The stimulus parameters for each condition.

TSG and SSG stand for temporal and spatial size gradient, respectively. All conditions are exemplified in Movies S1, S2, S3, S4, S5, S6, S7, and S8. In Condition 1, uniformly distributed stationary dots are presented. As they do not cue the location of the singular point, this condition was not used in our experiment. In Condition 2, stationary dots with the size gradient imply a 3D scene structure, signifying the location of the singular point. In Condition 3, all the stationary dots expand their diameter at the same rate. As they do not cue the location of the singular point, this condition was not used in our experiment. In Condition 4, static dots are initially arranged with the size gradient, implying a 3D depth structure. Each dot changes its size as if it looms or recedes without changing its position. Condition 5 corresponds to a conventional random dot movie with uniform dot size, which does not change over time. In Condition 6, the initial frame has the size gradient to imply the 3D depth structure. However, each dot does not change its size as it moves, which is unlikely to happen in the real situation. In Condition 7, all the dots have the same size in the initial frame. As they start to move, they change the size together at the same rate, regardless of the distance to the singular point, which is unlikely to happen in the real situation. In Condition 8, the dots are arranged to have the size gradient to imply the 3D depth structure. Each dot changes its size as it moves so that its diameter is proportional to the distance from the singular point. This is closest to the real situation where an observer moves in a 3D environment, which has the 3D depth structure.