Ragnar Granit was awarded the 1967 Nobel Prize in Medicine with Haldan Keffer Hartline and George Wald "for their discoveries concerning the primary physiological and chemical visual processes in the eye". Ragnar Granit describes the beginning of electronic retinal neurophysiology with the advent of recording electronic impulses in nerves in the late 1920s. He measured the fusion frequency (the frequency at which an intermittent light or flickering light appears steady) and found that contrast in background illumination played an important role, as did the area of illumination. He then began working on the electroretinogram (ERG). He developed micro-electrodes for retinal recording. He determined that light adapted eyes utilize inhibition, but he had to figure out how impulses might be stopped in the optical nerve if a flash of light were present. He verified that this was the case by looking at the cornea electrical deflection. He also measured the response to lights of different frequencies (colors) and found properties related to the rods and cones of several animals, including humans.