Duplicity Theory of Vision: From Newton to the Present

18 Contribution of G. Wald:photochemical sensitivityregulation mechanismsof rods and conesThis hypothesis of Kühne and Hecht must be considered an importantinsight with regard to biochemical processes underlying dark adaptation.Yet, it only represented a first step towards an understandingof the highly complex processes involved in the photochemicalsensitivity regulation mechanisms of rods and cones. Obviously, adeeper understanding would require more information on both themolecular structure of the rod and cone photopigments and thebleaching and regeneration processes generated by light.18.1 Molecular basis of bleachingand regeneration of photopigmentsin rods and conesInspired by Hecht, Wald, in the early 1930s, set out to throw lighton these largely unexplored research topics (see Wald, 1949a, 1958,1968). His profound discoveries and insights earned him the NobelPrize which he shared with Granit and Hartline in 1967.Firstly, he discovered vitamin A in the retina (Wald, 1933,1934/1935). Shortly thereafter, he concluded that the photopigmentrhodopsin was a conjugated carotenoid-protein engaged in a bleachingregenerationcycle when acted upon by light (Wald, 1934, 1935/1936).Thus, in line with the hypothesis of Kühne and Hecht that lightdecomposes rhodopsin into its two precursors, Wald presumed thatthe carotenoids, all-trans retinal (vitamin A aldehyde) and vitamin Arepresented both photoproducts and precursors of rhodopsin.A few years later, in 1937, Wald made two new importantdiscoveries: