Biology Reference
In-Depth Information
epithelium, a tissue lining the fundus of the eye and in intimate contact
with the rods and cones, played a crucial role in the regeneration
process, since the synthesis of rhodopsin from vitamin A did not occur
in a retina detached from the pigment epithelium. The significance
of this dependence was, however, unknown (see Wald,
1935
/1936,
p. 367). (Even today the transport pathway from the outer segment
of the receptors to the pigment epithelium and back again, and the
specific mechanisms by which the transformation from vitamin
A to 11-cis retinal occurs are not fully known.) Nevertheless, Wald
presumed that opsin trapped 11-cis retinal as fast as it appeared to
form the visual pigment and hence regulated how much vitamin A
was oxidized and visual pigments synthesized. In fact, Wald held that
with different opsins went differences in both the kinetics of bleaching
and regeneration, and in the absorption spectrum (see Wald,
1968
).
18.2 S e r i o u s c h a l l e n g e s t o t h e
photochemical theory
Despite the great contributions of Hecht and Wald, the photochem-
ical theory of adaptation presented did not gain general acceptance.
The main obstacle was the well-known fact that sensitivity measured
psychophysically by threshold intensity was dependent upon the
size of the test field. This finding had generally been explained by
assuming that impulses from widely separated retinal areas converged
on common pathways and thereby increased the sensitivity. Thus,
in addition to the concentration of photoproducts, this convergence
factor based on neural summation might influence sensitivity during
dark and light adaptation (see e.g. Lythgoe,
1940
).
18.3 The neural factor refuted
The view that neural summation influenced the sensitivity increase
during dark adaptation was challenged by Hecht, Haig and Wald
(1935/1936). The evidence they presented indicated that changes in
sensitivity with test area were due principally, not to the change in
the area itself, but to variation in the rod-cone composition of the
