Salience from feature contrast: variations with texture density.

The salience of popout targets was measured in regular line arrays as a function of texture density. Test targets (singletons with orientation, motion, or luminance contrast) presented at different raster widths were compared with reference lines (lines brighter than surrounding lines) presented at fixed raster width. The luminance at which the reference target appeared as salient as the particular test target was taken as a measure of the relative salience of the test target. For orientation or motion contrast, targets at medium to small raster widths were far more salient than targets in sparse or very dense line arrangements. For targets defined by luminance contrast, salience variations with texture density were less pronounced. Some subjects also reported salience for lines in sparse arrangements even when these did not display feature contrast. When such non-specific saliency effects were subtracted from the actual measurements, salience curves for orientation or motion contrast revealed peaks of increased sensitivity at line spacings below 2-3 deg and flat curves at larger grid sizes. In an additional experiment, saliency effects from orientation contrast were measured using texture lines of different size. Salience variations were commonly observed. However, the curves were not found to scale with the different sizes of texture elements but were constantly related to the free space between neighbouring lines. This suggests that peaks in the salience profiles reflect the limited spatial extent of the underlying neural mechanisms.