Biology Reference
In-Depth Information
The important hypothesis of Nagel (
1911
) that the colour
quality of scotopic vision may vary with conditions of adaptation
was also suggested by the results of a simultaneous contrast experi-
ment performed by Willmer (
1950
). In this experiment he had found
that the colour quality of a test field presented at an intensity level
below the specific-hue threshold (but not necessarily below the
absolute cone threshold) changed from achromatic to bluish when a
chromatic long-wave inducing field was applied. Presupposing that
the test field activated rods only, he assumed that the change from
achromatic to blue resulted from interacting rod and cone activities,
supporting his suggestion that the rods constituted the 'primary' blue
receptor of photopic vision. Thus, he speculated that the blue colour
observed in daylight was due to rod activity that escaped inhibition
from cones (Willmer,
1961
).
As we have seen, this speculation of Willmer was disproved by
spectrophotometrical measurements that demonstrated the existence
of 'blue' cone receptors. Yet, the more general presumption of Willmer,
that rods and cones may interact to produce hue sensations, was
supported by a preliminary study of Stabell and Stabell designed to
determine the first appearance of rod activity following substantial
bleaching. Thus, the subject should decide when the colour of the test
field presented at a mesopic intensity level first changed during the
long-term dark-adaptation period. On the basis of Lie's (
1963
) results,
it was expected that the rod intrusion would be marked by a sudden
desaturation change (Stabell,
1967
b). Surprisingly, however, it was
found that a test colour that appeared green during the cone-plateau
period suddenly changed markedly towards blue-violet in addition to
being desaturated. To ascertain that this blue-violet colour obtained
was actually generated by rods, the test intensity was dimmed to a
scotopic intensity level. The results obtained confirmed the sugges-
tion, and hence revealed that rods could trigger both blue-violet and
achromatic colours depending on the chromatic state of adaptation.
This finding raised the question of whether the mechanisms
underlying achromatic colour in scotopic and photopic vision actually
