Biology Reference
In-Depth Information
The intensity of the 656 nm light was then increased 1.2 log
units and the observer asked to adjust the intensity of one of the
ten monochromatic lights until the multicoloured image was judged
to appear neither too red nor too blue-green. This judgement was
termed 'optimum colour'. Under these conditions the observer saw
a larger variety of colours, including whites, reds, oranges, yellows,
blue-greens and browns. When the 'optimum colour'-threshold
intensity was plotted as a function of wavelength, the curve again
matched the scotopic luminosity function.
Lastly, a 510 nm light was directed either through the centre or
periphery of the pupil while the observer adjusted the intensity of the
510 nm light to obtain 'optimum colour'.
The results showed that the 'optimum colour'-threshold was
obtained at very similar intensity levels under these two conditions.
Thus, the Stiles-Crawford effect associated with cone vision was
absent, indicating that the 510 nm light stimulated rods only. On
the other hand, a Stiles-Crawford effect became apparent when
the intensity of the 656 nm light was further increased, so that the
intensity of the 510 nm light reached above cone-threshold level
before the 'optimum colour' threshold was reached.
The authors concluded that L-cones may interact with rods to
produce a multicoloured image and that, therefore, rods may be as
much a part of colour vision as cones.
A clue to an understanding of this remarkable ability of the
rod receptor system was found in an observation made by McCann
and Benton (
1969
). These authors had shown that when a
photopic
495 nm test light was added to the 656 nm multicoloured display,
the subject observed the same change in colour as obtained with the
scotopic test light. The only difference was that the colours obtained
with the photopic 495 nm test light appeared somewhat brighter,
sharper and slightly more saturated. Accordingly, since the photopic
495 nm test light could reasonably be assumed to activate both the
S-wave and M-wave cone receptors, McKee, McCann and Benton
