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otherwise. Hence, they concluded that the rate of 'diffusion' to the
locus of excitation, not the time constant of the process of rhodopsin
regeneration, was the significant factor for sensitivity regulation.
The amount of rhodopsin in the store was only indirectly involved
by participating in building up the excitable part.
Granit
et al
. (
1939
) concluded that their hypothesis could explain
both the well-known delay of increase in rod sensitivity obtained
psychophysically during long-term dark adaptation and the rapid and
slow rod dark adaptation found when the degree of light adaptation
varied (Kühne,
1879
), i.e. all these results could be accounted for by
the rate of diffusion, but not by regeneration of rhodopsin.
The explanation of Granit, however, was rejected by both
Lythgoe (
1940
) and Wald (
1954
). Lythgoe argued that each molecule
of rhodopsin was a potential initiator of a nerve impulse when broken
down by light, but that the final electrical response of the retina and
optic nerve was determined by the existing synaptic arrangements.
Accordingly, he regarded dark and light adaptation as two different
processes where the receptors acquired, respectively, progressively
more and less nerve fibres in common.
19.4 W a l d ' s e x p l a n at i o n : c o m pa r t m e n t
theory
Wald (
1954
), in accord with Lythgoe, argued that
any
molecule of
rhodopsin in a
completely dark-adapted
rod could excite it - each of
the molecules was equally able to contribute to excitation. On the
other hand, he accepted the finding of Granit that there was a non-linear
relationship between concentration of rhodopsin and sensitivity.
To get a more accurate idea of this relationship, he measured
both the percentage of bleached rhodopsin in cattle and the threshold
level in humans during light adaptation. He found that when about
0.6% of rhodopsin was bleached, the corresponding threshold level
increased more than 3 log units, i.e. more than a thousand times.
This result was in accord with available evidence and clearly
demonstrated that the simple photochemical hypothesis of Parinaud
