Rod and cone ERGs and their oscillatory potentials.

Normal human ERGs were recorded from a dark-adapted subject using white and colored test flashes. Oscillatory potentials (OPs) were studied after high-pass digital filtering. When blue and red responses were compared at equivalent photopic intensities, OPs were visible at much lower intensities for the blue flashes. As the intensity was reduced from maximum, the first (negative) wave for red flashes maintained a latency of 20-25 msec before being lost in noise, whereas the first wave for blue flashes increased its latency progressively from 25 to 60 msec. These differences between photopically matched red and blue responses are interpreted to be due to rod-generated responses. When blue, orange, and white responses were compared at equivalent scotopic intensities, the latency of the largest negative wave was found to be similar for all three colors. The authors interpret this wave to be the beginning of the rod-generated OPs, so that the preceding waves (particularly evident for orange flashes) are cone-generated OPs, and they propose that the existence of separate rod and cone OPs should be borne in mind when investigating clinical changes in OPs.