Constraints on human stereo dot matching.

Statistical efficiency was used as a model-free measure of stereo performance to explore how well human observers perform for a range of stereoscopic stimuli with varying amounts of added disparity noise. When the stimulus was a random dot stereogram in which perturbations of disparity were applied independently to each dot, most of the dots had very different disparities from their neighbours and efficiency was found to fall as the level of disparity noise was increased. It was hypothesized that the fall of efficiency was due to difficulty with dot matching, so random-dot stereograms were created in which there was a significant correspondence problem but in which there were groups of dots that had locally the same disparity. For these targets, efficiency was fairly constant as noise was increased. When the stimulus consisted of a pair of vertical columns of dots, efficiency was also generally constant as noise was increased, perhaps because such stimuli do not present a severe correspondence problem. These experiments may allow the separation of one source of efficiency loss--that due to the difficulty of dot matching--from other sources of efficiency loss. These separate components of efficiency loss may describe components of performance that are suitable for comparison with the performance of stereo dot matching algorithms.