Biology Reference
In-Depth Information
This last assumption could also easily explain the remarkable
finding of Granit
et al
. (
1938
,
1939
) that following high light adaptation,
dark adaptation measured by the b-wave of the electroretinogram
lagged markedly behind the regeneration of rhodopsin. Thus the
compartment theory implied that light- and dark-adaptation threshold
curves were related mainly to the bleaching of the first small fraction
and the re-synthesis of the last small fraction of rhodopsin.
The compartment theory could also be applied to light and
dark adaptation of
cones
. New considerations, however, had to be
taken into account. This was because cones were assumed to possess
very little visual pigment compared to rods. To illustrate the differ-
ence between rod and cone adaptation, Wald (
1954
) considered a
hypothetical cone where each of the compartments contained only
a single molecule of visual pigment. In such a cone each quantum
absorbed would involve a new compartment and would result in a
response. The sensitivity in such a cone receptor would, therefore,
always be directly proportional to the concentration of its visual
pigment, and the rate of dark adaptation would measure directly the
rate of re-synthesis of the visual photopigment.
Hence, Wald assumed that the complication of light and dark
adaptation of rods was due to the relatively high concentration of
rhodopsin and that a simpler picture would emerge if concentra-
tion of photopigment were reduced, as in the cones. Indeed, in the
limiting case, the simplest photochemical theory would apply: dark
and light adaptation would follow in detail the concentration of the
photochemical substance in the receptor.
Although Wald (
1954
) held that factors central to the photochem-
ical systems of the receptors under special conditions could play an
important role in determining visual threshold and adaptation, he
suggested that under all ordinary conditions the photopigments in
rods and cones were the major determinants - after all, he argued, the
entire visual apparatus hung upon the initial action of light.
A few years later Wald (
1958
) attributed more weight to neural
factors. Besides the obvious neural pupil factor, he also argued that
