Analysis of the Photopic Hill : Testing the Glasgow Model

Abstract

Purpose:: With progressively brighter stimuli, the b-wave of the photopic ERG first increases in amplitude to a maximal value and then decreases to finally form a plateau, a phenomenon known as the Photopic Hill (PH). A mathematical model combining a Gaussian function and a logistic growth function was developed by a team in Glasgow to fit this unusual luminance-response curve. This model is defined with five independent parameters. We examined whether these parameters could help us differentiate the PHs from patients with CSNB and cone dystrophy as well as those from normal subjects during the photopic light adaptation effect.

Methods:: PHs were obtained from seven patients (3 CSNB, 4 cone dystrophy) and from 10 normal subjects (LKC UTAS E-3000 system, background: 30 cd.m^-2; flash stimuli: -0.8 to 2.84 log cd.sec.m^-2). For the normal subjects, photopic ERGs were recorded prior to and at 0, 5 and 10 minutes following a 30-minute period of dark adaptation. The parameters fit to these data were compared to standard parameters calculated from a representative sample of 40 normal subjects using z-score statistics.

Results:: PHs obtained from CSNB patients were best fit using only the Gaussian function; whereas those from the cone dystrophy patients showed a significant reduction of the Gaussian (p<0.05), but a normal logistic growth function (p>0.05). Normal subjects showed a significant (p<0.05) time-related increase in the amplitude of the Gaussian function during light adaptation with a non-significant (p>0.05) modulation of the logistic growth function.

Conclusions:: The human PH is well described by the combination of two independent curves, supporting the utility of the Glasgow model to quantify the PH. Given that CSNB presents with selective dysfunction of the ON retinal pathway while our cone dystrophy patients exhibit a defect in the OFF pathway, our results suggest that the Gaussian function reflects the contribution of the functional OFF retinal pathway whereas the logistic growth is more likely the result of the ON pathway activation. This being said, our results would also support our earlier suggestion that the light adaptation effect most probably results from a gradual release of inhibition of the OFF pathway contribution to the ERG b-wave that was enhanced by the preceding period of dark-adaptation.

Keywords: electroretinography: clinical • electroretinography: non-clinical • pathology: human