A breakthrough in our understanding of the underlying mechanisms of dark and light adaptation of rods and cones would, of course, have represented an important step forward in the development of the duplicity theory. Yet, due to the complexity of the task involved, the progress in our understanding has proceeded at a very slow pace. Indeed, no general agreement about basic sensitivity regulation mechanisms of rods and cones has yet been reached (see Cameron et al., 2006; Baehr et al., 2007).

In line with Parinaud's (1885) assumption that both light and dark adaptation were determined by a changes in the amount of rhodopsin in the rods, most of the leading research workers during the hundred years that followed tended to conceive of adaptation as controlled by photochemical processes in the outer segment of the receptors. Of course, the research workers knew that this presumption was an oversimplification. Thus, it had long been known that sensitivity regulation of the visual system could not be controlled by one single mechanism operating at one site only, but rather was the result of different mechanisms engaged at different sites in the visual pathway. Besides the obvious regulation of the incident photons by the variation of the pupil size (in humans the diameter of the pupil may vary from about 8 mm in scotopic to about 2 mm in photopic vision, reducing the light incident on the receptors by about 1.2 log units), evidence had been provided indicating that important light- and dark-adaptation mechanisms were located more centrally than the outer segment of the receptors (e.g. Kuffler, 1953 ; Lipetz, 1961 ; Rushton, 1965a, 1972 ; Barlow, 1972).