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the rapid phases of the adaptation processes could reasonably well be accounted for by neural factors. Hence, he admitted that both neural and photochemical components may be involved in light and dark adaptation. However, the relatively slow components of the ­adaptation processes measured psychophysically during long-term light and dark adaptation he still assumed were determined by the amount of visual photopigments.

19.5  A logarithmic relationship between sensitivity and amount of bleached photopigment

Substantial evidence in favour of this photochemical theory was obtained by comparing the rate of photochemical regeneration in solution and changes in visual sensitivity (see Wald, 1958). Thus, in accord with the rapid cone and slow rod dark adaptation obtained psychophysically in humans, he found the cone photopigment iodopsin in chicken to be formed at much greater speed than rhodopsin. Furthermore, just as frog and alligator rhodopsin was synthesized slowly and rapidly in solution, the course of rod dark adaptation of the frog and alligator, measured by the height of the b-wave of the ERG diagram, was correspondingly slow and rapid.

He found even more striking evidence in favour of the photochemical theory in results provided by Rushton and co-workers. They had obtained direct measurements of the fall and rise of visual photopigment concentrations in the human retina during light and dark adaptation by means of light reflected from the retina (Rushton & Campbell, 1954; Campbell & Rushton, 1955; Rushton et al., 1955; Rushton, 1957).

Wald (1958) pointed to three supporting results. (1) The steady state level of bleached rhodopsin in the living human eye by light adaptation was attained within about 5 min – about the same time it takes to completely light adapt the human rod system. (2) The re-synthesis of rhodopsin in the living human eye following strong bleaching displayed much the same course as the dark-adaptation

rhodopsin and sensitivity during dark adaptation 153

curve of rods. (3) The regeneration of human cone photopigments, in contrast to the rod photopigments, was completed in 5–6 min, just as human cone dark adaptation.

Previously, Hecht (1919/1920c, 1921/1922) had given the photochemical theory an exact mathematical expression by assuming a direct proportionality between bleached cone photopigment concentration and sensitivity, and a proportionality between amount of bleached rod photopigment and the logarithm of sensitivity. Analyzing the available evidence, Wald (1958) concluded that visual sensitivity during light and dark adaptation in both rod and cone vision was logarithmically related to the rise and fall of the photopigment concentration.

19.6  Contribution of J.E. Dowling

In his important 1954 paper, where he presented his compartment theory, Wald had calculated the relationship between amount of bleached rhodopsin and corresponding threshold level by comparing measurements of the amount of bleached cattle rhodopsin in solution and threshold levels in humans. Ideally, however, both types of measurement should be obtained from the same species.

Such measurements had first been provided by Granit et al. (1938,1939 see above) and were later applied by both Dowling and Rushton in order to formulate the relationship between threshold level and amount of bleached photopigments in mathematical terms (see Dowling, 1960; Dowling & Ripps, 1970; Rushton, 1961a, b, 1965a, b).

Dowling (1960) at first compared the b-wave threshold of the electroretinogram and the concentration of rhodopsin in rats on a vitamin A-deficient diet. Later, he made the same comparison in rats during long-term dark adaptation. Finally, he made a similar ­comparison between threshold measurements obtained during long-term dark adaptation and concentration of rhodopsin in the rod retina of the skate (Dowling & Ripps, 1970). Under all these conditions he found, in accord with the theoretical model of Hecht (1921/1922)

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that log threshold rose linearly with reduction in concentration of rhodopsin. The relationship obtained for the rat was expressed by the equation:

Log It/I0 = 3.6 (R0 – Rt) / R0

where It and Rt are, respectively, the threshold and rhodopsin concentration in vitamin A-deficient or partly dark-adapted animals, and I0 and R0 are, respectively, the threshold and rhodopsin concentration in dark-adapted control animals. For the skate, he obtained a proportionality factor of 5.

19.7  Contribution of W.A.H. Rushton: relationship between sensitivity and amount of bleached rhodopsin in humans

W. A. H. Rushton may be considered the leading authority within vision research in the 1960s and 1970s. His main interest was sensitivity regulation of the rod and cone systems. Shortly after Dowling’s (1960) investigation, he addressed the extremely difficult task of determining the relationship between sensitivity and amount of bleached rhodopsin in humans (see Rushton, 1961a, b). Actually, he was the first to measure both the change in rod sensitivity during long-term dark adaptation and rhodopsin regeneration in the same subject.

In this endeavour he encountered a major obstacle in that cones are much more sensitive than the rods during the first phase of the long-term dark adaptation (Fig. 19.1). In fact, following ­substantial bleaching, about 90% of rhodopsin has regenerated before rod sensitivity surpasses cone sensitivity and hence can be measured below cone threshold. In order to overcome this limitation, Rushton used a so-called photanope as subject (i.e. a subject with deficient cone vision). Thereby, he could measure the change in rod threshold during dark adaptation over a range of nearly 7 log units (Rushton, 1961b). Using his densitometer, he also measured the regeneration of rhodopsin of the photanope. The relationship between the amount

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ways. Firstly, weak bleaches produced threshold recovery curves that were more rapid than predicted by his formula. Secondly, the shape of the rod dark-adaptation curve changed markedly with the size of the test field, whereas regeneration of rhodopsin, of course, does not. Rushton concluded that the relationship obtained was valid only for substantial bleaching and that the constant of the linear relationship depended upon the size of the test field.

Rushton (1965a, 1966) also made an attempt to find the relationship between the amount of bleached cone photopigment and threshold level. Following strong bleaches, regeneration of cone photopigments (measured with his densitometer) and the ordinary cone dark-adaptation curve were recorded. Protanope, deutranope and normal trichromats were used as subjects, and the measurements were confined to the rod-free fovea.

At variance with Hecht (1921/1922), who proposed a linear relationship between threshold intensity and the amount of photoproduct of cones, Rushton found threshold level T to be logarithmically related to amount of bleached photopigment B. For the ‘green’- and ‘red’-related cone photopigments the relationship was given by the equation:

Log T = 3 B

Unfortunately, due to the scarcity of ‘blue’ cone receptors within the central fovea, Rushton was unable to measure the relationship for the ‘blue’ cone photopigment (see Rushton, 1965a).