Mechanisms of chromatic rod vision in scotopic illumination - Duplicity Theory of Vision - page 115

basic assumption that rods mediate achromatic colours only. On
either presumption, therefore, the duplicity theory would have to be
reformulated.
Evidence obtained in the 1960s and early 1970s favoured the
second alternative that rods triggered the scotopic contrast hues, but
its validity was first conclusively demonstrated by Stabell and Stabell
( 1975 ) when it was found that:

1. The spectral threshold and brightness curves of scotopic contrast hues
coincided with the scotopic visibility curve ( Fig 12.1 ).
2. Scotopic contrast hues could be obtained at an intensity level of nearly
3 log units below the absolute threshold level of dark-adapted cones.
3. Scotopic hues remained invariant of test wavelength and test intensity
in accord with the Principle of Univariance.
On this evidence, the conclusion had to be drawn that the scotopic
contrast hues were triggered by impulses originating in the rod
receptors.

12.3 Scotopic contrast colours depend on
s e l e c t i v e c h ro m at i c a da p t at i of n of f
cones
The next important insight into the mechanisms underlying the
scotopic contrast colours came when it was discovered that the
disposition for the colour-related activity triggered by rod impulses
was generated by selectively chromatic pre-stimulation of cones.
Thus it was found that:

1. The scotopic contrast hue was approximately opponent to the hue of the

pre-stimulation ( Fig. 12.2 ).

2. The lowest intensity of the pre-stimulation light that could produce

scotopic contrast hues was closely linked to the intensity level of the
specific hue threshold of the pre-stimulation light.

3. Under simultaneous contrast conditions, scotopic contrast hues could

be obtained when only cones were activated in the inducing field.

On this evidence the conclusion was drawn that the scotopic contrast
hues were based on rod-cone interactions - that selectively chromatic
stimulation of cones generated the disposition for the hue-related
response triggered by rod signals (Stabell & Stabell, 1973 a, 1978 ).