The perception of depth in simple figures.

When subjects with good stereoscopic acuity are given the task of judging which of two vertical lines lies nearer, the presence of other features nearby alters the perceived depth within the test pair. In the presence of a single flanking line shown with disparity, the test line pair is seen as fronto-parallel when it has disparity in the direction which tends to align it in depth with the flanking line. The notion of "salience" is introduced. This is the summed disparity--weighted approximately inversely with distance--between test objects and their neighbours. We make the hypothesis that objects appear at equal depths when they have equal salience. The salience hypothesis accounts for a variety of depth interaction effects between test lines and adjoining features, such as one or more other lines and a lattice of dots with a disparity gradient. Whether features other than nearest neighbours influence depth judgments depends on the individual. For five good stereo subjects, in two a single line completely masked all effects beyond the nearest neighbour, two others had partial masking, and one had none. If the visual system is interested in corners between planes in depth and in objects protruding from such planes, then salience constitutes a useful indicator for this purpose.