The photopic hill: A new phenomenon of the light adapted electroretinogram

Abstract

Utilizing a high intensity photographic flash unit, electroretinograms were recorded from normal adults under fully light adapted conditions over a 5 log unit range of stimulus luminance (-1.35 to 3.34 log cd-s/m2). At lower luminance levels b-wave amplitude increased with increased luminance until it reached a maximum (Vmax of the Naka-Ruston equation) in agreement with previous work. At higher luminance levels, the b-wave amplitude decreased to 33% of Vmax and then plateaued. This previously unreported phenomenon has been named the photopic hill. There was no appreciable change in b-wave amplitude with increased interstimulus intervals from 15 sec to 5 min and luminance-response functions serially recorded for increasing and for decreasing stimulus luminance were very similar. These latter results indicate that the photopic hill is not due to light adaptation. The reason for the photopic hill and possible clinical implications are discussed.

The relationship between stimulus intensity and response amplitude for the photopic negative response of the flash electroretinogram

“…In this study, the data indicated that a reduction in bwave and PT PhNR amplitude was apparent in most participants when the stimulus intensity reached 2.11 log phot tds (equivalent to 0.53 log cd.s.m -2 ). This is comparable to the stimulus intensity reported in the literature as corresponding to Vmax for the photopic b-wave luminance response function (from around 0.5 log cd.s.m -2 [40] to 0.86 log cd.s.m -2 [27]) However, the fact that the reduction in amplitude at the highest stimulus intensity was more marked for the PT PhNR data than the b-wave data, and the apparent reduction in BT PhNR amplitude in 10 individuals at the highest stimulus intensity, both suggest that this effect is not solely attributable to the well-recognised 'photopic hill' of the b-wave. responses at low stimulus intensities, but the intensity-response functions converged for all colours at the highest intensities [11,12].…”

“…Other studies have demonstrated that the photopic b-wave intensity-response data are well characterised by the Naka Rushton function at low to moderate stimulus intensities (up to around 0.8 log cd.s.m -2 ) [36,27], a finding which we have replicated in the b-wave and report for the first time in the PhNR.…”

“…This reduction in amplitude to high intensity stimuli might be expected with respect to the modelling of PT PhNR data, as the peak-to-trough measurement is the sum of b-wave and PhNR amplitudes, and the photopic b-wave is known to show a reduction in amplitude at high stimulus intensities, a phenomenon known as the 'photopic hill' [34,37,35,38,39,27,22]. In this study, the data indicated that a reduction in bwave and PT PhNR amplitude was apparent in most participants when the stimulus intensity reached 2.11 log phot tds (equivalent to 0.53 log cd.s.m -2 ).…”

“…The Naka-Rushton function (see Equation 1) is a sigmoidal curve which has been shown to describe the increase in b-wave amplitude, measured from the minimum point of the a-wave, with increasing stimulus intensity [24][25][26]. This reportedly provides a good fit to data across a range of intensities, although the relationship breaks down at high intensities for both light [27] and dark-adapted [28] b-waves. The intensity-response data of the d-wave of the flash ERG has also been fitted with the Naka-Rushton function [29].…”

Functional observations in vitamin A deficiency: diagnosis and time course of recovery

“…The relative changes in a-and b-wave amplitude with increasing stimulus intensity resemble those described under photopic conditions as the 'photopic hill'. 18,19 This phenomenon thus appears to be a property of cones rather than be related to the adaptive state of the retina. Perlman et al suggested that the 'negative' ERG could indicate that signal transmission in the inner retina was affected, it is likely that their data merely reflect the 'photopic hill' phenomenon in a dark-adapted functionally coneisolated retina.…”