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bleaching and background light, and Naka and Rushton ( 1968 ) had
found that background light, but not bleaching, produced a steady
hyperpolarization of the intracellular recorded S-potential.
Moreover, psychophysical measurements had shown that the
log difference between incremental threshold curves obtained with
large, long duration test flashes and those with small, brief test
flashes was markedly reduced as background intensity was increased.
Barlow ( 1972 , p.10) concluded:
Clearly changes of performance of this sort cannot result solely
from photochemical bleaching or alterations of receptors, and must
involve changes in the neural mechanism. The loss of summation
ability at higher backgrounds is accompanied by improved resolu-
tion; it is as if the retina changed to resemble a finer grain film
being operated at a faster frame repetition frequency at higher
luminance.
Similar conclusions had previously been reached by Granit and
Lythgoe (see Granit, 1947 ; Lythgoe, 1940 ).
Thus, although his original theory had received striking confir-
mation from several sources, Barlow ( 1972 ) acknowledged that
neural-adaptation mechanisms had also to be taken into account to
explain adaptation processes.
Another potential complication was that the human retina
contained both rods and cones and that the cones were of three
different kinds. Yet, Barlow ( 1972 ) obtained a great simplification
by accepting Du Croz and Rushton's ( 1966 ) suggestion that Stiles's
'red', 'green' and 'blue' cone mechanisms with regard to threshold
determination operated nearly independently of each other, both
under bleaching and background adaptation conditions.
22.5 Sensitivity difference between rods and
cones explained
In his summary paper, Barlow ( 1972 ) also attempted to explain the
marked difference known to exist between rod and cone sensitivity.
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