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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
