The efficiency of speed discrimination for coherent and transparent motion

Transparent motion involves the integration and segmentation of local motion signals. Previous research found a cost for processing transparent random dot motions relative to single coherent motions. However, this cost can be the result of the increased complexity of the transparent stimuli. We investigated this possibility by measuring the efficiency of transparent and coherent motions. Since efficiency normalises human performance to that of an ideal observer in the same task, performance can be compared fairly across tasks. Our task, identical in both transparent and coherent conditions, was to discriminate the fastest speed between two opposite motion directions. In two experiments where we varied dot density and speed, we confirmed the cost in human sensitivity for transparent motion but also found a cost for the ideal observer. The outcome was a consistent residual cost in efficiency for transparent motion. This result points to a processing limitation for transparent motion analogous to previously suggested inhibitory mechanisms between opposite directions of motion. Furthermore, we found that both transparent and coherent motion efficiencies decreased as dot density increased. This latter result stresses the importance of the correspondence problem and suggests that local motion signals are integrated over large areas.