Duplicity Theory of Vision: From Newton to the Present

98 development of the duplicity theory from 1930–1966Similarly, a review on change in brightness discrimination withchanges in background intensity showed no clear breaks in the extrafovealcurves. The results, therefore, were judged to indicate overlapping andinteraction of the brightness functions of rods and cones.Furthermore, evidence presented by Lythgoe (1940) stronglyindicated that rod and cone impulses may also interact to determinethe fusion frequency of a flickering light. Measuring the fusionfrequency in the extrafoveal retina during dark adaptation whilst theillumination of the flickering light was kept constant, he found, inaccord with the duplicity theory, that the fusion frequency fell or rosedepending on whether the test illumination activated the photopicor scotopic mechanism, respectively. However, the results displayedno clear-cut change-over from fall to rise during the dark-adaptationperiod. Instead, they strongly indicated that cones determined thecritical frequency measurements during the first part of the darkadaptationcurve, both receptor mechanisms during the middle partand only rods during the last part.Saugstad and Saugstad (1959) also presented evidence thatchallenged the basic assumption of Schultze (1866) that rods mediatedachromatic vision only. Thus, evidence in support of the assumptionthat rods may mediate colours of short wavelengths was found in theresults obtained by Hecht, Haig and Chase (1936/1937). Measuringthe dark-adaptation curve following moderate light adaptation, theyshowed that the change from coloured to colourless perception for aviolet test light in the extrafoveal retina could be obtained below thecone plateau level.Furthermore, Kohlrausch (1931) obtained dark-adaptationcurves with only one branch in the extrafoveal retina despite the factthat colours could be observed at the threshold level during the firstpart of the curve. Saugstad and Saugstad (1959) interpreted this resultto mean that either the form of the curves was due to interactionbetween rod and cone activities, or, presuming that the curves weredetermined by rods alone, that rods mediated the colour sensationsobserved.