EACH RECEPTOR TYPE HAS A SEPARATE 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 independence 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.