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Michael D. Crossland, Adnan Tufail, Gary S. Rubin, Andrew Stockman; Mesopic Microperimetry Measures Mainly Cones; Dark-adapted Microperimetry Measures Rods And Cones. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4822.
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Conventional microperimetry using the MP-1 microperimeter (Nidek, Italy) is typically performed under mesopic conditions in a dim, but not completely dark, room. We have recently described a modification to this microperimeter that enables it to be used for dark-adapted microperimetry (Crossland, Luong, Rubin and Fitzke, BMC Ophthalmology, 2011). The purpose of this study was to determine the extent to which cones are measured using conventional (mesopic) microperimetry, and the extent to which rods are measured using our modified microperimetry.
Five healthy volunteers with good vision participated in the study. Mesopic microperimetry was performed in a dim room without dark adaptation. Dark-adapted microperimetry was performed after pupil dilation and 30 minutes of dark adaptation in a completely dark room (<0.1 lux). Each type of microperimetry was performed using Goldmann III targets of two wavelengths: 483nm and 562nm. Thresholds for each wavelength were compared for each microperimeter. Comparison with established spectral sensitivity curves for cone (Stockman and Sharpe, 2000) and rod (CIE, 1951) photoreceptors identified whether our results were consistent with cone or rod responses.
For conventional mesopic microperimetry, mean sensitivity at 483nm was 0.36 log unit lower than at 562nm (sd: 0.38). This is consistent with a photopic or mainly cone response (a cone-only response would predict the response at 483nm to be 0.57 log unit lower). For dark-adapted microperimetry, mean sensitivity at 483nm was 0.07 log unit higher than at 562nm (sd: 0.09). This is not consistent with the cone-only response prediction of 0.57 log unit lower or with the rod-only response prediction of 0.38 log unit higher.
These data suggest that, in people with good vision, conventional mesopic microperimetry is mainly photopic. Stimuli presented using our in-house modified dark-adapted microperimeter are truly mesopic, since they are detected by rods and cones. The cone contribution to dark-adapted microperimetry thresholds is approximately 3dB.
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