Primate cone sensitivity to flicker during light and dark adaptation as indicated by the foveal local electroretinogram

Abstract

During light and dark adaptation the primate foveal local electroretinogram (LERG) was monitored in response to yellow sinusoidally flickering stimuli. Since the foveal LERG is dominated by the late receptor potential (LRP) under these stimulus conditions, we were most likely monitoring cone photoreceptor activity. (1) Relative threshold vs intensity ( tvi) functions were obtained for stimuli flickering at 40, 20 and 5 Hz. Each of these functions achieves a unit slope at moderate to high intensities. Changes in phase relations between the stimulus and response occur only at those adaptation levels where photopigment bleaching is relatively ineffective as an adaptational mechanism. (2) The time course of dark adaptation was monitored for stimuli flickering at various frequencies between 5 and 50 Hz. At frequencies between 5 and 20 Hz, the time course of recovery of log relative sensitivity follows a nonexponential course; a break at 1.5 min sometimes occurs in the 5 Hz function. At higher frequencies the functions are exponential with time constants smaller than found for photopigment regeneration. These results are inconsistent with a simple linear relation relating log sensitivity and photopigment regeneration. (3) A Crawford type of transform indicates a non-equivalence between the effects of prior bleaching and steady-state light adaptation. (4) Absolute amplitude sensitivity functions obtained at various times after a bleach have low and high frequency branches. After compensating for the reduction in photon catch due to photopigment depletion the high frequency sensitivity changes little during dark adaptation, but the low frequency sensitivity changes markedly.1