Biology Reference
In-Depth Information
as intensity of the background field increased. They employed a
blue-green test stimulus superimposed on a large red-adapting field
to prevent cones from detecting the test stimulus. By this famous,
so-called 'two-colour threshold technique', they were able to follow
the incremental threshold of the
isolated
rod receptor system over
most of its functional range before the cone receptors intruded.
Previously, Flamant and Stiles (
1948
) had provided strong
evidence that the two-colour threshold technique could be used to
isolate the rod receptor system. They had found that the reciprocal
of the intensity of the adapting field required for different test
wavelengths to raise the increment threshold by a fixed factor (1 log unit
above the dark-adapted threshold) was very similar to the spectral
sensitivity curve of the rod mechanism.
Stiles had obtained similar curves to the incremental rod curve
under a variety of conditions intended to isolate the incremental
curves of the different kinds of cone receptor. The results were
judged to indicate that the different underlying adaptation
mechanisms reacted independently of each other in that the most
sensitive mechanism determined the threshold level except for a
small probability summation factor operating when the two most
sensitive underlying mechanisms approached equality (see Stiles,
1978
). On this evidence Rushton (
1965a
) concluded that the different
types of adaptation mechanism must operate independently of each
other when threshold level is determined under light-adaptation
conditions.
He then, in cooperation with Du Croz, investigated whether
the different types of adaptation mechanisms also operated independ-
ently when threshold level was determined during dark adaptation
(see Du Croz & Rushton,
1966
). If so, they argued, it should be
possible to obtain breaks in cone dark-adaptation curves similar to
the breaks found in the incremental threshold curves obtained by
the two-colour threshold technique and to the cone-rod break, by
changing the relative sensitivity of the three cone-receptor types
during dark adaptation. Their prediction was confirmed.
