A pioneering study by J. M. Harris and A. J. Parker (1995) found that disparity judgments using random-dot stereograms were better for stimuli composed of mixed bright and dark dots than when the dots were all bright or all dark. They attribute this to an improvement in stereo correspondence. This result is hard to explain within current models of how stereo correspondence is achieved. However, their experiment varied task difficulty by adding disparity noise. We wondered if this might challenge mechanisms subsequent to the solution of the correspondence problem rather than mechanisms that solve the correspondence problem itself. If so, this would avoid the need to modify current models of stereo correspondence. We therefore repeated Harris and Parker's experiment using interocular decorrelation to vary task difficulty. This technique is believed to probe stereo correspondence more specifically. We observed the efficiency increase reported by Harris and Parker for mixed-polarity dots both using their original technique of disparity noise and using interocular decorrelation. We show that this effect cannot be accounted for by the stereo energy or by simple modifications of it. Our results confirm Harris and Parker's original conclusion that mixed-polarity dots specifically benefit stereo correspondence and point up the challenge to current models of this process.