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and steady background adaptation procedures. No equivalence was
found. With background adaptation the semi-saturation constant σ
remained invariant, whereas with bleaching adaptation it increased.
These results, then, further strengthen the suggestion that background
and bleaching adaptation were fundamentally different.
21.5 Limitations of Rushton's photochemical
theory
Previously, both Rushton and Dowling had provided strong evidence
that long-term dark adaptation was controlled by photochemical
processes. Both had found that log rod threshold was proportional to
the amount of bleached rhodopsin, i.e. log T = cB . Serious problems,
however, soon became apparent. Rushton ( 1972 ) pointed out two
important limitations: (1) rod responses to light could not be registered
before 50% of the rhodopsin had regenerated (as Granit, 1938 , 1939
had shown), (2) when the adapting light bleached less than 10% of
rhodopsin, the threshold level was found to be raised and to fall far
more than expected on the basis of the Dowling-Rushton equation
during the early phase of dark adaptation.
Rushton ( 1972 ) and Rushton and Powell ( 1972a , b ) attempted to
account for these limitations in line with the conclusions previously
reached by Donner and Reuter ( 1968 ). These authors had obtained
results with frogs that suggested that accumulation of meta II
(a photoproduct of rhodopsin not yet degraded to leave free opsin)
caused rod saturation under light adaptation, and that the decay of this
photoproduct determined the first rapid phase of recovery during rod
dark adaptation. Similarly, Rushton presumed that meta II produced
strong afterimages and caused the observed deviations between the
Rushton-Dowling formula and the actual threshold measurements.
However, about 5 min after a strong bleach, the amount of meta II and
free opsin were assumed to reach a state of equilibrium and thereafter
to be proportional to each other, so that they both remained propor-
tional to the log threshold elevation. He found supporting evidence
for the photochemical explanation of dark adaptation in that the
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