Still moving: The double-drift illusion survives smooth pursuit.

Abstract

If a gabor pattern drifts in one direction while its internal texture drifts in the orthogonal direction, observers see a remarkable shift in its perceived direction when it is viewed in the periphery. The reported direction of the double-drift stimulus (also known as the infinite regress and curveball illusions) is some combination of the actual external motion of the gabor envelope and the internal motion of its texture (Tse & Hsieh, 2006). Here we find that if the observers track a fixation point that moves in tandem with the gabor, the illusion is undiminished. The pursuit of the moving fixation spot keeps the gabor roughly fixed at one location on the retina, cancelling its external motion, leaving only the internal motion. The gabor is seen to move in the world at roughly its actual speed as the motion of the eye is discounted at some point to recover velocities in world coordinates (e.g. Wallach, 1959). Our finding indicates that the combination of internal and external motion that produces the double drift illusion must happen after the eye movement signals have been factored into stimulus motions. We also test the double drift effect at various path lengths, durations, and speeds, with both mid-grey and black backgrounds, all with a static fixation. These results confirm that a simple vector combination of the two speeds alone accounts for virtually all the direction shifts on the grey background. On the black background, the illusion is eliminated. These results place constraints on where perceived spatial coordinates arise in the visual processing hierarchy to locations at or beyond where compensation for pursuit eye movements arise, specifically V3A, V6, MSTd, and VIP (e.g., Nau et al, 2018).