This animation shows two rotating "wheels." If you look directly at the wheel on the left, it appears to rotate counterclockwise, as does the wheel on the right. However, focus your eyes on the right wheel while paying attention to the left. Keep your attention on the left-hand wheel while moving your gaze toward or away from it. Does its direction of rotation appear to change? For most observers, the wheel on the right always appears to rotate clockwise, while the wheel on the left changes depending on whether it is in central or peripheral vision.
This illusion builds on two different mechanisms that humans have for detecting visual motion. One (the first-order motion system) responds to the momentary local brightening or darkening that an object makes as it passes over a particular location. Another (which I call the position-defined system) explicitly tracks changes in position of objects.
Moving objects produce both first-order motion signals and position changes, and the two sources of information are normally in agreement. This stimulus is constructed so that in the left-hand wheel, the flicker motion is in the opposite direction to the position-defined motion. When the two sources of information are in disagreement, what you see depends on which provides the stronger signal.
When in central vision, the position-defined movement clearly wins. However, in peripheral vision, and under conditions of crowding (five elements instead of one) the position-tracking mechanism can no longer track distinct objects. In contrast, the first-order motion system is happy to add together the signals from all five objects. In peripheral vision, first-order motion wins out, which causes the change in motion as you took toward or away from the wheel.
This illusion suggests that position-defined motion is crucial for explaining our perception. However, the inner workings of the first-order motion system are well understood, while position-tracking motion is less well understood. In my research I used this illusion to probe which conditions help or hinder each motion system, and how each contributes to the perception of motion.