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

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.