Biology Reference
In-Depth Information
As we have seen, Lamb's (
1981
) solution to this problem was
to suggest that the long-term rod dark-adaptation curve consisted of
three
different segments, each with a different slope, and that these
three different slopes were direct plots of the decay of three different
photoproducts.
Strong support of Lamb's assumption that the long-term rod
dark-adaptation curve is best described by different consecutive
segments was provided by Nordby
et al.
(
1984
) and Stabell
et al
.
(
1986b
). Using a complete rod monochromat (K. N.) as subject, these
authors could measure the sensitivity increase of the rod system
during dark adaptation for about 7 log units and found, unexpectedly,
that a major, middle part of the curve closely followed a straight line
with a slope that was independent of size, exposure time and retinal
location of the test stimulus.
Similar results were also obtained with trichromats (Stabell
et al
.,
1986a
). The slope of the straight line component was found
to be remarkably constant, not only in a given individual, but
in different individuals as well. Hence, the evidence appeared to
support the notion of Lamb (
1981
,
1990
) that the normal long-term
rod dark-adaptation curve consisted of several segments each with its
own underlying mechanism.
24.2 The search for a new formula
The obvious next step was to relate the sensitivity change obtained
during the middle, straight line part of the long-term rod dark-
adaptation curve to concomitant change in amount of bleached
rhodopsin (see Boll, 1877; Parinaud,
1885
; Hecht,
1921
/1922;
Wald,
1954
; Dowling,
1960
; Barlow,
1964
; Rushton,
1965a
; Lamb,
1981
; Pepperberg,
1984
). By relating the psychophysical data of the
rod monochromat and the trichromats to previous measurements
of rhodopsin regeneration (Rushton & Powell,
1972b
), a formula
quite different from the Dowling-Rushton equation emerged. Thus,
the results suggested that for both rod monochromats and normal
trichromats the relationship between threshold elevation
T
and
