We measured the regions of the equiluminant plane that are exploited by observers during a Yes/No detection task. The signal was a 640-ms Gaussian modulation (sigma(t) = 160 ms) of a Gaussian spatial patch (sigma(s) = 2.4 deg) presented in chromatically bivariate uniform noise. One component of the noise was along the direction axial with the signal in color space, the other perpendicular. Four signal directions were tested: along cardinal LM and S axes and two intermediate directions to which the cardinal axes were equally sensitive. The distribution of noise chromaticities from each trial was correlated with the observers' responses and the presence and absence of the signal to build a classification image of the distribution of chromaticities on which the decision of the observer was based. The images show a narrowly selective peak in the signal direction flanked by regions with a broader selectivity. These results raise the possibility that detection judgments are mediated by both linear and nonlinear mechanisms with peak sensitivities between the cardinal directions.