Biology Reference
In-Depth Information
21
Rushton's AGC model
21.1 E ac h r e c e p t o r t y p e h a s a s e pa r at e
and independent adaptation pool
Presupposing that the site of adaptation is located in summation
pools situated centrally to the receptor level, one might expect that
different kinds of receptor (rods, 'red', 'green' and 'blue' cones) would
tend to interact in determining the adaptation process. In opposition
to this view, however, Rushton (
1965a
) presented evidence suggesting
that each class of receptor had a separate and independent AGC
pool. Thus, he suggested that for rods and for each kind of cone
the dependence of threshold on background and on bleaching were
private, i.e. separate and independent.
Strong evidence in favour of independence between rod and
cone adaptation pools was given by the well-known fact that the
threshold level obtained during long-term dark adaptation may
remain unchanged for several minutes during the cone-plateau
period, while the sensitivity of the rods may increase by several log
units. Indeed, with deep-red test light, the absolute threshold level
of the cones may remain invariant for more than 20 min during the
cone-plateau period before the rods eventually influence the threshold
level. Furthermore, well-founded evidence in favour of the indepen-
dence of the adaptation pools for
all
the different receptor types was
found by Rushton (
1965a
) in the extensive research work of Stiles on
incremental threshold (see Stiles,
1978
). This research work may be
illustrated by Stiles's well-known experiment together with Aguilar in
1954
on light adaptation of the rod mechanism. They briefly exposed
a test stimulus in the extrafoveal retina superimposed on a steady,
large adapting field. The threshold of the test stimulus was measured
