Electrical signals from the retina and the visual cortex were recorded using a checkerboard pattern stimulus. The signals obtained from normal subjects indicate that, at the retinal level, both macular and nonmacular cones contribute to the electroretinogram (ERG). At the cortex, however, only macular cones are responsible for the electrical signal. Records obtained from a patient with macular degeneration show the absence of a visually evoked cortical potential (VECP).
Sokol S. An Electrodiagnostic Index of Macular DegenerationUse of a Checkerboard Pattern Stimulus. Arch Ophthalmol. 1972;88(6):619-624. doi:10.1001/archopht.1972.01000030621010