Purpose

The electroretinogram (ERG) reflects changes in electrical activity across the entire retina, in response to light stimulation, and can be recorded non-invasively from the human eye. The amplitude of cone-driven ERG responses to flashes increases with flash luminance up to a maximum and then declines with further increases in flash luminance, termed the “photopic hill”. In this study we investigated the cone-driven response to 30 Hz flickering stimuli of increasing luminance as well as the effect of changes in background luminance on the response.

 

Methods

Extended recordings were conducted in 2 healthy adults, both male, aged 20-21. The study had Research Ethics Committee approval and conformed to the tenets of the Declaration of Helsinki. Subjects’ pupils were dilated pharmacologically with 1% tropicamide, and conductive fibre electrodes were placed into the lower conjunctival fornix of only one eye for recording, whilst the other eye was patched. Participants were exposed to a standard white light-adapting, rod-saturating background (30 photopic cd.m^-2; 86 scotopic cd.m^-2) and ERGs were recorded in response to white 30 Hz flicker stimuli of increasing luminance (ranging from 0.5 to 50 cd.s.m^-2). The same flicker stimuli were also delivered on other background luminances (50 and 100 photopic cd.m^-2; approximate scotopic luminances, 143 and 287 cd.m^-2 respectively) following initial adaptation to each background for one minute.

 

Results

Each participant underwent five repetitions of the same experiment over several days, with recordings made from right and left eyes (yielding a total of 20 experiments for analysis). Findings were broadly consistent for both subjects. For the 30 cd.m^-2 background, a photopic hill effect was observed with maximal amplitudes to a stimulus c.5-10 cd.s.m^-2. With increasing background luminance, the hill effect was also observed, such that response amplitudes to the strongest stimuli for each background were lower than responses to weaker stimuli; the response elicited by the 50 cd.s.m^-2 stimulus was smaller than that elicited by the 10 cd.s.m^-2 stimulus on each background (p<0.0002, paired t test). Also, as background strength increased, a rightward shift was seen in the luminance-response relation. For the 1 cd.s.m^-2 stimulus (consistently on the ascending limb of the photopic hill in each background), the response-amplitude was significantly smaller on backgrounds of greater luminance (p<0.01 for all pair-wise comparisons between backgrounds, paired t test).

 

Conclusion

Our findings show a photopic hill effect can be observed in responses to 30 Hz flicker stimuli. The decline in response amplitudes to a fixed stimulus strength delivered on brighter backgrounds is consistent with a reduction in sensitivity with greater background luminance, reflecting retinal light adaptation. The flicker ERG arises largely from signals in cone-driven ON and OFF bipolar cells, and concurrent changes in amplitude and response kinetics of the two pathways are likely to underlie the trends observed.