Perceptual entrainment of individually unambiguous motions.

When two bistable spheres defined by dots oscillating around a common axis (coaxial condition) are placed near each other, their motions become entrained such that they appear to rotate in the same direction. When the dots in the two spheres oscillate around parallel axes (non-coaxial condition), entrainment is much reduced, suggesting that this phenomenon is driven by interactions between the global stimulus representations rather than between the locally ambiguous motion signals. In order to examine where these interactions may be arising, we created highly unambiguous spheres by introducing size and contrast cues on the dots. We find that coupling is strong for the two nearly unambiguous spheres rotating in opposite directions of motion in the coaxial condition but, importantly, that this effect is subject to attentional control. Forcing observers to withdraw their attention from the rotating spheres by performing a demanding task at fixation significantly reduces the amount of coupling, arguing against mediation by low-level interactions between the spheres. These results reveal a disjunct between the local motion signals that are unambiguous and the global percept, implicating a process that requires active suppression of reliable visual information. We propose that this is a dynamic process that requires feedback from higher levels to bind opposite motions into a unified directional percept.