Biology Reference
In-Depth Information
On the basis of their reformulation, Saugstad and Saugstad
(
1959
) then made a thorough and systematic evaluation of the
empirical basis of the duplicity theory. They concluded that
although there was ample and well-founded evidence in support
of a duality of function between the two basically different types
of receptor, there was also ample, if not equally well-founded,
evidence pointing to a need for a revision and elaboration of the
theory. In particular, they stressed the need for its statements to be
qualified by being developed into a more comprehensive theory by
including known structures and functions of the nervous substrate
of the visual system.
9.2.2 E v i d e in c e i in s u p p of r t of f S c h u l t z e 's d u p l i c i t y
theory
In support of the duplicity theory they presented the following
evidence:
1.
The spectral sensitivity curves obtained under photopic and scotopic
conditions had been found to differ markedly with maximum sensitivity
at about 560 nm and 510 nm, respectively.
2.
Dark adaptation curves obtained by measuring the change in absolute
threshold or the critical frequency of a flickering light (i.e. the fusion
frequency) displayed one branch under photopic test conditions and
another, quite different one, under scotopic test conditions.
3.
The Purkinje shift obtained within the central fovea had been found to
be much smaller than that obtained in the extrafoveal regions.
4.
Studies of night blind and complete colour-blind subjects indicated
that one of the two receptor types may be left intact while the other is
impaired.
5.
The retinal directional sensitivity, i.e. the change in sensitivity with a
change of incident light angle, was mainly connected to cones.
6.
Two different photochemical substances had been found; the rod
photopigment rhodopsin that determined the scotopic-luminosity curve
and the cone photopigment iodopsin that determined the photopic-
luminosity curve (Wald, 1937b).
7.
Pure cone and pure rod retinas had been found only in diurnal and in
nocturnal species, respectively.
