Neuronal adaptation in the human retina: a study of the single oscillatory response in dark adaptation and mesopic background illumination

Abstract

The single oscillatory response in complete dark adaptation (DA) and the effect of mesopic illumination were studied in order to investigate the behaviour of the neuronal adaptation system as reflected in the oscillatory potentials (OPs) of the electroretinogram (ERG). The rapid oscillatory and slow components (a- and b-waves) of single ERGs were simultaneously recorded in nine healthy, young subjects in response to first flash after both DA of 45 mins and light adaptation to a steady background light (BGL) of low mesopic intensity. Two low-amplitude oscillatory peaks were present in the single response to the first flash recorded in DA. There was no increase in the summed amplitudes of the OPs (SOP) when recorded in the single response to the first flash in mesopic BGL. However, the morphology of the oscillatory response altered. The first OP was reduced and a third oscillatory peak appeared. We conclude that early, scotopically related OPs may indeed be activated in the single response to the first flash in DA (i.e. without using conditioning flashes). Secondly, on its own, adaptation to mesopic BGL does not seem to trigger enhancement of the overall oscillatory response. The altered single oscillatory response to the first flash apparent in the mesopic BGL comprises a third cone-associated OP and seems to reflect a reorganization of the retinal microcircuitry from a predominantly rod-activated system to one of mixed rod/cone neuronal activity in the inner part of the retina at the level at which individual OPs have their respective origins.