Time-course of S-cone system adaptation to simple and complex fields

We examine the temporal nature of adaptation at different stages of the S-cone color system. All lights were restricted to the S-cone-only (a constant L and M) cardinal axis in color space passing through mid-white (W). The observer initially adapted to a steady uniform field with a chromaticity on the -S end of the axis or on the +S end of the axis or a complex field composed of chromaticy -S and +S (+/-S adaptation). The observer then readapted to a steady uniform field of chromaticity W for a variable length of time (i.e., 0, 0.1, 0.25, 0.5, 1.0, or 2.0 s). A probe-flash technique was used to measure S-cone discrimination at various points along the S-cone-only cardinal axis. This allowed estimation of the response of the S-cone system over an extended response range. Following exposure to the -S and +S uniform fields, sensitivity was maximal at or near the chromaticity of the initial adaptation field and decreased linearly away from the adapting point. The shift from +S to W occurred more rapidly than the shift from -S to W; both of these shifts can be described by a multiplicative scaling of the S-cone signal. Following +/-S adaptation the threshold curve initially had a shape similar to that measured following -S adaptation, but returned rapidly to the W adaptation state. The shift following +/-S adaptation cannot be described by the multiplicative model, but can be explained by a change in the shape of the non-linearity. The results suggest the existence of fast post-receptoral processes.