In this demo there is an array of gratings each its own diagonal motion. Look at the centre of the figure on the right, and without moving your eyes, pay attention to the combination of centre, upper and lower patches. You might be able to see that they form a vertical zig-zag pattern moving horizontally. Now pay attention to the combination of left and right and centre patches. Now you might see a horizontal /\/ zig-zag pattern, moving vertically. The direction of motion may now appear to switch from horizontal to vertical. Practice alternating your attention every five seconds, and see how easily you can switch between horizontal and vertical motion.

 

I have found that observers can voluntarily control their perceived direction of motion by alternating their attention in response to a periodically switching cue (Freeman & Driver, 2008, Journal of Vision).

 

This illusion demonstrates that we can integrate different parts of a visual scene selectively. This integration is entirely under voluntary control, and results in a striking transformation in visual appearance