Before revision:  The behavior of individual subjects is highly consistent: correlations beyond the second order are hard to distinguish from random noise, and detection thresholds within this salient second-order subspace are quantitatively predicted across 26 two-dimensional texture planes. We additionally find non-trivial symmetries of natural images that leave psychophysical thresholds unchanged, further supporting our hypothesis. These findings significantly extend previous results obtained for binary images, and they provide deeper insights into the relationship between natural signal statistics and human perception .
After revision:  The behavior of individual subjects is highly consistent: correlations beyond the second order are hard to distinguish from random noise, and detection thresholds within this salient second-order correlations match psychophysical thresholds closely (median fractional error <0.13) .