Pupil reflexes generate the peripheral drift illusion due to ON/OFF motion responses

In the peripheral drift illusion, a static pattern containing asymmetrical spatial luminance gradients can appear to move at its onset and then whenever the viewer moves their eyes or blinks. Recent evidence indicates that the illusion is due to changes in retinal luminance created by pupillary reflexes. Can computational models of human cortical motion sensing account for the illusory motion? An implementation of the Adelson-Bergen motion-energy models was unable to account for the illusion. However, a variant of the model incorporating half-wave rectification (half-squaring) applied to the response of the model's direction-selective sensors can account for the illusion. Half-squaring creates parallel ON and OFF channels. The output of the OFF channel was found to signal motion in the direction of the illusion whereas the ON channel signalled the opposite direction. If the two channels' outputs were combined in a way that is biased in favour of the OFF channel, the net output accounted for the illusion. A bias in favour of OFF responses is supported by evidence from physiological and psychophysical studies of human vision.