DARK AND LIGHT ADAPTATION BASED ON SIMILAR MECHANISMS

In contrast to the assumption of Rushton that light and dark adaptation 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.

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).