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Anirbaan Mukherjee, Richard A Bone, Miguel A. Escanelle; Validation of the Mapcat for lens density measurements. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1062.
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© ARVO (1962-2015); The Authors (2016-present)
To compare lens density measured by heterochromatic flicker photometry under photopic conditions with that obtained under scotopic conditions at absolute threshold.
The LED-based Mapcat flicker photometer, primarily designed for measuring macular pigment optical density (MPOD), was used to measure lens density or lens equivalent age (LEA). A 150 stimulus alternating between blue and green was viewed centrally and subjects adjusted the blue intensity to minimize flicker in the periphery. 25 subjects participated using their right eye only. We also measured LEA in the same subjects by determining their absolute thresholds when fully dark adapted. A 10 stimulus was positioned at 7.50 eccentricity from a fixation target. The stimulus was illuminated by either a blue or green LED, similar to those used in Mapcat. It was presented in the form of square wave pulses, 0.5 s on and 1.0 s off. Subjects counted pulses during a 20 s period for incrementally decreasing luminance. An inverse sigmoidal curve was fit to the data to obtain the 50% threshold. By comparing the thresholds with the corresponding rhodopsin absorbance, we calculated the corresponding lens transmittances, which were converted to LEA using a published model [Sagawa K & Takahashi Y JOSA 18, 2659 (2011)].
Analysis of a plot of photopic LEA, P (yr), vs. scotopic LEA, S (yr), yielded a regression line P = 0.942S + 0.23, (r2 = 0.66, p < 0.0001). Although the slope was close to unity and the intercept small, individual differences between pairs of measurements were sometimes large.
Measurement of LEA under scotopic conditions was found to be time consuming and imprecise. LEA measured by Mapcat was fast and precise, and therefore much more practical. In an attempt to reconcile differences between the results, where these occurred, we examined the effect on the Mapcat results of varying the long (L)- to medium (M)-wavelength cone ratio used in the calculation of LEA. Using L:M ratios between reported extremes [ Sharpe et al. J. Vis. 5, 948 (2005)], we were still not able to explain all the differences. Other factors such as eye movements causing poor fixation in the scotopic test might be responsible. Nonetheless, on average, the scotopic results validated the use of the Mapcat as a valuable instrument for LEA as well as MPOD measurement.
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