The efficiency of depth discrimination for non-transparent and transparent stereoscopic surfaces

The perception of transparency in binocular vision presents a challenge for any model of stereopsis. We investigate here how well human observers cope with stereo transparency by comparing their efficiency between transparent and opaque depth judgments. In two experiments, the efficiency measure was computed relative to an ideal observer to take into account the larger correspondence ambiguity in the transparent condition. We found that thresholds for human and ideal observers were consistently higher in the transparent condition than in the opaque condition, across a range of dot densities (Experiment 1) and disparity ratios (Experiment 2). Efficiencies (the ratio of human to ideal performance) were approximately equivalent for the opaque and transparent conditions across all stimulus conditions. Therefore, the cost for stereoscopic transparency can be accounted for by the greater correspondence problem in that condition. Indeed, the fact that efficiencies were very low, around 1%, and decreased with increasing dot density demonstrates that human observers use far less information than is available to perform the task. This account contrasts with previous interpretations for the cost in stereoscopic transparency in terms of inhibitory interactions specific to transparent configurations. We relate our findings to a previous and comparable study of motion efficiency, and discuss our findings in terms of a physiologically plausible model.