Biology Reference
In-Depth Information
that the long-term dark-adaptation curves of the cones, like those of
rods, contained a middle, straight-line segment (Stabell & Stabell,
1989
; Stabell
et al.
,
1990
). Yet, the speed and duration of the middle
sections of the rod and cone systems suggested that their sensitivity
regulation mechanisms were controlled by somewhat different
enzymatic processes. Thus, the relationship between threshold
elevation
T
and fraction of bleached photopigment
B
for cones could
be expressed by the equation
T
=
cB
2.5
and for rods
T
=
cB
4
.
24.4 Light and dark adaptation are not
equivalent
As we have seen, the important question of whether or not the
underlying processes of light and dark adaptation were equivalent
had not been settled (see Rushton,
1972
; Barlow,
1972
; Lamb,
1981
,
1990
). It was, therefore, of interest to attempt to test the equivalence
hypothesis using the rod monochromat K. N. as subject (Stabell
et al
.,
1987
). K. N. was considered an ideal subject for this purpose, since his
light- and dark-adaptation curves could be followed for more than 6
log units. Relying on the regeneration curve of rhodopsin published
by Rushton and Powell (
1972b
), both the relationship between the
amount of bleached rhodopsin
B
and threshold level
T
measured
during long-term dark adaptation, and the relationship between
background field illumination
I
and incremental threshold level
T
measured during light adaptation were calculated.
The two functions were found to be vastly different: the
relationship for the dark-adaptation curve was
T
= c
B
4
and for the
light-adaptation curve
T
=
I
0.65
. When tested for cooperativity by
double reciprocal plots (so-called Lineweaver-Burk plots), the
results showed that the dark-adaptation function was indicative of a
positively cooperative enzymatic underlying mechanism, while the
light-adaptation function was indicative of a negatively cooperative
enzymatic underlying mechanism. These results, then, supported
Rushton's (
1965a
,
1972
) position suggesting that light and dark
adaptation were based on different underlying mechanisms.
