The purpose of the study was to determine whether visual search can be mediated by an achromatic, or luminance, mechanism in which signals are independent of the chromaticity of the stimuli. Experiments were designed to determine whether variability in the chromaticity of distractor stimuli made it more difficult to search for a target that differed from the distractor stimuli in luminance. Variability in the chromaticity of the distractors had little or no effect on search times when the target stimulus was white. Variability in the chromaticity of the distractors increased search times when the target was a reddish or bluish chromaticity. Results obtained with white targets suggest that these searches are mediated by an achromatic mechanism in which the signals are independent of the chromaticity of the stimuli. Results obtained with reddish and bluish targets suggest that searches for those targets may be mediated by mechanisms tuned to both chromaticity and luminance. Further experiments in which observers searched for targets that differed from distractors in both chromaticity and luminance provided additional support for the second conclusion.
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