was used to estimate the fringe acuity limit.
Mesopic interferometric acuity significantly decreased with increasing myopia at a rate of 0.025 log unit per diopter of myopia (p=0.029). This rate is steeper than the rate of about 0.013 log unit per diopter previously published for photopic interferences fringes (Coletta and Watson, 2006; Atchison et al., 2006). When acuity was plotted against axial length, mesopic interference fringe acuity fell at a rate of about 0.067 log unit per mm increase in axial length (p=0.028). This slope is steeper than the comparable data for photopic interferometric acuity (0.03 log unit per mm) and much steeper than the prediction based on decreased retinal illumination, which is about 0.01 log unit per mm (based on a change of acuity of 0.25 log unit per log unit change in illumination).
Mesopic interferometric acuity decreases with increasing myopia and axial length at rates that are steeper than those observed at photopic light levels. These results imply that factors other than decreased retinal illumination and optical quality affect visual acuity in myopia.
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