Biology Reference
In-Depth Information
25
Several mechanisms involved
in sensitivity regulation
A breakthrough in our understanding of the underlying mechanisms
of dark and light adaptation of rods and cones would, of course,
have represented an important step forward in the development
of the duplicity theory. Yet, due to the complexity of the task
involved, the progress in our understanding has proceeded at a very
slow pace. Indeed, no general agreement about basic sensitivity
regulation mechanisms of rods and cones has yet been reached (see
Cameron
et al
.,
2006
; Baehr
et al
.,
2007
).
In line with Parinaud's (
1885
) assumption that both light and
dark adaptation were determined by a changes in the amount of
rhodopsin in the rods, most of the leading research workers during
the hundred years that followed tended to conceive of adaptation as
controlled by photochemical processes in the outer segment of the
receptors. Of course, the research workers knew that this presump-
tion was an oversimplification. Thus, it had long been known that
sensitivity regulation of the visual system could not be controlled by
one single mechanism operating at one site only, but rather was the
result of different mechanisms engaged at different sites in the visual
pathway. Besides the obvious regulation of the incident photons by
the variation of the pupil size (in humans the diameter of the pupil
may vary from about 8 mm in scotopic to about 2 mm in photopic
vision, reducing the light incident on the receptors by about 1.2 log
units), evidence had been provided indicating that important light-
and dark-adaptation mechanisms were located more centrally than
the outer segment of the receptors (e.g. Kuffler,
1953
; Lipetz,
1961
;
Rushton,
1965a
,
1972
; Barlow,
1972
).
Yet, the photochemical explanation of sensitivity regulation
remained dominant for nearly a century. Gradually, however, evidence
