Electroretinogram (ERG) and visually evoked cortical potentials (VECP) are used to determine the refractive condition of the human eye. A special form of test pattern is used in which the overall intensity of light remains constant while individual elements of the pattern exhibit sinusoidal fluctuations of luminance. With optimal focusing, the eye differentiates these separate fluctuations and large ERG and VECP waves are recorded. Small errors of refraction act to reduce the average amplitude of these waves. Large refractive errors, introduced by the use of ophthalmic lenses, cause the waves to approach zero amplitude. The VECP is more sensitive than is the ERG to small refractive changes, perhaps because the VECP heavily emphasizes the foveal region, while the ERG is more broadly representative of the entire stimulus field.
Millodot M, Riggs LA. Refraction Determined ElectrophysiologicallyResponses to Alternation of Visual Contours. Arch Ophthalmol. 1970;84(3):272-278. doi:10.1001/archopht.1970.00990040274003