Abstract
Monocular regions in binocularly viewed scenes are usually found near a step-change in depth, between a foreground object and the background scene. They occur because one eye can see a portion of the background that is occluded in the other eye's view by the foreground object itself. That such regions have a role in the perception of depth is clear, but what is less well understood is the nature of the visual mechanisms that deliver the perceived depth. For example, for most configurations, viewing geometry predicts that monocular regions do not specify a unique depth. Instead, the possible depth interpretations can be expressed in terms of a ‘depth constraint region’. This specifies the minimum possible depth, and sometimes a maximum. Previous research has shown that perceived depth is often close to the minimum possible depth. Here we used a depth discrimination experiment to directly compare thresholds and perceived depth, for both conventional binocular disparity and depth from monocular occlusions. Forced-choice psychophysical methods were used, where observers were shown a target and comparison stimulus, and asked which contained the greater depth step. Targets contained either conventional binocular disparity, or depth from monocular regions. Comparison stimuli contained conventional binocular disparity. Depth discrimination thresholds were considerably elevated for depth from monocular regions compared with conventional binocular disparity. Depth biases were also found. There were large individual differences, but some biases were consistent with observers perceiving less depth from monocular occlusions than the depth constraint region would predict. Our experiments suggest that a different, less precise, mechanism is at work in the perception of depth from monocular occlusions, than that available for the perception of depth from conventional binocular disparity.