Stiles's II5 color mechanism: tests of field displacement and field additivity properties

The II5 color mechanism was isolated in two observers with a 667 nm, 200 ms, 1 deg foveal test flash. Stiles's field displacement law and the law of field additivity were tested with increment thresholds upon monochromatic and bichromatic adapting fields. The data of both observers obey the displacement law and exhibit no systematic deviations from additivity. Our results cannot reject the hypothesis that the first log unit of adaptation observed under II5-isolation conditions is determined exclusively by the quantum catch rate of the long-wavelength-sensitive cones.