Purpose: S-cone discrimination depends on the chromaticity difference between the test field and the surrounding area. The current study investigated the effect of the spatial and temporal chromatic contrast of a surround to the test field on S-cone discrimination.

Methods: The experimental paradigm isolated spatial, temporal, and spatial-and-temporal chromatic contrast effects on discrimination (Zele, Smith & Pokorny, 2006). S-cone discrimination thresholds were assessed by a four-alternative spatial forced choice procedure (Smith, Pokorny & Sun, 2000). Stimuli were either metameric to the equal energy spectrum, or varied in S-cone activation along a line of constant L/M-cone activation. A model based on primate Koniocellular pathway physiology described the data (Pokorny & Smith, 2004).

Results: Spatial and temporal contrast produced equivalent reductions in chromatic discriminability as the chromatic difference between the test and surround increased. S-cone discrimination in the absence of chromatic contrast was similar to that with spatial, temporal and spatial-and-temporal contrast.

Conclusions: S-cone discrimination was determined by either the spatial or temporal contrast component of the signal. In contrast to L/M-cone discrimination, which was best in the absence of chromatic contrast (Zele, Smith & Pokorny, 2006), S-cone discrimination did not vary with spatial or temporal contrast manipulation. This may reflect differences between the receptive field organizations of the Koniocellular and Parvocellular pathways.