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M.-L. Garon, C. Jauffret, R. K. Koenekoop, R. C. Polomeno, J. M. Little, P. Lachapelle; Estimating the On-Off Contribution to the Human Short Flash Photopic ERG With the Photopic Hill: Normative Data and Clinical Applications. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3804.
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© ARVO (1962-2015); The Authors (2016-present)
A mathematical model combining a Gaussian (G) and a logistic growth (L) function (Hamilton & al., 2007) was recently fitted to the photopic ERG luminance-response curve or photopic hill (PH). At last ARVO, we showed that the G and L functions seem to depict the OFF and ON contributions to the building of the PH. The purpose of this study was: 1- to determine the clinical utility of our finding with the GL ratio, 2- to confirm that the GL ratio reflects ON-OFF contribution to the PH.
The ON/OFF contribution to the PH was determined with the GL ratio [GL=Gb/(Gb+Vbmax)] where Gb and Vbmax represent the amplitude of the G and L functions respectively. We also obtained long flash (200ms) and short flash (5ms) ERGs from 6 normal subjects (bg: 30 cd.m-2; intensities 5 to 500 cd.m-2 in 8 steps; bandpass: 0.1-500 Hz) from which we derived the OFF/(ON+OFF) amplitude ratio. We also considered the amplitude ratio of the long flash (ON + OFF) ERG / short ERGs to estimate the interaction of the ON and OFF resulting in the short flash b-wave.
Significantly different GL ratios of 0.60±0.08, 0.78±0.07 and 0.33±0.06 were obtained from normal subjects, CSNB-1 and cone pathway anomaly patients respectively. In contrast, RP patients showed a much broader distribution (0.43 ±0.23). The mean OFF/(ON+OFF) ratio was 0.39 ± 0.05, a value significantly smaller (p < 0.05) than the average GL ratio, while the long flash/short flash amplitude ratio was 1.55±0.25.
Use of the GL ratio distinctly segregated normal subjects from patients affected with various retinopathies suggesting potential diagnostic values. The GL ratio appears to overestimate the true contribution of the OFF component to the building of the PH as per the OFF/(ON+OFF) ratio analysis. This could reflect our inability to accurately estimate ON and OFF contributions to short flash ERGs. Interestingly, this estimate is significantly improved with the use of the long/short amplitude ratio [e.g. GL= 1.55 X (OFF/ON+OFF); Normals: 0.60=1.55 X 0.39]. Supported by CIHR and Réseau Vision.
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