Biology Reference
In-Depth Information
colour sensations may operate independently of each other.
Thus, as already pointed out by Tschermak (
1902
,
1929
) and von
Kries (
1911
), a chromatic sensation may change to an achromatic
sensation when, for example, size or intensity of a test field is
reduced. This observation, of course, clearly opposed the Young-
Helmholtz colour theory, but could easily be explained by Granit's
dominator-modulator theory on his assumptions that (1) modula-
tors (mediators of hue sensation) and dominators (mediators of
achromatic sensation only) were activated by different receptor
units, and (2) dominators were much more numerous than
modulators. (Granit, though, admitted that Hering's colour theory,
with its 'white-black' independent colour mechanism, could also
explain the apparent independence of achromatic and chromatic
sensations in a straightforward manner.)
Granit's dominator-modulator theory also offered a quite
new explanation of various forms of colour-blindness. Thus, Granit
(
1947
), in opposition to the trichromatic colour theory, suggested that
deficiency of colour vision may depend on the absence of modulator
mechanisms or their sensitivity reduction, changes in the relative
number of modulators and dominators, and changes in dominator
mechanisms. Rod monochromacy, for example, could be explained
simply on the assumption that the photopic dominator and all the
modulators were missing.
8.3 Schultze's duplicity theory challenged
Interestingly, in mixed rod-cone retinas the photopic dominator
response obtained from single units was often found to change to a
scotopic dominator response during dark adaptation. This evidence
strongly indicated that both rods and cones could activate the same
nerve fibre, and that a given fibre, therefore, could carry messages
from both rods and cones as had previously been suggested by Polyak
(1941). In the dark-adapted state the response would be determined
by the properties of rhodopsin, and the scotopic sensitivity curve
would be obtained, while in photopic vision the sensitivity curve
