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S. Dewhirst, A. Fournier, B. C. Leonard, B. Hurley, S. G. Coupland; Photopic Hill Revisited. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1501.
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The photopic ERG has a luminance-response curve which shows b-wave amplitudes increasing to a maximum, and then decreasing to reach a non-zero plateau. This asymptotic luminance-response function has been referred to as the "Photopic Hill". The Photopic hill has been shown in primates to result from luminance dependent interaction of cone on- and off-responses. Recently, the Photopic hill has been modeled by the sum of a Gaussian function and a logistic growth function using a 16 step protocol (Hamilton et al. 2007) and has been tested in a small number (n=7) of patients. Herein we describe a photopic hill protocol using fewer steps which yields results that fit a combined Gaussian-logistic model and which are in good agreement with results previously obtained by Hamilton et al.
Thus far Photopic hills have been obtained from 9 eyes of 6 normal subjects with dilated pupils using DTL fiber electrodes and an Espion e2 system (Diagnosys LLC), with background: 30 cd.m-2; flash stimuli: -1.2 to 3.0 log cd.sec.m-2 in eight steps. Two artifact-free ERGs were averaged at each step and b-wave amplitude values were fitted using a non-linear regression analysis (SigmaPlot v11.0).
Photopic hills were obtained from all subjects within 5-8 minutes. The mean Gaussian component amplitude Vgmax was 125.3 µV with a 95% confidence interval =(85.28, 155.36) and the mean Logistic component amplitude Vbmax was 62.7 µV with a 95% confidence interval =(36.75, 88.79). Using a 1:1.7 ratio of Burian-Allen:DTL amplitude ratio we found clear comparison with the Hamilton et al. model’s Vgmax and Vbmax parameters.
The human Photopic hill is well described by a combined Gaussian and logistic growth function. We have developed a brief protocol taking 5 minutes which can be used to generate a combined Gaussian-logistic function that compares favorably with the Hamilton et al. model in normal subjects. Clinical utility of this protocol is under investigation.
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