Biology Reference
In-Depth Information
9.3.2 The colour-mixing hypothesis
Presuming that the sensitivity of cones stayed essentially constant,
while the sensitivity of rods increased during the rod phase of dark
adaptation, Lie (
1963
) explained his results (the rise of the specif-
ic-hue threshold during the rod phase of dark adaptation and the
effects on saturation and brightness of dark adaptation obtained with
the green test light, Experiment 7) by a colour-mixture hypothesis,
where the cone and rod mechanisms contributed a chromatic and an
achromatic component, respectively.
Presuming that the relative strength of the achromatic rod
and the chromatic cone components varied with changes in the test
conditions, he could also explain the changing relationship obtained
between the fall in rod threshold and the rise in the specific-hue
threshold when test conditions were varied.
9.3.3 An alternative explanatory model
Although Lie (
1963
) held that his simple colour-mixture hypothesis
could account reasonably well for his results, he presented a more
sophisticated, alternative explanatory model where he combined the
colour-mixture hypothesis with a hypothesis of antagonistic retinal
interaction between the rod and cone activities. In defence of this
antagonistic interaction he argued that rod and cone impulses when
activating the same nerve fibre simultaneously must be mutually
exclusive since they mediate qualitatively different colour sensations.
With the alternative model he could explain change in colour
with intensity observed in a dark-adapted state as follows:
When the eye is test stimulated with a monochromatic light
between the absolute threshold and the cone-plateau level in a
dark-adapted state, there is no antagonistic rod-cone interaction,
since only the rod system is activated. The subject will therefore
observe the test flash as achromatic.
When the test intensity reaches the cone-plateau level, fibres
connected to both rods and cones will still be occupied with rod
activity, since the cone impulses are inhibited by the stronger rod
