Relationship between amount of rhodopsin and sensitivity during dark adaptation - Duplicity Theory of Vision - page 156

ways. Firstly, weak bleaches produced threshold recovery curves that
were more rapid than predicted by his formula. Secondly, the shape
of the rod dark-adaptation curve changed markedly with the size of
the test field, whereas regeneration of rhodopsin, of course, does not.
Rushton concluded that the relationship obtained was valid only for
substantial bleaching and that the constant of the linear relationship
depended upon the size of the test field.
Rushton (1965a, 1966 ) also made an attempt to find the
relationship between the amount of bleached cone photopigment
and threshold level. Following strong bleaches, regeneration of cone
photopigments (measured with his densitometer) and the ordinary
cone dark-adaptation curve were recorded. Protanope, deutranope
and normal trichromats were used as subjects, and the measurements
were confined to the rod-free fovea.
At variance with Hecht ( 1921 /1922), who proposed a linear
relationship between threshold intensity and the amount of
photoproduct of cones, Rushton found threshold level T to be
logarithmically related to amount of bleached photopigment B . For
the 'green'- and 'red'-related cone photopigments the relationship
was given by the equation:
Log T = 3 B
Unfortunately, due to the scarcity of 'blue' cone receptors within
the central fovea, Rushton was unable to measure the relationship
for the 'blue' cone photopigment (see Rushton, 1965a).