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Lastly, Schultze ( 1866 ) suggested that cones provided better
spatial resolution than rods. However, presupposing that acuity was
based on the number of receptors per unit retinal area, he encountered
a serious problem when attempting to explain the reduction in acuity
with retinal eccentricity. If each receptor sent an independent signal
to the brain, acuity should increase, not deteriorate with eccentricity,
due to the increasing number of small rods that far outnumbered the
cones. In want of a better explanation of this apparent inconsistency,
Schultze ( 1866 ) speculated that the better acuity performance of
cones, somehow, was related to their ability to mediate colour vision.
Thus, cones were assumed to perform better than rods in acuity tasks,
because several fibres were connected to the inner segment of each
cone and thereby provided greater information processing capability.
3.4 Phototransduction is photochemical
in nature: Boll and Kühne
At the time Schultze ( 1866 ) published his duplicity theory, there was
no consensus on phototransduction. Although it was known that
light energy was transformed into nervous energy in the receptors,
the transformation mechanism was completely unknown. Indeed,
Newton's idea, presented nearly two centuries earlier, that light
produced physical vibrations in the nerve fibres was still considered
a viable alternative (Helmholtz, 1867 , pp. 214-215).
The works of Boll and Kühne, published a few years later,
however, were a breakthrough in our understanding of the phototrans-
duction process. Boll ( 1877 ) discovered that the photopigment
rhodopsin, situated in the outer segment of the rods, was bleached by
light and regenerated in the dark and suggested that these photochem-
ical processes might be a foundation of visual colour processing.
Kühne ( 1879 ) disagreed with this suggestion and instead, on the basis
of very extensive research, put forward his general 'optochemische
Hypothese' (optochemical hypothesis) where he assumed the existence
of photochemical see-substances (Sehstoff) both in rods and cones
(Kühne, 1879 , pp. 326-332).
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