Depth discrimination of a line is improved by adding other nearby lines.

Depth discrimination thresholds are shown to be lowered by up to a factor of 10 when a few reference lines are added to a stimulus containing a single isolated test line. Four reference lines are better than two, which are better than one, and the improvement in performance is greater when the test line lies between the two reference lines in depth. Stereoacuity for the relative depth of a target line, relative to other nearby reference lines, is shown to be insensitive to changes of disparity of the whole pattern of up to about +/- 5 arc min and is only weakly sensitive to larger displacements of up to +/- 10 arc min.