Biology Reference
In-Depth Information
binocular colour mixture. G. E. Müller (
1923
) found this similarity
consistent with a rod origin, since he assumed the rod and cone
pathways were separated, perhaps to beyond the optic radiation, i.e.
to the visual cortex.
The chromatic rod system would, of course, differ markedly from
the chromatic cone system by operating at low light levels and subserving
a green-blue colour invariant of wavelength. In order to explain the
origin of such an odd chromatic rod system, G. E. Müller speculated
that the rod system in a previous state of phylogenetic development, in
fact,
could
discriminate between different coloured lights, but that the
development of the photopigment rhodopsin, necessary for the increase
of sensitivity during the second phase of long-term dark adaptation, was
antagonistic to the formation of chromatic-related red-green and yellow-
blue substances in the rods, so that only a primitive form of colour vision
remained for the rod system.
5.2.4 T h r e e t y p e s of f c of n e s a n d f i v e p a i r s of f
opponent processes
The cone system, in contrast to the rod system, was assumed to have
retained the ability to mediate the full range of colour sensations.
In order to explain this marvellous ability, G. E. Müller made two
basic assumptions: (1) the phototransduction in the retina, where
light generates nervous activity, involved a triplex cone-receptor
mechanism, and (2) colour processing was based on opponent activi-
ties. Accordingly, he considered both the Young-Helmholtz and the
Hering colour theories to be over-simplifications (see foreword G. E.
Müller,
1930
).
A simplified diagram of his theory is shown in
Fig. 5.1.
For
illustrative purposes, only activation of the rod and one of the three
cone systems (P1) are shown. As can be seen, though, the difference
between the rod and cone systems is marked. In fact, the rod system
involves two white-black systems and possibly an undeveloped green-
blue colour system, while the cone systems with photon-absorption
processes P1 (sensitive from the extreme longwave end of the visual
