Visual sensitivity is a process that allows the visual system to maintain optimal response over a wide range of ambient light levels and chromaticities. Several studies have used variants of the probe–flash paradigm to show that the time course of adaptation to abrupt changes in ambient luminance depends on both receptoral and postreceptoral mechanisms. Though a few studies have explored how these processes govern adaptation to color changes, most of this effort has targeted the L–M-cone pathway. The purpose of our work was to use the probe–flash paradigm to more fully explore light adaptation in both the L–M- and the S-cone pathways. We measured sensitivity to chromatic probes presented after the onset of a 2-s chromatic flash. Test and flash stimuli were spatially coextensive 2° fields presented in Maxwellian view. Flash stimuli were presented as excursions from white and could extended in one of two directions along an equiluminant L–M-cone or S-cone line. Probes were presented as excursions from the adapting flash chromaticity and could extend either toward the spectrum locus or toward white. For both color lines, the data show a fast and slow adaptation component, although this was less evident in the S-cone data. The fast and slow components were modeled as first- and second-site adaptive processes, respectively. We find that the time course of adaptation is different for the two cardinal pathways. In addition, the time course for S-cone stimulation is polarity dependent. Our results characterize the rapid time course of adaptation in the chromatic pathways and reveal that the mechanics of adaptation within the S-cone pathway are distinct from those in the L–M-cone pathways.
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