Biology Reference
In-Depth Information
Secondly, they interpreted the duplicity theory to mean that each
of the two receptor types functioned identically in different species,
and that the retina did not contain any other visual receptor type.
Finally, they interpreted the theory to mean that rods and cones
formed two completely separated visual systems without any interac-
tion and that there, consequently, was
no
common intensity interval
where the two receptor types functioned together. This interpreta-
tion of the duplicity theory was not intended by Schultze (
1866
), who
had suggested that there was a mesopic intensity range where both
receptor types were activated producing a combined effect. Yet, from
Schultze's suggestion, several important, specific questions naturally
arise:
1.
What is the extent of the overlapping mesopic intensity interval?
2.
What is the mechanism underlying the rod-cone interaction?
3.
Do the answers found for one visual function apply to all?
The somewhat extreme interpretation of the duplicity theory
by Saugstad and Saugstad (
1959
) may thus be seen as an attempt to
highlight these basic questions not addressed by Schultze (
1866
).
Saugstad and Saugstad (
1959
) also drew attention to yet another
serious weakness inherent in Schultze's (
1866
) theory: it attributed
the duplex functioning of the visual system to differences between
the rod and cone
receptors
. In view of Polyak's (1941) investigation
of the connections of the various types of neural cells in the retina
they found, in accord with Willmer (
1946
), this basic assumption to
be quite untenable. Indeed, the thorough histological study of Polyak
(1941) had revealed that rods and cones had common pathways.
Obviously, these common pathways provided strong support for the
assumption that the two receptor systems interacted even as early
as at the retinal level, and that interaction might be found under
a variety of conditions. Yet, Saugstad and Saugstad (
1959
) did not
preclude the possibility that rods and cones might operate within
different intensity intervals through mechanisms of neural inhibi-
tion and facilitation.
