Biology Reference
In-Depth Information
then a given test stimulus should produce the same perception under
the two procedures
irrespective
of the nature of the test.
The test of equivalence between dark and light adaptation
had been carried out and confirming results had been obtained by
both Crawford (
1947
) and Blakemore and Rushton (
1965
). Thus, both
studies had found that variation in test parameters that produced
marked changes in threshold level during dark adaptation produced
the same changes in threshold level with light adaptation.
21.3 A decisive experiment
Yet, Rushton (
1965a
) was not quite convinced and therefore made an
additional, so-called decisive test of equivalence, where he investi-
gated whether the
spatial
organization of the adaptation pool for
bleaching and light signals was the same. In this experiment, he
measured dark-adaptation and incremental-threshold curves with
the interposition of a perforated plate in front of both the bleaching
and background lights. The holes of the plate were in sharp focus on
the retina subtending an angle of about 0.5º from hole to hole.
The results showed that the dark and light adaptation produced
vastly different adaptation effects: with dark adaptation, the log
threshold elevating effect was found to be proportional to the average
amount of bleached photopigment while with light adaptation the
desensitizing effect (the increase of test intensity) was proportional
to the average luminance of the background light confirming his
general rule that the threshold is raised simply by the average light flux
independent of spatial distribution (within the summation area) of
the background light. Apparently, there had to be a marked differ-
ence between the spatial spread of the desensitizing influence of real
and 'dark' light. Consequently, Rushton (
1965a
) concluded that light-
and dark-adaptation processes were not equivalent.
To describe the two different processes, he developed his AGC
model in more detail (see Rushton,
1965a
,
b
). Thus, he assumed that the
background-light signal when entering the input of the gain box became
logarithmically transformed as indicated by Fechner's famous law:
