Biology Reference
In-Depth Information
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 photopig-
ment 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,
1961
a, b,
1965
a, 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)
