Biology Reference
In-Depth Information
Finally, in support of his basic assumption that the retina
contained more than one type of rod, Willmer pointed to the observa-
tion of Kühne that rods in the extreme periphery of the human retina
and also in a region close to the central fovea were devoid of rhodopsin
(Kühne,
1879
, see pp. 263-264). He also mentioned the observation of
Boll (
1878
) that the frog retina contained two different rod receptor
types each with its own photopigment.
9.1.3 Reformulation of Willmer's duplicity theory
In 1961, Willmer reformulated his theory. He still maintained the
hypothesis that the ordinary dark-adapting rods by being inhibited
by cones constituted the 'blue' receptor mechanism in photopic
vision, but abandoned his hypothesis of 'day-rods', since the results
of Rushton (
1957
) strongly indicated that the retina contained at least
two different types of cone photopigments. Thus, by measuring the
capacity of different spectral lights to bleach photopigments in the
human fovea, Rushton obtained evidence of two photo-labile cone
pigments with spectral sensitivities similar to Stiles's 'red' and 'green'
primaries. Willmer (
1961
), in accord with this finding, presumed that
the retina, in addition to the primary 'blue' rod mechanism, contained
a primary 'red' and a primary 'green' cone-based mechanism.
Another important improvement of the theory was his determi-
nation of the spectral sensitivity functions of these two primary cone
receptor mechanisms. For this purpose he exploited an ingenious
technique. Taking advantage of the finding that the central fovea lacked the
primary 'blue' mechanism, he could single out and measure the spectral
sensitivity of the 'red' and 'green' primaries by examining, respectively,
deutranope (green-blind) and protanope (red-blind) subjects, since they all
became monochromatic when test stimulated in the central 'blue-blind'
fovea (the fovea of deutranope and protanope being subserved, respec-
tively, only by the 'red' and 'green' primary mechanism).
The procedure employed to obtain the spectral sensitivity of
the 'blue' primary mechanism, on the other hand, was much more
complicated and expressed by the mathematical equation:
