Rod and Cone Contributions to the Off-Effect of the Human ERG

¹ Department of Ophthalmology and Institute for Research in Vision, Ohio State University Columbus, Ohio.

From previous research, it has been unclear as to whether the off-effect is a purely cone response. Spectral sensitivity was measured here both in dark and light adaptation with a response-averaging computer. A large stimulus field and a slow frequency of flickering light stimuli ivith equal light and dark times were used. Curves of measured amplitudes of both on- and off-effects against stimulus luminance were plotted for each of 12 test wavelengths. The b-wave rapidly and the off-effect only slowly increased in amplitude with stimulus luminance. In dark adaptation, the spectral sensitivity (amplitude criterion) for both b-wave and off-effect was predominantly scotopic (rod-dominated). It was found that spectral sensitivity curves for the off-effect could also be based on a constant latency criterion. This spectral sensitivity also was scotopic. In light adaptation (2 FL) spectral sensitivity based on an amplitude criterion was a fair fit to the parafoveal cone sensitivity curve (Wald) for both b-wave and off-effect. It was concluded that rods mainly contributed to the off-effect in dark adaptation, while in the light-adapted condition cones became dominant.