The orientation-difference cue in figure-ground separation: border ownership and timing

Abstract

Figure-ground separation, a crucial component of visual processing, can be driven by differences in contrast, orientation, and other local cues. We previously showed (VSS 2020) that the orientation cue has distinctive characteristics: figure-ground separation depends not only on the magnitude of the orientation difference, but also on whether the oriented component is present in the figure vs. the ground. Here we show that this dependence is invariant with respect to convexity vs. concavity of the figure, and determine how it evolves over time. In a 2-AFC task, subjects (N=3) distinguished a target image containing five randomly-positioned figures rendered by one synthetic texture superimposed on a background defined by a second synthetic texture, from a non-target image consisting of a uniform texture whose image statistics matched the spatial average of the target’s statistics. Target and non-target images consisted of 64 x 64 arrays of 10-min black and white checks, and the figures covered 25% of the target’s area. Texture construction enabled control of the spatial frequency content of figure and ground in one or two orientations. For figures consisting of circular discs, figure-ground separation was facilitated when the figure texture was oriented, compared to when the ground was oriented. This difference was linked to the degree of anisotropy, as determined from the textures’ spectra (r=0.82, p<0.001). When figure boundaries were changed from convex to concave, results were similar, indicating that figure content and border ownership, but not border shape, determined performance. In preliminary results (N=1), we find that this asymmetry develops slowly; it is largely absent for a presentation time of 125 ms but close to maximal at 250 ms. In sum, figure-ground separation uses orientation differences in a way that depends on figure-ground assignment, suggesting a recurrent process that evolves over time.