Biology Reference
In-Depth Information
The formula was found to hold for a large range of
N
, φ and θ
variations and was therefore considered to be of fundamental import-
ance for the understanding of the adaptation processes.
One important achievement of the formula was that it provided
a quantitative explanation of receptor saturation like the saturation
Aguilar and Stiles (
1954
) had found for the rod receptor. Previous
evidence had indicated that there existed a basic difference between
rod and cone background adaptation in that the rod response saturated
when only a small fraction of rhodopsin was bleached, while the
incremental threshold of the cones could be followed to very high
background intensity levels where nearly all the cone pigments were
bleached and yet showed no upward turn away from the straight
Weber law incremental threshold behaviour that would indicate the
start of the saturation process.
Their formula, on the other hand, predicted saturation both
for rods and cones. Thus the equation shows that however great φ
becomes,
N
will never reach [
θ
D
/ (
θ
D
+
θ
)].
They performed a test that confirmed the prediction: cone
saturation occurred as expected when a background light was
presented only briefly - at a time when the increment φ was
flashed upon it. Thereby the bleaching of the cone pigment
was slight and the threshold rise could be obtained almost
uncontaminated by any impoverishment of quantum catching
through bleaching. The reason for previous failures to obtain
cone saturation was thereby revealed: cones at high background
intensities bleach much more quickly than rods. As a
consequence, in attempts to demonstrate cone saturation under
background equilibrium or so-called steady state conditions,
much of the cone pigment is bleached away so that the capacity
for absorbing light quanta becomes smaller, limiting the adapta-
tion effect of the background light.
An interesting extension of their data was obtained by
bleaching
the photopigment in the annular surround to various degrees. They
could then compare the test threshold-elevation effect of bleaching
