The conspicuousness of orientation and motion contrast

Subjects were asked to compare the salience of different targets in a texture array. Elements that popped out from local differences in orientation or motion were compared with texture elements that popped out from luminance because they were brighter than all other elements in the pattern. The luminance of a comparison target that produced an equal preference rating for both targets was taken as a direct measure of the salience of the target under test. Saliency matches were made for targets in different background conditions; orientation and motion defined targets were tested in separate experiments. The data show that the salience of pop-out targets does not linearly increase with increasing orientation or motion contrast but reveals nonlinear properties from threshold and saturation effects. With increasing variation of background elements, target salience decreases continuously. Already small variations in background texture make a given target appear less compelling than when presented on a homogeneous background. The data also show that orientation and motion properties behave very similarly in these aspects and suggest that a similar neuronal mechanism may underly pop-out in both visual dimensions.