Studying visual adaptation of the human retina by using the electroretinogram (ERG) provides insights into the function of the photoreceptors and probably the retinal pigment epithelium, both likely candidates for containing the genetic defect in a large variety of different hereditary retinal degenerations. Visual adaptation depends on a number of different processes working together. They can be broadly grouped into those involving the transduction machinery of the photoreceptor and those involving regeneration of the photopigment. The former has been called the “neural” and the latter the “photochemical” components of adaptation.^7,8