Biology Reference
In-Depth Information
1.
Rods in photopic vision would be saturated over a large part of the
spectrum from violet to yellow and, hence, be unable to discriminate
between wavelengths in this region.
2.
A match between two colour fields made under photopic conditions would
break down under scotopic conditions (the Purkinje phenomenon).
3.
The Stiles-Crawford directional effect, found in the blue region of the
spectrum, was absent under scotopic conditions.
4.
The spectral sensitivity curve for rhodopsin could not be obtained by
any linear transformation of the colour-mixture curves.
Willmer's (
1961
) inquiry led him to conclude that each of the
objections could be surmounted, since they all presumed that the
dark-adapting rods constituted the primary 'blue' receptor mechanism,
while his theory implied that the dark-adapting rod activity was drasti-
cally modified at photopic intensity levels by cone activity.
Although Willmer's ingenious but speculative colour theory
adequately explained many of the basic facts of colour vision, its distin-
guishing mark, that the ordinary dark-adapting rods under photopic
conditions represented the primary 'blue' receptor mechanism, was
shown to be wrong only a few years after the second version of his
theory had been published. In 1964, it was conclusively demonstrated
that the retina actually contained three types of cone as had been
generally presumed (see Marks
et al
.,
1964
; Brown & Wald,
1964
).
By measuring the absorption spectra of single parafoveal receptors
from the primate retina, these researchers found three types of cone
each with its own photopigment. The maximum absorptions of the
pigments were found in the violet, yellow-green and green-yellow
parts of the spectrum. Furthermore, Wald (
1964
,
1967
), using a limiting
case of the two-colour threshold technique of Stiles, presented strong
evidence for three kinds of cone within the rod-free fovea of the
human retina. From now on it became universally accepted that
the sensation of hue in photopic vision in normal trichromats was
mediated by three types of cone.
In the early 1900s most of the research workers presumed that
the desaturation effect of rod activity under mesopic test conditions
was due to some unknown brain processes or was of a psychic nature
