Biology Reference
In-Depth Information
17
Contribution of S. Hecht
17.1 Hecht's photochemical theory
The finding of Loeser (
1904
) that cones also had the ability to increase
their sensitivity during early dark adaptation was confirmed by Hecht
(
1921
/1922). He found that cones could increase their sensitivity
markedly even during the first few
seconds
after bleaching.
More importantly, however, Hecht developed a photochem-
ical theory for dark and light adaptation of rods and cones that had a
strong influence on a whole generation of research workers. Certainly,
he has played a central role in the developmental history of the
duplicity theory. In a series of papers he provided an array of evidence
supporting his photochemical theory (see Hecht,
1919
/1920a, b, c,
1921
/1922). In its essence and in its most simple version, the theory
runs as follows: light acts on a photosensitive substance
S
and
decomposes it into two precursors called
P
and
A
. The sensitivity of
the eye, then, depends on the concentration of these precursors, not
on the quantity of the photosensitive substance
S
. Thus, the model
states that the amount of fresh precursors necessary for a threshold
response is always a constant fraction of the amount of the precursors
already present in the system. Hence, dark adaptation was thought to
depend on the regular decrease in the concentration of the residual
precursors present in the sensory system. This decrease was assumed
to proceed according to the dynamics of a bimolecular reaction, to
be independent of light stimulation, and, in accord with Kühne's
(
1879
) 'Optochemische' hypothesis, to result in a reformation of the
photosensitive substance
S
.
The fresh precursors, on the other hand, served to catalyze a
simple chemical conversion of an inactive substance
L
into an active
