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would depend on the integrative response of a number of different
types of cone receptors connected to the fibre under investigation.
An even more interesting finding was that a dominator
(assumed to mediate achromatic sensation) and a modulator (assumed
to mediate hue sensation) could be obtained from the same fibre. To
solve this problem, Granit suggested that the quality of the dominator
and modulator responses was served by specialized fibre response
patterns. Thus, he suggested that single nerve fibres were able to
convey qualitatively different messages to the brain, a possibility
also acknowledged by Helmholtz ( 1867 ) and Polyak (1941). In order
to account for this violation of Helmholtz's ( 1896 ) specific fibre-
energy doctrine (i.e. that a given nerve fibre transmitted a certain
'quality' independently of the nature of the stimulus), Granit ( 1947 ,
1955 ) presumed that excitatory and inhibitory impulses in the retina
may interact and, thereby, produce a variety of frequency patterns
in single nerve fibres conveying different kinds of information.
Indeed, he suggested that different kinds of information could be
transmitted simultaneously in the same fibre (see Granit, 1955 , pp.
287-291). Such a powerful frequency-coding mechanism would, of
course, allow rods and cones to function independently of each other
in common pathways when both the receptor systems were simulta-
neously active.
Yet, in opposition to this view, Granit ( 1947 ) held that cone
and rod activities tended to suppress each other when activated
simultaneously. Clearly, one would expect the fast reacting negative
electrical potential of the cones (PIII), chiefly concerned with inhibi-
tion, to suppress rod activity, but Granit also found strong evidence
that the slow reacting positive electrical potential of the rods (PII)
may inhibit cone activity. For instance, he showed that the fast cone
component of the ERG obtained under photopic conditions tended
to disappear after the eye had become completely dark adapted (see
Granit, 1938 , pp. 64-66, 1963 , pp. 146-147).
The assumption of the orthodox duplicity theory of Schultze
( 1866 ) that rods only mediate achromatic vision was also challenged
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