Abstract: PURPOSE: The perceived position of a visual object could be deviated by the presence of motion signals, a phenomenon known as illusory motion-induced position shift (MIPS). While the prominent object-tracking model casts illusory MIPS as the result of optimal integration of the positional and the motion information, which are weighted differently by the visual system based on their relative uncertainty, it remains unknown how sensory uncertainty might affect such integration. Here, we investigated this issue by examining how the magnitude of illusory MIPS varied as a function of the positional and motion uncertainty.
METHODS: To enable effective and independent manipulation of the positional and motion uncertainty of the visual object, we modified the classic double-drift illusion that has been shown to give rise to compelling illusory MIPS. The visual stimulus was a random dot kinematogram (RDK) containing presented within a circular aperture that was moving on a tilted path (45° or 135°). The directions of the individual moving dots conformed to a von Mises distribution whose mean (internal motion) was set orthogonal to the external aperture path. To measure the magnitude of MIPS, a psychophysical staircase procedure was employed to find the point of subjective verticality of the external aperture path. Critically, the uncertainty of the RDK motion was manipulated by changing the variance of the distribution, while the positional uncertainty was manipulated by changing the dot density of the aperture. According to the object-tracking model, we predicted stronger illusory MIPS effects for lower internal motion uncertainty (smaller variance of the von Mises distribution) and higher external positional uncertainty (lower dot density).
RESULTS: Consistent with our predictions, the perceived tilt of the external aperture path deviated towards the internal RDK motion, as was previously reported in the classic double-drift illusion. More importantly, we found that the magnitude of MIPS was constrained by the positional and motion uncertainty. The illusory effect was significantly weakened by the increased variance of the von Mises distribution, yet was strengthened as the dot density increased.
CONCLUSIONS: Our findings showed that information with higher certainty plays a dominant role in the integration process, which generates coherent perceptual estimates that enable the tracking of visual objects.

Key words: Motion-induced position shift, Sensory uncertainty, Double-drift illusion, Object-tracking model