Biology Reference
In-Depth Information
For example, bleaching the 'red' and 'green' cone photopigments
completely with an orange light that was hardly absorbed at all by
the 'blue' cone photopigment was found to favour the sensitivity of
the 'blue' mechanism to such an extent that it could be measured
in isolation with a blue test light during the first 90 s of the dark-
adaptation period. Thereafter, the sensitivity of the 'green' cone
mechanism was found to surpass that of the 'blue' producing a break
in the dark-adaptation curve.
Similarly, by selective chromatic adaptation, breaks between
the 'red' and 'green' cone dark-adaptation curves could be obtained,
and isolated 'red' and 'green' branches could be plotted in their
entirety.
On this evidence Du Croz and Rushton (
1966
), in harmony with
Stiles's hypothesis that the four receptor-adaptation mechanisms
acted independently in increment threshold measurements,
suggested that they also acted independently of each other during dark
adaptation. Thus, the apparent independence of the dark-adaptation
curves found for the four receptor mechanisms was assumed to
imply separate and independent AGC organization: for each of the
four dark-adaptation mechanisms the threshold was thought to be
determined only by its own photopigment and its own AGC pool
(see Rushton,
1965a
).
21.2 Are light and dark adaptation really
equivalent?
The similarity between light and dark adaptation found could
easily be accounted for if they were based on the same underlying
mechanism. To address this problem, Rushton made an extensive
investigation to determine whether light and dark adaptation were,
in fact, equivalent (see Rushton,
1965a
).
The concept of equivalence of visual adaptation had been
analyzed previously in a remarkable paper by Stiles and Crawford
(
1932
) where they also put forward a test of equivalence: if two
different adaptation procedures caused the same state of adaptation,
