The statistical efficiency of stereopsis was investigated by measurement of the
discriminability d′ of a step edge in depth in a random-dot stereogram and its comparison
with d′ calculated for an ideal observer model. Efficiencies of~ 20% were found for stimuli
with very few (< 30) dots. As the number of dots was increased, efficiency fell markedly to 2%
and below. Over the ranges explored in this study, there was little variation in efficiency with
changes in dot density (with dot number constant) or stimulus size. Efficiency loss was also …

… 2 Examples of stereograms used in the experiment. The two stereohalves are presented
side by side in each example, and one can view them by fusing the white fixation crosses. If
they are cross fused, both (a) and (b) show the right-hand side standing out farther in depth.
(a) Shows a stereogram before noise is added. The dots in each half have the same
disparity, and a central depth discontinuity is easily seen. (b) Shows a … 2 Examples of
stereograms used in the experiment. The two stereohalves are presented side by side in …