HECHT'S PHOTOCHEMICAL THEORY

The finding of Loeser (1904) that cones also had the ability to increase their sensitivity during early dark adaptation was confirmed by Hecht (1921/1922). He found that cones could increase their sensitivity markedly even during the first few seconds after bleaching.

More importantly, however, Hecht developed a photochemical theory for dark and light adaptation of rods and cones that had a strong influence on a whole generation of research workers. Certainly, he has played a central role in the developmental history of the duplicity theory. In a series of papers he provided an array of evidence supporting his photochemical theory (see Hecht, 1919/1920a, b, c, 1921/1922). In its essence and in its most simple version, the theory runs as follows: light acts on a photosensitive substance S and decomposes it into two precursors called P and A. The sensitivity of the eye, then, depends on the concentration of these precursors, not on the quantity of the photosensitive substance S. Thus, the model states that the amount of fresh precursors necessary for a threshold response is always a constant fraction of the amount of the precursors already present in the system. Hence, dark adaptation was thought to depend on the regular decrease in the concentration of the residual precursors present in the sensory system. This decrease was assumed to proceed according to the dynamics of a bimolecular reaction, to be independent of light stimulation, and, in accord with Kühne's (1879) ‘Optochemische’ hypothesis, to result in a reformation of the photosensitive substance S. Thus, the model states that the amount of fresh precursors necessary for a threshold response is always a constant fraction of the amount of the precursors already present in the system.