We investigated the effects of attention on visual processing of luminance and colour contrast by measuring contrast discrimination thresholds for peripherally displayed gratings (1 c/deg) in attended and unattended conditions. In the attended condition the task was performed on its own, in the unattended condition together with a central search task. Stimuli for the search task and for the contrast discrimination were modulated in either luminance or colour (equiluminant red-green). When both the central and peripheral stimuli were modulated in luminance, contrast discrimination thresholds in the unattended condition were about 3 times higher in the unattended than the attended condition for mid to high base-contrasts, but not for low or zero base-contrasts (agreeing with previous results). Similar results were obtained when both central and peripheral stimuli were modulated chromatically. The attentional effects were well modeled by a multiplicative gain change of the contrast response functions underlying the discrimination data. However, when the stimuli in the central search task were modulated in luminance, that task had no effect on chromatic contrast discriminations; and when they were modulated in colour, the task had no effect on luminance contrast discriminations. The results suggest that luminance and colour discriminations are controlled by separate attentional processes that modulate independently their gain mechanisms. They may also imply that the magno- and parvo-cellular pathways, preferentially stimulated by the luminance and chromatic stimuli, have independent attentional resources.
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