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 atthe same constant speed on a stripey background are seen as alternatelyaccelerating and decelerating like footsteps. The cortical mechanisms thatgive rise to footsteps and similar illusions remain to be fully understoodand may reveal important neural computations. Using an implementationof 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 modeloutputs. Third, inferring from the successful model, the perceptual roleof multi-scale spatio-temporal channels. We developed a 2-DimensionalMotion Detector implementation adding a global (single value) frame-by-framedynamic readout to quantify continuous and oscillating responsecomponents. We confirmed that an expected signature oscillatory motionresponse corresponded to the footsteps illusion, demonstrating that itsamplitude varied according to empirically measured illusion strength.We showed that with a global readout, the inherent pattern and contrastdependence of correlation detectors is sufficient to reproduce the surprisingperceptual illusion. This evidence suggests spacetime correlation may bea fundamental sensory computation across species, with complementaryfiltering and global pooling operations adapted for various complex phenomena