The consequence of modulating background on the luminance-response function of the human photopic electroretinogram

To study the consequences of a modulating background on the luminance-response function of the human photopic flash electroretinogram for different relative timings of the flash relative to the background luminance.

Seven healthy subjects (age: 29-63 years; four females) participated in the study. We measured the response to flashes (9 flash strengths in total between 0.12 and 29.76 cd.s/m² while doubling the strength at each step; 5 ms flash duration) on a steady background (24 cd/m²) and on a 1 Hz modulating background (24 cd/m² mean luminance; 100% contrast). The flashes were presented at 6 different phases during the sine wave (0°, 90°, 180°, 225°, 270°, and 315°). Responses to a 1 Hz sinusoidally modulating stimulus were subtracted from the responses to the combined flash plus sine-wave stimuli to obtain the flash ERGs at different phases.

The a-wave and PhNR amplitudes monotonically increased with increasing flash strength. The b- and i-wave amplitudes displayed a maximum at 1.9 cd.s/m², above which they decreased again (the so-called "Photopic hill effect"). The responses could be described by an addition of a logistic growth and a Gaussian. The parameters of these functions depended on the time of flash presentation relative to the background. The dependency of the a-, b- and i-wave on flash presentation time with constant flash strength could be described by a model that assumes that the amplitude depends on the Weber fraction of the flash including a delay and a saturation.

The use of modulating backgrounds may increase the signal-to-noise ratio of flash ERGs and thus its diagnostic value. The dynamics of the response as a function of flash presentation time gives additional information about the retinal processing of flash and background. The photopic hill model allows the separation of processing in retinal On- and Off-pathways.