The effect of variability of background elements on the conspicuity of objects

The effect of complex backgrounds on target visibility was studied by asking observers to locate a disc target presented on a background of randomly arranged discs. A previous paper (Vision Res. 22, 1241-1252, 1982) explored the effect of increasing the density (numerosity) of the background elements when the elements were all the same size and luminance. In this investigation the size or the luminance of the background elements varied although mean size and mean luminance was held constant. Variability of the size of the background elements had a substantial effect on the size contrast necessary to detect the target. The data are well described by a contrast metric CT which assumes that observers detect the target by comparing it to a weighted average of the larger elements. The larger elements that determine the weighted average are those that all appear the same size because, at the eccentricity of target, the observers are unable to distinguish their differences in size. The threshold values of CT for detection of the target at a given level of probability were dependent on eccentricity but independent of the degree of variability of the background elements. The threshold contrast was also the same as the necessary for the more elementary task of discriminating size differences of two discs. In a second experiment it is shown that variability in the luminance of the background elements did not affect target visibility. This result is explained by reference to the data in our earlier paper from which it can be shown that the observers were unable to perceive the differences in luminance in their peripheral visual field so that, within limits, backgrounds that vary in luminance can be validly characterised by mean luminance.