The basic stimulus display was the same as in
Experiment 1, with a visual search for higher contrast targets (see
Figure 1). The background had CIE1931 coordinates of 0.3052, 0.3154, and 42.1 cd/m
2. In this experiment, we used the same fixed contrast levels for all participants. Adjusting perceived contrast for color/luminance combined stimuli would pose problems, because they could be manipulated both along the luminance and along the color dimension. We selected a color and a luminance direction for use in the experiment based on conditions with the most stable results in
Experiment 1; the criteria were based on a combination of the largest amplitude, least inter-participant latency variability and noise within the difference waveforms, smallest number of removed observations, and greater monitor gamut range. These were found to be light gray and greenish. Seven distractor Gaussians were homogeneous distractors, as in
Experiment 1, with the contrast of either the individual mechanisms (1
L or 1
C) or the combination of both (1
L/1
C), depending on the condition. The distractors were equivalent to the low contrast light gray and greenish, averaged over all participants, in
Experiment 1. On average, this should ensure equal salience of color and luminance distractors. Two contrast ratios were then used to generate targets, which were twice and three times the contrast of the distractors; multiples higher than three could not be used as they would have put the chromatic stimulus outside the gamut of the monitor. These conditions will be referred to as color only with twice the contrast of distractors (2
C), luminance only with twice the contrast of distractors (2
L), color only with three times the contrast of distractors (3
C), and luminance only with three times the contrast of distractors (3
L). Three combination stimuli were created from these luminance and chromatic stimuli: triple contrast in luminance and double contrast in chromaticity (3
L/2
C), double contrast in luminance and triple contrast in chromaticity (2
L/3
C), and triple contrast in both luminance and chromaticity (3
L/3
C). The distractors for these conditions represented a combination of the luminance and chromaticity of the light gray and greenish used for the distractors in the isolating conditions, which could be labeled as 1
L/1
C. This enabled analysis of differences between lower and higher salience levels within the same mechanism, between different mechanisms, and between combinations of different levels of the two mechanisms.