Comparing the photopic ERG i-wave in different species

The i-wave, a post b-wave component of the human photopic electroretinogram (ERG), is claimed to originate at the level of the retinal ganglion cells (RGC) or more distally. We investigated whether this wave is a feature common to all species. Photopic ERGs were obtained from the following species: Beagle dog, European cat, New Zealand white rabbit, Göttingen minipig, Cynomolgus monkey, Sprague-Dawley and brown Norway rats, Hartley guinea pig, and CD1 and C57BL6 mice. Results were compared with those obtained from normal human subjects. Except for rats and mice, all species yielded a well-demarcated i-wave, easily identifiable and separated from the a-b-wave complex by approximately 20 ms. Our sample suggests that the i-wave is a feature common to the photopic ERG of most species including humans. In view of its suggested origin, the i-wave would offer a unique opportunity to test, with the flash ERG, the functional integrity of the retinal ganglion cells in animals where use of a pattern stimulus is not always easily obtained.