Discovery and the first analyses

The potential voltage difference that occurs between the cornea and the fundus was discovered by Du Bois Reymond in 1849.²³ He showed that it persisted for long periods in the isolated eye. In 1878, Kühne and Steiner⁵⁶ and de Haas²¹ measured the voltages after successively removing the cornea, iris, lens, vitreous, and retina. Only when the retinal pigmented epithelium (RPE) had been damaged did the potential vanish, and this localized the source of the current production. Although illumination was known to affect the potential recorded between cornea and fundus,⁴⁴ the capillary electrometers that were used in early work were not sufficiently sensitive or stable to analyze the changes in detail. With the advent of electronic amplification, condenser coupling prevented recording the changes caused by light. It was not until the 1940s that Noell⁷⁶ was able to employ stable d.c. recording systems and follow the slow changes; he related the c-wave of the electroretinogram (ERG) to the later and still slower responses. He used poisons that selectively damaged the RPE, as demonstrated by histological changes. He found that azide acting on the RPE increased the “standing potential” and the c-wave of the ERG, while iodate, which damaged the RPE selectively, not only caused a fall in the standing potential, but also reduced the azide increase and reduced the c-wave. Faster changes caused by illumination, that is, other components of the electroretinogram, were less affected.