Double Drift

In this illusion, a gabor patch moves in one direction while its internal texture moves in the orthogonal direction. The effect is based on the Infinite Regress illusion of Tse and Hsieh (Vision Research 2006) and the curveball illusion of Art Shapiro (Shapiro et al., PLoS ONE 2010). What is different here is that each time the gabor reverses direction, its internal motion reverses as well. This stabilizes the location and perceived path enabling a new range of measures.

Surprisingly, this robust perceptual illusion does not affect eye movements to the gabor (Lisi & Cavanagh, 2015 PDF). There is no other illusion, as far as we know, that has such markedly different effects on perception and saccades. More recent results show that the illusion is present for pointing (Lisi & Cavanagh, 2017 PDF), and memory-guided saccades (Massendari et al, 2018 PDF) indicating that illusion does not dissociate perception from all action, only from immediate saccades. Interestingly, the illusion is unaffected by attentional load (Haladjian & Cavanagh, 2018 PDF).

Beyond smooth pursuit

Also surprisingly, this perceptual illusion is maintained even if you track a fixation point that moves in tandem with the Gabor. This keeps the Gabor more or less stationary on the retina, eliminating its retinal motion and yet the illusion is unaffected (Cavanagh & Tse, 2019 PDF).

Follow the left hand fixation point with your eyes

While you are following the fixation point, try to notice the orientation of the moving gabor's path. You may find that it is still vertical — the illusory direction. It is actually moving obliquely to the left and the illusion you see while tracking has the same strength as the illusion with fixed gaze. This indicates that the combination between the external and internal motions must occur after the gabor's motion is recovered from the eye movement signals that drive the pursuit. The illusion occurs in world coordinates not retinal coordinates (the gabor envelope is not moving much on the retina if your tracking is accurate).

The captured flash

Fixate the dot on the left, judge the location of the two green flashes.

Here is a version generated by Matteo Lisi where he placed a brief green flash at each end of the trajectory. Just as in the flash grab, these flashes are also grabbed by the illusion and shifted to match the perceived locations of the endpoints of the trajectory. Judge whether the top flash is to the left or right of the bottom flash. It may appear to be slightly to the right, but it is actually to the left. This indicates that the illusion distorts the space around it.

Double-Drift Stimulus from 2019 Current Biology article

Fixate the left hand black dot

Video S1. Start the movie and fixate on the black dot at the left. The small patch on the right moves up and down vertically, however, it may appear tilted to the left. It is the internal motion of the patch that drives its perceived path to appear tilted leftward compared to its physically vertical path.