Rod and cone components in the dog electroretinogram during and after dark adaptation

In anesthetized beagle dogs, electroretinograms (ERGs) were recorded with full-field stimuli during, and after, dark adaptation. With blue stimuli, a cornea-positive b-wave appeared at 13 min in the dark, and b-wave increased in amplitude and peak latency with dark adaptation, reaching near-plateau level approximately 61 min after bleaching. This b-wave was considered to be derived from rods (scotopic b-wave) based on its spectral sensitivity function. After about 31 min, oscillatory potentials and a small cornea-negative wave preceding the b-wave appeared. With red stimuli, a small cornea-positive b-wave with a short peak latency appeared immediately after the start of dark adaptation. The amplitude and peak latency of this wave remained substantially unchanged for 16 min. It then gradually became obscure after the appearance, and increase in amplitude, of scotopic components. After 2 hr dark adaptation, ERGs elicited by blue and red stimuli showed essentially the same waveforms, and in contrast to the situation in humans and monkeys, the cone component was not easily distinguished in the waveform elicited by red stimulus. It was confirmed that the ERG elicited by red stimulus, under blue-green background light sufficient to eliminate rod components, included a cone-derived photopic b-wave. The ERGs elicited by red stimuli were not different from those elicited by blue stimuli, because the photopic b-wave had a very small amplitude relative to the scotopic b-wave and a peak latency very similar to that of the scotopic cornea-negative small wave.