Walsh & Hoyt: Electroretinogram (ERG)

The Swedish physiologist Holmgren reported in 1865 that in vertebrates and higher invertebrates, an alteration in the electric potential occurred when light fell on the retina. Dewar (234) subsequently recorded this electric response and called it the electroretinogram. Einthoven and Jolly (235) identified three main components of the ERG: (a) an early cornea-negative "a-wave"; (b) a cornea-positive "bwave"; and (c) a slower, usually cornea-positive "c-wave" (Fig. 2.55). Granit (236) identified three fundamental processes that he called P-I, P-II, and P-III from the ERG of a cat. He proposed that these components interact to produce the ERG waveform, with the P-II giving rise to the leading edge of the a-wave, P-II and P-III interacting to produce the b-wave, and the interaction of P-I and P-III producing the c-wave.