Biology Reference
In-Depth Information
that the higher the initial light adaptation, the longer the eye required
to reach a given threshold intensity during the dark-adaptation
period, irrespective of whether cone or rod threshold was measured.
Moreover, the dark-adaptation curves of the cones had a fixed shape.
It was merely the vertical extent of the cone curves and their position
on the time axis that changed, as if more and more of the same curve
became apparent with increasing light adaptation.
Similarly, with strong bleaches, the rod dark-adaptation curves
appeared the same, only moved along the time axis, as if the rod curve
as a whole were moved to appear later the higher the light-adaptation
level. Also, the curves of the three subjects closely coincided when
shifted along the time and intensity axes, and only a slight day to day
variation in threshold intensity for the same subject was obtained
when the experimental conditions were strictly controlled. Certainly,
all these results may easily be explained by the photochemical theory
of Hecht presuming that: (1) the cone photopigments regenerate more
rapidly than the rod photopigments, and (2) more photopigment is
bleached as light adaptation increases. As a consequence, cone dark
adaptation would proceed more rapidly than rod dark adaptation,
and increasing light adaptation would produce more extended dark-
adaptation curves, since more of the photopigment would have to be
regenerated.
This simple photochemical view, however, broke down when
dark-adaptation curves of rods at the highest and lowest light-adapta-
tion levels were compared: The curves of the lowest adaptation levels
proceeded much quicker than those of the highest levels. Apparently,
rod dark adaptation could follow two different courses depending
upon the level of pre-adaptation: a rapid and a delayed type. The
delayed type was found to be established by light adaptation levels
above 20 000 photons (troland) and the rapid type at levels below
200 photons. On this evidence they concluded, in accord with Kühne
(
1879
), that the regeneration of rhodopsin may occur in two different
ways: rapidly from the photoproducts and more slowly from new
substances.
