Cone-dominated ERG luminance-response function: the Photopic Hill revisited.

Abstract

In response to progressively brighter stimuli, the b-wave of the photopic ERG gradually augments in amplitude, reaches a plateau for a narrow range of intensities and then rapidly decreases with further increments in the luminance of the flash. This unique luminance-response function was originally introduced as the Photopic Hill. The purpose of this study was to further characterize this unique feature of the cone ERG, investigate if it was only limited to b-wave measurements and if it could be obtained under different photopic background luminances. Photopic ERGs and oscillatory potentials were generated in response to flashes of light ranging from 0.5 to 16 cd m(-2) s in intensity and presented against photopic backgrounds varying from 18 to 525 cd m(-2) in luminance. All but the brightest background yielded a clear Photopic Hill like luminance-response function which could only be evidenced with the b-wave, the i-wave and OP4 amplitude measurements. Interestingly, the maximal amplitude reached remained almost identical irrespective of the background luminance. Our results suggest that the retinal mechanisms at the origin of the Photopic Hill effect could represent a voltage limitation mechanism, intimately tied to the OFF pathway. The latter would however be intrinsic to the cone system only and not to the entire retinal network since significantly higher peak amplitudes are reached with dark adaptation.