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C.D. Bhatia, D.C. Hood, M. Lyons, S.G. Coupland, D. Birch; An Easy and Automated Method for Estimating Receptor Parameters From A–Waves of the ERG . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5676.
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The leading edge of the human a–wave of the electroretinogram (ERG) is determined by the sum of the underlying receptor activity. The parameters of a model of rod phototransduction , while usually determined with a set of 4 or more flash intensities and a fitting program [3,4], can be estimated with only one or two flash intensities and a simple, cursor method. Here this simple cursor method is automated in a widely available spreadsheet program.
Full–field ERGs were recorded from 50 patients with RP with a contact lens electrode (GoldLens, DiagnosysLLC). Four white flashes, 4.4, 4.0, 3.7 and 3.3 log scot td–s were presented in the dark and on a rod–suppressing background. Rod–only responses were isolated with a subtraction method.[4,5] An equation based upon a model of rod transduction was fitted and yielded two parameters, S (sensitivity) and RmP3, maximum response. The cursor (C) method: Using only the response to the 4.0 log td–s flash (1–flash method) or 4.4 and 3.7 log scot td–s (2–flash method), a horizontal line was drawn at the maximum a–wave peak to estimate Amax. To estimate tAmax, a diagonal line was drawn along the leading edge of the a–wave of the 4.0 log scot td–s flash (1–flash method) or the 3.7 log scot td–s flash (2–flash method). The location on the x–axis of the intersection of the two lines defines tAmax. An EXCEL program (X method) was developed to automatically estimate tAmax and Amax.
The cursor and X methods showed extremely good agreement with correlation r–values of essentially 1.0 for the parameters, Amax and tAmax, for both the 1– and 2–flash methods. The slopes of the regression lines were essentially 1.0 for Amax, and were 1.02 (1–flash) and 0.98 (2–flash) for tAmax.
The automated, spreadsheet method can be used to estimate rod receptor parameters. It should be relatively easy to incorporate the algorithm into the software of commercial equipment so that estimates of these parameters can be easily obtained in the clinic with either the 1– or 2–flash method. 1. Hood & Birch (1990) VNS. 2. Lamb & Pugh (1992) JP; 3. Breton et al (1994) IOVS. 4. Hood & Birch (1994) IOVS; 5. Hood & Birch (2005), chapter
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