Biology Reference
In-Depth Information
The results obtained for the cones were found to closely
fit a curve calculated from an equation of a bimolecular reaction.
Since he had obtained similar photosensory behaviour both for
the invertebrate
Mya
and for the
rod
dark adaptation of humans,
he came to the conclusion that both cone and rod dark adaptation
and the photosensory reactions in the invertebrate
Mya
could all be
accounted for in terms of his photochemical model.
Yet, one important difference between rod and cone dark
adaptation was stressed: while
threshold intensity
of cones measured
during dark adaptation was assumed to be proportional to the amount
of fresh photoproducts formed by the
test light
, the
log threshold
intensity
of the rods was assumed to be proportional to this amount.
Furthermore, presuming that this amount of fresh decomposition
produced by the test light was proportional to the concentration of
photoproducts
already present
in the sense organ, he could conclude
that during dark adaptation:
T
= k
B
for cones and
log
T
= c
B
for rods
where
T
= threshold intensity, k and c = constants and
B
= concentration
of residual
P
and
A
(see Hecht,
1919
/1920c,
1921
/1922).
Strong supporting evidence for the photochemical model was
also obtained in a classical psychophysical study by Hecht, Haig and
Chase (
1936
/1937). They measured the dark adaptation of the eye
following light adaptation by utilizing a test procedure analogous to
that employed on
Mya
. Thus, they used a test flash of short duration
(0.2 s) and measured the lowest test intensity necessary to produce
the threshold response. The major experimental variable was degree
of light adaptation.
When the subjects were strongly light adapted (400 000 photons,
i.e. troland, for 2 min), the cone and rod dark-adaptation curves were
separated in time: the cone part started at once and was completed
within a few minutes of dark adaptation, while the rod part showed
up later and took more than 30 min to complete. They also found
