Two-stroke Apparent Motion


This animation involves a repeating cycle of two different pattern fr ames (hence 'two-stroke'), with a blank grey frame after the second frame. The motorcycle repeatedly moves forward (pattern frames 1-2) and then backward (frames 2-1), but the blank frame intervenes between each backward shift from frame 2 to frame 1. Use the movie's playback control to examine individual frames.
This sequence of displacements would normally lead to the perception of to-and-fro motion. But the effect of the blank frame is to reverse the direction of perceived motion, so instead of appearing to move backwards from frames 2-1, the motorcycle still appears to move forward. Consequently the impression is of continuous forward motion. This kind of display was first described by Mather (2006).
The illusion seen in two-stroke apparent motion can be explained by responses in specialised neurons in the visual system which respond selectively to motion direction and stimulus contrast. For a detailed account of the explanation for this effect and for a related effect (four-stroke apparent motion), see Mather (2013).
Challinor, K., & Mather, G. (2010). A motion-energy model predicts the direction discrimination and MAE duration of two-stroke apparent motion at high and low retinal illuminance. Vision Research, 50, 1109-16. PDF
Mather, G. (2006). Two-stroke: A new illusion of visual motion based on the time course of neural responses in the human visual system. Vision Research, 46, 2015– 2018. PDF
Mather, G. (2013). Two-stroke apparent motion. In: A. Shapiro & D. Todorovic (Eds.) Oxford Compendium of Visual Illusions. OUP, Oxford. PDF
Shioiri, S., & Cavanagh, P. (1990). ISI produces reverse apparent motion. Vision Research, 30, 757–768.