Illusory speeding-up and slowing-down of objects moving at constant speed emerges from natural motion detection algorithms.

Abstract

The footsteps illusion is a perceptual illusion in which two bars moving at the same constant speed on a stripey background are seen as alternately accelerating and decelerating like footsteps. The cortical mechanisms that give rise to footsteps and similar illusions remain to be fully understood and may reveal important neural computations. Using an implementation of the biologically inspired correlational model of motion detection, the 2-Dimensional Motion Detector, this study had three aims. First, reproducing perceptual speed oscillations in model simulations. Second, mapping empirical reports of multiple illusion configurations onto model outputs. Third, inferring from the successful model, the perceptual role of multi-scale spatio-temporal channels. We developed a 2-Dimensional Motion Detector implementation adding a global (single value) frame-by-frame dynamic readout to quantify continuous and oscillating response components. We confirmed that an expected signature oscillatory motion response corresponded to the footsteps illusion, demonstrating that its amplitude varied according to empirically measured illusion strength. We showed that with a global readout, the inherent pattern and contrast dependence of correlation detectors is sufficient to reproduce the surprising perceptual illusion. This evidence suggests spacetime correlation may be a fundamental sensory computation across species, with complementary filtering and global pooling operations adapted for various complex phenomena.