Several mechanisms involved in sensitivity regulation - Duplicity Theory of Vision - page 187

accumulated which clearly revealed that the photochemical theory
was too elementary a conception. An early, important psychophys-
ical study that challenged the theory was performed by Craik and
Vernon ( 1941 ). They found that following light adaptation the ratio
of thresholds of large to small test fields increased markedly with
time in the dark, suggesting that the summation effect increased
during dark adaptation and that the threshold curve, therefore, partly
depended on an increase in the amount of interaction in the retina.
Furthermore, by measuring the reduction of the brightness level of the
positive afterimage with time in the dark following light adaptation,
they could provide evidence that afterimages might also affect the
threshold level and thereby influence the form of the dark-adaptation
curve.
Strong psychophysical evidence opposing the simple
photochemical theory of the adaptation phenomena was also
provided by Wright in the early 1930s by his ingenious binocular
matching technique. Using this technique he could measure the
change in brightness and colour quality of a test light presented
to the right eye fovea by a mixture of his three colorimetric
stimuli (650, 530 and 460 nm light) presented to the completely
dark-adapted left eye fovea. The changes of the test light measured
were produced by pre-adapting the right eye to various coloured
lights.
Summing up his results, Wright ( 1946 ) reported three different
findings that clearly revealed that the photochemical adaptation
theory was too simplistic.
Firstly, in cooperation with Walters, he found that his results
did not obey the coefficient law. Thus, the effects of adaptation on
two different test stimuli measured separately and then summed were
significantly different from the adaptation effects on the mixture of
the two lights. As an explanation of this breakdown of the coefficient
law, Wright ( 1946 ) suggested that the three primary responses tended
to interact in that stronger responses would tend to suppress or
inhibit weaker ones.