Light adaptation in cone photoreceptors: the occurrence and significance of unitary adaptive strength.

The behavior of "peak response-log intensity" functions generated by the "dark glasses model" is examined and is shown to describe previously observed light adapted behavior in cone photoreceptors. The models of Boynton and Whitten (1970) and Norman and Werblin (1974) are closely related to the dark glasses model--the Boynton-Whitten model being more specific and the Normann-Werblin model more general. For the models, a certain parameter relationship will produce systems which have optimal intensity discriminative capacities. When the data are fitted, this parameter relationship--unitary adaptive strength--seems to emerge. Possible evolutionary and psychophysical implications are discussed.