Figure 1.
Schematic explanation of ambiguous second-order motion displays, reproduced from Werkhoven et al. (12). (Left) A series of eight frames (f1, f2, … ) is shown in succession, each frame lasting 125 ms. Frame fn+2 is similar to frame fn but rotated clockwise by a 45° angle. (Right) Unfolding the annular stimulus, the horizontal axis represents the angular positions φ, and the vertical axis represents time. Two different textures (spatial frequencies) s and v are present in the odd frames, and the even frames only contain sectors with texture s. Phase is randomized in each frame, each sector. When frame fn and frame fn+1 are presented in succession, two opposing interpretations of the motion direction are possible, as indicated by the arrows: (i) homogeneous motion path: matches between sectors of identical texture s. In this case, motion will be perceived in the counter-clockwise direction; (ii) heterogeneous motion path (counter-clockwise): matches between sectors of textures s and v. In this case, motion will be perceived in the clockwise direction. Spatial frequency for texture s was 1.26 c/deg and for texture v was 0.83 c/deg. Outer radius was 3.00 deg. Inner radius was 1.50 deg.