Border Ownership as a Special Case of Contrast Ownership

Abstract

The classification of boundaries as figure and ground is one of the fundamental computations performed by the visual system. This computation requires classifying one side of a boundary, as an occluding or ‘intrinsic’ contour, and the other side of the contour as occluded or ‘extrinsic’ (also described as the problem of border ownership). A recent analysis of stereoscopic contour junctions revealed that the classification of edges as intrinsic or extrinsic to a neighbouring surface depends critically on the relative depth of the contours at the image junction. Specifically, it was argued that near edges are intrinsically one-sided (ie border ownership is uniquely specified by the junction geometry), whereas far contours are inherently two-sided (ie the border ownership of the far contour cannot be determined on the basis of the local junction geometry). Here I present new stimuli that reveal that the asymmetry of near and far contours in specifying border ownership reflects a more general asymmetry in the classification of near and far contrasts.

A variety of stimuli were created. In one set of patterns, an array of five circular discs was embedded on a homogeneous background (either white or black) and was filled with a texture. A uniform disparity was added to the textured region such that it appeared nearer than the outer edges of the disc. When the power spectrum of the texture was systematically varied as functions of 1/fⁿ, the texture did not appear as a solid surface. Rather, the discs containing the texture gradients were decomposed into two sources: a constant background colour of the discs, and transparent clouds that appeared to float in front of the discs. Remarkably, the apparent colour of the clouds and the discs could be transformed simply by changing the colour of the background of the regions that neighboured the discs; the texture itself was unchanged. Specifically, when the textured discs were placed on white backgrounds, the pattern appeared as white clouds hovering in front of dark discs; but when the textured discs were placed on black backgrounds, the percept was of black ‘smoke’ hovering in front of white discs. It is argued that these results reveal that the problem of border ownership is a special case of contrast ownership. The two-sided qualities of far contrasts determine the colour of the background, which in turn determines which side of a near gradient is considered to be more or less opaque (ie which side is more occlusive).