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Sensitivity regulation due to
rod-cone interaction
Perhaps the most profound change in our conception of the sensitivity
regulation mechanisms, however, came when it was realized that the
rod and cone activities were not mutually independent, but that rod
activity could influence cone sensitivity and vice versa. Anatomical,
electrophysiological and psychophysical studies all contributed to
this change in view.
Well-founded histological evidence in favour of the rod-cone
interaction hypothesis was provided by Polyak's investigations of
primates, summarized in Polyak (1941/ 1948 ). He discovered that,
with the exception of the midget cone system, rods and cones were
connected to the same pathways in the retina. Furthermore, he showed
that the retina of primates was an extremely complex structure
composed of a variety of different types of neural networks strongly
indicating that the retina, and not just the brain, was involved in
analyzing and synthesizing visual information.
More direct evidence in favour of rod-cone interaction was
provided by Granit's ERG measurements. These results were
interpreted to mean that rods and cones competed for 'a final common
path' when excited simultaneously, and that the more active receptor
types tended to exclude the other (see Granit, 1938 , pp. 65-66).
Later on, Gouras and Link ( 1966 ) came to a similar conclu-
sion. They investigated rod-cone interaction in Rhesus monkeys by
analyzing the discharge pattern of ganglion cells connected to both
rod and cone receptors. Their results showed that impulses from
rods or cones arriving first at the ganglion cell would tend to block
impulses from the other receptor type when these arrived shortly
thereafter. When, for instance, rods and cones were activated simulta-
neously by a light stimulus, the faster cone signals would antagonize
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