It is easily recorded in humans using a non- invasive contact lens electrode. ERG waveforms have several sub-components which are associated with the responses of different retinal cell types. The ERG is a standard clinical test to diagnose rod monochromatism.
Rod monochromats have few or no functional cones, but supposedly normal rods.
Therefore we expect an extinguished cone ERG and normal rod ERG in rod monochromats. The extinguished cone ERG is found, but clinical observation shows that the dark adapted ERG is reduced in amplitude in rod monochromacy compared to that of the normal eye. This finding has gone unexplained for decades. There are several possible causes for the relative reduction in the rod monochromat's dark adapted ERG response. We find that the rod PIII components of normal eyes and achromatic eyes behave identically.