Biology Reference
In-Depth Information
threshold level during dark adaptation remained invariant as long
as total bleaching exposure (the product of bleaching intensity and
exposure time) remained constant, obeying the Bunsen-Roscoe law of
photochemical reactions. Even under conditions where the bleaching
intensity was above the rod saturation level, the results were found
to be in accord with the Bunsen-Roscoe law.
Rushton's conclusion that the early dark-adaptation process was
photochemical and not 'nervous' was also in accord with the finding
of Donner and Reuter (
1968
) that the sensitivity regulation effect of
meta II was restricted to receptors that contained this substance, that
is, meta II did not reduce sensitivity of neighbouring rods that had
not been bleached.
Background adaptation, on the other hand, was assumed to be
governed by a neural mechanism. The plainest evidence supporting
this view was still found in that a background so weak that only
a small fraction of the rods caught a single quantum raised the
threshold several-fold.
Rushton's (
1972
) conclusion that bleaching and background
adaptation were governed by fundamentally different mechanisms
(chemical and nervous) was in sharp contrast to the theories of
Parinaud (
1885
), Hecht (
1921
/1922), Wald (
1954
) and Barlow (
1964
)
who all presumed that light and dark adaptation were based on
similar processes. His strong rejection of this view is shown by his
forceful statement on p. 503,
The nature of visual adaptation is an extremely complex question
and our understanding of it received a mighty set back when Hecht
and Wald used their outstanding force and eloquence to insist that
everything depended simply upon the level of rhodopsin in the
rods and the photopic pigments in the cones.
And further, on p. 512,
Hecht was correct in his conjecture that the dark-adaptation curve
was correlated with the regeneration of rhodopsin, but he was
