Directional Bias for Vertical Integration of Motion Trajectories: Investigation of an Ambiguous Bouncing-Bouncing Display

Here we report a new ambiguous continuous motion display, in which two objects appear at the diagonally opposite corners of an imaginary square, move along the diagonal axis toward each other, and after meeting in the center, shift their trajectories to the other two diagonal corners. This display can be seen as two objects' colliding and bouncing off each other, with two competing interpretations of trajectory configuration requiring either vertical or horizontal integration of trajectory segments. Despite the fact that both percepts are equally plausible, the current study revealed a perceptual preference toward a vertical integration interpretation. We compared this bias with the similar vertical bias in a bistable apparent motion quartet, which suggests that the directional anisotropy found here is quite a new, and distinct phenomenon in both its perceptual characteristics and underlying mechanism.