Biology Reference
In-Depth Information
9.1.2 E v i d e n c e s u p p o r t i n g Wi l l m e r 's d u p l i c i t y
theory
Willmer (
1946
) pointed to three different experimental findings in
support of the view that ordinary dark-adapting rods under photopic
condition constituted the primary 'blue' receptor mechanism:
1.
Experiments had shown the rod-free fovea to be tritanopic, or
'blue-blind', i.e. unable to discriminate between wavelengths in the
short-wave region of the spectrum (König,
1894
; Willmer,
1946
).
2.
The tritanopic state was also found to emerge during dark adaptation
in the extrafovea when the test condition changed from
photopic
to
mesopic
, i.e. when the cone inhibition of the ordinary, dark-adapting
rod receptors was assumed to decrease.
3.
Finally, Willmer pointed to the finding of Granit that the rod-retina of
the guinea pig showed clear evidence of a relative increase in sensitivity
to the blue part of the spectrum when the eye became light adapted.
Moreover, Willmer could present a plausible case in evolutionary
terms. His reasoning ran as follows: in the course of the evolutionary
development, rods had gained their remarkable sensitivity by
increasing their content of rhodopsin and so had become useful
under conditions of low illumination. They had also become inordi-
nately sensitive under daylight conditions causing dazzling at high
intensity levels - as clearly evidenced by the visual behaviour of rod
monochromats. If, then, the less sensitive cones had developed an
inhibitory mechanism that counteracted the extreme increase in
rod sensitivity and at the same time had created the photopic 'blue'
primary, this would surely have been a phylogenetic development of
great survival value.
He also found evidence for the inhibitory cone mechanisms in
the results obtained by Granit. Thus, by examining the waveform
of the electroretinogram (ERG) of the retina, Granit had provided
evidence that the mixed rod-cone retina was more inhibited in the
light-adapted than in the dark-adapted eye, and that in the light-
adapted eye the mixed rod-cone retina was more inhibited than the
pure rod retina.
