Biology Reference
In-Depth Information
On this evidence, Barlow had to conclude that, in addition to the
noise factor, another factor was involved causing the threshold to rise
faster than simple noise consideration would have led one to expect.
22.3 Evidence in support of the noise
theory
In 1972 Barlow re-evaluated his noise theory. He agreed with Rushton
(
1965a
) that the relation between the amount of bleached rhodopsin
and rod threshold during long-term dark adaptation was given by the
well-known Dowling-Rushton formula (log
T
=
c B
), and that the
threshold level measured during the dark-adaptation period, therefore,
was somehow determined by the amount of bleached photopigment.
If this suggestion were correct, there had to be a very rapid increase
of the desensitizing reaction with a rise in the amount of rhodopsin
bleached; the reaction would have to depend exponentially upon
the concentration of bleached rhodopsin. No mechanism that could
explain this reaction had yet been demonstrated, but Barlow argued
that the receptors were structures with a high degree of organization,
and that, therefore, a small proportion of altered molecules might
cause disproportionately large effects. Bleaching a single molecule
might significantly change the condition of other molecules in its
vicinity, and cooperative action of this kind might give rise to an
exponential law (Barlow,
1964
).
Opposed to the suggestion of Rushton, however, that the dark-
and light-adaptation mechanisms were fundamentally different,
Barlow stressed the similarity, suggesting that dark and light adapta-
tion were equivalent processes. Thus, he suggested that receptors in
the dark, containing a proportion of bleached photopigment, signalled
messages indistinguishable from those caused by the illumination of
the receptors. The so-called 'dark' and real lights, therefore, had the
same adaptation effect; they both enhanced the noise level of the
receptors and, thereby, reduced their sensitivity.
He found supporting evidence in the results obtained by
Crawford (
1947
) and Blakemore and Rushton (
1965
), who had
