Rotating Structure-From-Motion Experiment

In all experiments, observers viewed two rotating structure-from-motion (SFM) objects. Spherical and “gear” shapes were used in Experiment 1. Spheres only were used in all Experiments 2 and 3. All experimental conditions for Experiments 1 and 3 are illustrated in Videos 1-31.
Experiment 1
Two types of shapes – a sphere and a gear – were used in Experiment 1 (see Videos 1-30). Individual shapes subtended approximately 6.5° of the visual angle vertically and horizontally and consisted of 500 dots distributed randomly over their surface. For the ambiguously rotating shapes, the dot diameter was equal to 0.23° (see Figure 1A-C). When we biased the direction of rotation via perspective cues, the dot diameter was systematically varied between 0.10°

The diameter of the individual dots was 0.06°. Spheres rotated around either vertical or horizontal axis with an angular speed of 72°/s (0.2 Hz). Objects were placed to the left and to the right of the fixation (horizontal arrangement) or above and below the fixation (vertical arrangement). The gap between the objects was 0, 0.1, 0.25, 0.5, and 0.8 sphere widths or, respectively, 0.0° (the touching layout configuration in Experiment 1), 0.6°, 1.5°, 2.0° (the gap layout in Experiment 1), and 4.8°.
Displays were presented either on a uniform gray background (no background condition) or on the textured background (background condition). The textured background consisted of a randomly placed grayscale overlapping rectangles and was generated anew for each block.
Experiment 2 contained 40 conditions: five gap sizes (0, 0.1, 0.25, 0.5, and 0.8 sphere widths) x two layouts (horizontally or vertically arranged objects) x two directions of rotation (around the vertical or horizontal axis) x presence/absence of the background. Presentation order was randomized. The 40 blocks were split into four experimental