Biology Reference
In-Depth Information
22
Contribution of H. B. Barlow
22.1 D a r k a n d L i g h t a da p t at i o n b a s e d o n
similar mechanisms
In contrast to the assumption of Rushton that light and dark adapta-
tion were determined by quite different mechanisms, Barlow (
1964
)
put forward the hypothesis that their underlying mechanisms were
very similar; that the sensitivity regulation during dark adaptation
was mainly determined by so-called 'photon-like' events. Thus, he
presumed that the presence of bleached photopigment caused the
receptors to show effects similar to those induced by light. Both the
bleached photopigments and light increased the intrinsic noise level
of the receptors. In fact, the effect of bleached pigments was assumed
to add to that of light so that the separate effects of the two factors
could not be distinguished at the receptor output.
22.2 Both noise and neural mechanisms
involved
There was, however, a serious obstacle to Barlow's noise theory,
since the expected square root law was not fulfilled in incremental
threshold measurements. Thus, if a light flooding the retina elevated
the threshold solely by virtue of the intrinsic noise level of receptors
(i.e. the statistical fluctuation in the number of quanta absorbed)
the incremental threshold should rise in proportion to the square
root of the intensity of the flooding light (see De Vries,
1943
; Rose,
1953
; Barlow,
1957
). Opposed to this prediction, however, it had
been found that, over a large range of background intensities, the
incremental threshold for a large test field rose in direct proportion to
the background intensity (the Weber law). Only for small test targets
and moderate background intensities were the expectations fulfilled.
