Idiosyncratic preferences in translational and rotational motion transparency

Previous studies reported idiosyncratic direction preferences in motion transparency depth rivalry (Mamassian & Wallace, 2010; Schütz, 2014; Schütz & Mamassian, 2016), such that observers are more likely to perceive a certain motion direction in front. Here we investigated if such idiosyncratic direction preferences occur also with more complex motion types at higher levels of motion processing, such as rotation. We presented two overlapping dot clouds with opposite contrast polarity in a stationary circular aperture. In translational motion, the dot clouds were moving along a straight path in opposite directions. In rotational motion, one dot cloud was rotating clockwise and counter-clockwise each. Participants had to report the color of the dot cloud they perceived in front. In Experiment 1, rotational motion was presented inside a full circular aperture or only in a sector with varying angular size and location. In Experiment 2, we tested a mixed condition, with one dot cloud of translational and of rotational motion each. In both experiments, there were idiosyncratic directional preferences in transparent motion and, more interestingly, also in rotational motion within the full circular aperture. With smaller sectors in Experiment 1, perception depended on both the rotational and the translational direction preference, and the impact of the rotational compared to the translational preference increased with increasing sector size. This suggests that there is a transition from translational to rotational motion preference as more of the complex rotational motion field is visible. In the mixed condition of Experiment 2, rotational motion was preferred over translational motion in general, even when there were strong directional preferences in translational motion. These findings show that idiosyncratic direction preferences can occur at all levels of motion processing and that there are also preferences between different types of motion that are processed at different levels of the hierarchy.