Biology Reference
In-Depth Information
15
Introduction
The most basic assumption of the orthodox duplicity theory that
only rods function in night vision and only cones in day vision has
been generally agreed on during the whole developmental period
from Schultze (
1866
) to the present (2009).
Schultze had provided two strong arguments in favour of this
basic assumption. Firstly, rods and cones tended to dominate in
nocturnal and diurnal species, respectively. In fact, some nocturnal
species had a pure rod and some diurnal species a pure cone retina.
Secondly, colour vision, a characteristic feature of cones of the central
fovea, was absent in night vision.
Reasoning from this assumption, one would expect that a
reduction in light intensity from that of daylight would lead to the
functional appearance of rods at some particular intensity level.
Schultze, however, did not state where and how this transition from
cone to rod vision and vice versa happened. Furthermore, he did not
provide any theory to explain the difference in sensitivity between
the two receptor types. This reveals a serious limitation in his theory
construction and shows that the most basic assumption of his theory
was without any adequate explanation.
Several of the leading scientists within vision research have
attempted to bridge this gap in Schultze's theory construction by
developing theories of the sensitivity regulation mechanisms of the
rod and cone receptor systems. Actually, the development of these
theories may be considered a main thread in the history of the duplicity
theory. The theories offered by Hecht, Wald, Rushton, Barlow and
Lamb dominated the scene and will all be dealt with in some detail.
