CONTRIBUTION OF T.D. LAMB

The theories of Rushton and Barlow were very much alive in the 1960s and 1970s and dominated the discussion about visual adaptation. The most convincing defence of Barlow's view was published by Lamb (1981). He admitted that Rushton's theory had received stronger support from the scientific community, but argued that Barlow's theory could more easily be reconciled with known properties of the visual system. He supported his view by presenting three strong arguments against Rushton's theory: (1) intracellular electrical recording from rods and cones under conditions of light adaptation had shown that photoreceptors could become desensitized in a way approximating the Weber law, (2) Rushton's theory implied that the photoreceptor could generate two quite different signals: an incident light intensity signal and a signal of the fraction of bleached photopigment, but no distinguishing mechanism located in the receptor had been found, and (3) the uniform desensitizing effect found by Rushton and Westheimer (1962) and Rushton (1965a) could not be repeated in an experiment performed by Barlow and Andrews (1973). In fact, these authors found a marked difference between sensitivity in the dark and bleached areas.

Lamb (1981) then went on to propose his own specification of Barlow's theory. By analyzing a series of rod dark-adaptation curves of a trichromat previously published by Pugh (1975), he came to the conclusion that the long-term dark-adaptation curve of rods could best be described by three straight lines, each with a different slope.