Purpose: To evaluate rod and cone contributions to the dark-adapted 15-Hz flicker electroretinogram (ERG) across a broad range of stimulus luminances by comparing rod-isolating (ERGR), cone-isolating (ERGC), and non-receptor-specific (ERGR+C) responses.
Methods: Dark-adapted, full-field 15-Hz ERGs were obtained from four normally sighted subjects (ages 29-36 years) using a four-primary LED-based stimulating system. The primaries were either modulated sinusoidally in phase (ERGR+C) or were modulated in counter-phase to achieve rod isolation (ERGR) or cone isolation (ERGC) by means of triple silent substitution. Measurements were made for a broad range of luminances (-2.5 to 1.8 log scot. cd/m(2) in 0.2 log unit steps). Fourier analysis was used to obtain the amplitude and phase of the fundamental response component at each stimulus luminance.
Results: Stimulus luminance had different effects on response amplitudes and phases under the three paradigms. Specifically, ERGC amplitude and phase increased monotonically as luminance increased. The effects on ERGR+C and ERGR were complex: ERGR+C and ERGR amplitude was small and the phase decreased for low luminances, whereas amplitude and phase increased sharply at moderate luminances. For high luminances, ERGR+C amplitude and phase increased, whereas ERGR amplitude decreased and phase was approximately constant.
Conclusions: At low luminances, the ERGR+C and ERGR functions can be attributed to interactions between two rod pathways. At high luminances, the functions can be accounted for by interactions between rod and cone pathways (ERGR+C) or rod insensitivity (ERGR). The ERGR paradigm minimizes cone intrusion, permitting assessment of rod function over a large range of luminance levels.