Studies were made of the visual functions of subjects with complete and incomplete achromatopsia, with particular emphasis on measurements of the rates of dark adaptation in a parafoveal region of the retina and at the fovea. When adequate preadaptation to a high luminance is achieved, the dark-adaptation curves reveal the presence of some form of photopic receptors in addition to the normal scotopic rods. Both types of receptor are present, not only in the paracentral retina, but also at the fovea.
LOUISE L. SLOAN, "Congenital Achromatopsia: A Report of 19 Cases," J. Opt. Soc. Am. 44, 117-128 (1954)