In order to perceive transparent motions the visual system must compute and represent two (or more) motion signals at some level of spatial representation. We have developed performance-based measures of transparency, based on the precision of a joint directional judgment of two superimposed global motions in a random-dot kinematogram (RDK). Qian, Andersen, and Adelson (1994 Journal of Neuroscience 14 7357 – 7366) have reported that transparent motion is not perceived in RDK stimuli if each leftward moving dot is paired with a rightward moving dot that it meets at the midpoint of a short trajectory (locally balanced). Using performance-based measures of transparency we investigated the conditions for the occurrence of transparency in locally balanced stimuli. Using stimuli with the same parameters as Qian et al we found that the critical distance that the dots must travel to abolish transparency was 0.2 deg or less. Offsetting one set of dots, orthogonally to its motion direction, by 0.3 deg or more allowed for transparency-based judgments with the same degree of accuracy as for random distribution of the two motion directions. These values differ slightly from those reported by Qian et al (0.4 deg and 0.2 deg respectively), perhaps because Qian et al depended solely on subjective reports of transparency. The data suggest that different processes may be involved in detecting transparency when the trajectories are extended and when they are offset. When the trajectory length was varied, transparency-based judgments were possible when each dot pair had an average separation of approximately 0.1 deg over the course of their lifetimes. For the offset stimuli, transparency-based judgments required the dots to have an average separation of approximately 0.2,deg. However, our data are consistent with transparent motion signals not being represented at the most local levels of motion analysis, as proposed by Qian et al.
