Methods
Natural Image Analysis: 88 grayscale images were block averaged in N × N patches to remove camera pixel matrix artifacts and divided into R × R patches. Image patches were whitened and binarized at their pixel intensity median. Psychophysical Analysis: Subjects were asked to identify a target (4-alternative forced choice) in brief (120-160 ms) presentations of 64 × 64-check binary images subtending 15 × 15deg. The target/background distinction was defined by a specific set of local image statistics, whose strengths were varied parametrically across trials [2].