In this experiment, a target, when present, differed from the distractors in space-average chromaticity of the variegation. Target and distractor variegation was articulated along four chromatic directions (+
l (reddish), −
l (greenish), +
s (bluish), −
s-(yellowish)). The distractor variegation was always centered at the origin of the color space and articulated in the same chromatic direction as the target vector (
Figure 2a). Thresholds were estimated at four different levels of variegation noise contrast to determine the effectiveness of the noise to raise threshold.
Figure 3 shows squared threshold contrast as a function of the squared noise contrast for the various conditions. Each panel is for a different observer. As expected, threshold rose with noise contrast. Squared threshold contrast has been shown to rise approximately linearly with the square of noise contrast for achromatic patterns (Legge, Kersten, & Burgess,
1987) as well as for chromatic patterns (e.g., D'Zmura & Knoblauch,
1998; Gegenfurtner & Kiper,
1991; Hansen & Gegenfurtner,
2006; Lindsey & Brown,
2004). Legge et al. (
1987) and Pelli (
1981) show that such relationship can be captured by a linear function relating squared threshold contrast and squared noise contrast with the intercept and slope of the function capturing the intrinsic noise of the underlying mechanism and the effect of external noise, respectively. Since the slope of the function represents the effectiveness of the noise to raise threshold, it can be used to infer the efficiency of the mechanism to process, in this case, multiple variegated elements. Lines in
Figure 3 are such fits with the slopes (
k) for each function indicated in the figure. Although thresholds for variegated elements that were articulated along the S/(L + M) direction were elevated compared to L/(L + M) direction, the slopes of the functions were similarly shallow (mean slope: 0.0338) for the various conditions and across observers.
To test independence between the chromatic mechanisms mediating the searches for these variegated elements, measurements along the +
l and +
s directions were repeated at two levels of noise contrast (approximately 4× and 16× normalized threshold) with orthogonal noise added to both target and distractor elements. Since identical orthogonal noise was added to both the target and distractors, it provided no information about the target and could only impair search performance. Four levels of orthogonal noise contrast were tested (0×, 4×, 8×, and 22× normalized threshold). A noise vector of zero represents a control condition in which half of the small tiles (32) within each target and distractor element were set to gray (
Figure 1, middle column) and the other half were used to articulate the target and distractor chromaticities.
Figure 4 shows squared threshold contrast plotted as a function of orthogonal squared noise contrast for three observers (separate panels) and for two levels of noise contrast (solid symbols with solid lines and open symbols with dashed lines for 4× and 16× normalized threshold, respectively). As expected, thresholds for variegation articulated along the S-cone direction were elevated. Likewise, not surprisingly, threshold with higher noise contrast (i.e., 16× threshold) was elevated compared to lower noise contrast (i.e., 4× threshold), but most importantly, orthogonal noise had little effect on threshold. We therefore conclude that the measurements supported independence between the chromatic mechanisms mediating these searches.