Early discoveries

In 1849, DuBois-Reymond²⁷ discovered in excised tench (fish) eyes a potential of about 6 mV when using an electrode placed behind the eye and a similar electrode placed on the surface of the cornea. He found that the cornea was positive with respect to the posterior pole of the eye. The existence of this standing potential was soon confirmed by other authors.

In 1865, Holmgren³⁹ discovered that an excised frog eye showed an electrical response to light, and in 1880 he found by removing the anterior segment of the eye and placing the corneal electrode directly on the retinal surface that the retina itself was the source of the response.^38,40

About the same time, Dewar and McKendrick²⁶ independently reported the discovery of “action currents” with illumination of the eye; they concluded that there was a relationship between the amplitude of the electrical response and the logarithm of the stimulus intensity. Wavelengths that appeared brightest to the human eye evoked the largest amplitude response.

In 1877 Dewar²⁵ showed that electrical potentials could be recorded from an intact animal eye by applying the second (reference) electrode on the abraded skin. He also reported the first successful recording of a human electroretinogram (ERG). For this purpose, he used an elaborate instrumental setup.

A small trough of clay or paraffin was constructed around the margin of the orbit, so as to contain a quantity of salt solution, when the body was placed horizontally and the head properly secured. The terminal of a non-polarizable electrode was introduced into this solution and in order to complete the circuit, the other electrode was connected with a large gutta-percha trough containing salt solution, into which one of the hands was inserted.

The two electrodes were connected to a sensitive Thomson galvanometer. The resulting curves, however, were not published.

In 1880, Kuhne and Steiner,⁴⁷ working on isolated frog and fish retinas, claimed that the light-induced action currents originated in the receptor layer and not in the ganglion cell layer.