Bias and sensitivity of stereo judgements in the presence of a slanted reference plane.

The perceived depth of features is known to be affected by the presence of a slanted reference plane. Mitchison and Westheimer ((1984). Vision Research, 24, 1063-1070) reported that two lines appear to be at the same depth when they lie in a plane approximately parallel with the reference plane. We measured the perceived depth of two lines presented in front of a regular grid of dots that was either fronto-parallel or slanted about a vertical axis. The effect of the slanted grid on perceived depth diminished as the grid was moved further in disparity from the lines. We also found that the slanted grid affected the sensitivity to differences in disparity. The minimum threshold for detecting changes in disparity is normally lowest in the fixation plane and rises systematically with increasing pedestal disparity. In the presence of a slanted reference plane, the minimum threshold is at or close to the plane of equal perceived depth and rises with increasing disparity from this plane.