Dichoptic difference thresholds for uniform color changes applied to natural scenes.

It has recently been shown that the visual system is more sensitive to uniform color and/or luminance changes applied to raw compared to phase-scrambled images of natural scenes (A. Yoonessi & F. A. A. Kingdom, 2008). Here we consider whether the mechanisms responsible for the differential sensitivity operate before or after the point at which the signals from the two eyes are combined. Knowing this should help determine the types of nonlinearities responsible. Thresholds for detecting uniform color transformations applied to raw and phase-scrambled natural scenes were measured under two conditions: monocular, in which the discriminand pairs were placed side by side, and dichoptic, in which they were dichoptically superimposed. Subjects were required to select the pair of images that were transformed from two pairs of images in which the other pair was untransformed. In the dichoptic condition, the transformed image pair was identifiable by its lustrous appearance. In line with our previous findings, thresholds in the monocular condition were higher for the phase-scrambled compared to raw scenes. However in the dichoptic condition there was no significant difference between raw and phase-scrambled thresholds, suggesting that the differential sensitivity was mediated by mechanisms lying beyond the point of binocular combination. It is suggested that cortical neurons sensitive to edges but suppressed by neighboring texture might be responsible for the higher sensitivity to transformations applied to raw compared to phase-scrambled images of natural scenes.