ORIGINAL RESEARCH ARTICLE Asymmetrical growth of the photopic hill during the light adaptation effect Marie-Lou Garon • Marianne Rufiange • Ruth Hamilton • Daphne L. McCulloch • Pierre Lachapelle Received: 19 May 2010 / Accepted: 26 July 2010 / Published online: 15 August 2010 Ó Springer-Verlag 2010 Abstract In response to progressively stronger flashes delivered against a rod saturating background light, the amplitude of the photopic ERG b-wave first increases, reaches a maximal value (V max ) and then decreases gradually to a plateau where the amplitude of the b-wave equals that of the a-wave, a phenomenon known as the photopic hill (PH). The purpose of this study was to investigate how the PH grew during the course of the light adaptation (LA) process that follows a period of dark adaptation (DA): the so-called light adaptation effect (LAE). Photopic ERG (time- integrated) luminance-response (LR) functions were obtained prior to (control-fully light adapted) and at 0, 5 and 10 min of LA following a 30-min period of DA. A mathematical model combining a Gaussian and a logistic growth function, suggested to reflect the OFF and ON retinal contribution to the PH respectively, was fitted to the LR functions thus obtained. Our results indicate that the magnitude of the cone ERG LAE is modulated by the stimulus luminance, with b-wave enhancements being maximal for luminance levels that result in the descent of the PH. The Gaussian function grew significantly with LA while the logistic growth function remained basically unchanged. Our findings would therefore suggest that the LAE reflects primarily an increase in the retinal OFF response during LA. Keywords Human electroretinogram (ERG)  Light adaptation effect (LAE)  Photopic hill  Luminance-response function  b-wave Introduction The photopic, cone-mediated, electroretinogram (ERG) represents the electrical potential evoked from the retina in response to a flash stimulus delivered against a rod desensitizing background light. The photopic ERG response thus reaches maximal amplitude when the retina is fully adapted to the predetermined photopic environment. This is best exemplified with the light adaptation effect (LAE) that characterizes the gradual enhancement of the cone ERG amplitude observed during the course of light adaptation (LA) that follows a period of dark adaptation (DA). Nearly all ERG components [i.e. a-, b-, d-waves and oscillatory potentials (OPs)] are enhanced as a result of this process [1–5], with the b-wave demonstrating the most M.-L. Garon  M. Rufiange  P. Lachapelle (&) Department of Ophthalmology and Neurology- Neurosurgery, Montreal Children’s Hospital Research Institute, McGill University, 2300 Tupper Street, Montre ´al, QC H3H 1P3, Canada e-mail: pierre.lachapelle@mcgill.ca R. Hamilton Department of Clinical Physics, Yorkhill Hospital and University of Glasgow, Glasgow, UK D. L. McCulloch Vision Sciences, Glasgow Caledonian University, Glasgow, UK 123 Doc Ophthalmol (2010) 121:177–187 DOI 10.1007/s10633-010-9243-0