Flash Grab

When a stimulus moves back and forth along a trajectory, the apparent length of the trajectory is shortened (Sinico, Parovel, Casco, & Anstis, 2009). This is likely related to the extrapolation of the seen position of moving targets (Nijhawan, Nature, 1994). When flashes are presented at the ends of the trajectory, they too are shifted to appear at the trajectory end point — the Flash Grab. Here are two examples, one with expanding and contracting motion (Anstis & Cavanagh, 2017 PDF), the other with rotary motion (Cavanagh & Anstis, 2013 PDF).

Expanding and contracting squares

Here the red and blue outline squares have the same size and their borders have the same width. Try to place your finger on the bottom right corner of the red square to convince yourself that its location matches that of the blue corner. Subjects judged the blue square to be about twice the size of the red one.

Rocking Lines

Here the red and green bars are always vertical and parallel but when the background motion fades in, the bars are pulled along by the motion and appear strongly tilted.