• Background
  • Instructions
  • Illustration
  • Quiz


In motion perception, the correspondence problem refers to how the visual system knows if an object seen at Time 1 is the same object at Time 2. In most normal vision settings, we know that an object is the same because most objects do not go through rapid changes across time when they are in motion. Moreover, we may use eye movements to track that motion across time. But consider watching a flock of birds move across the sky. In this case, it may be difficult to track a particular bird, but we do not see some birds flying out of existence and others coming into existence. We see a pattern of birds flying.

A classic illustration of the correspondence problem is what is called the wagon wheel effect. In movies or television, perhaps you have see wheels that appear to be rotating backwards. The illusion is clearest in an old fashioned wagon wheel with clear distinct spokes. This illusion results from the fact that all forms of video are created from a sequence of still images with black periods in between. As each new frame of the video is presented, your eye/brain must match what occured in one frame to the next frame to determine the movement in the scene. In ambiguous situations, the eye/brain may make the wrong conclusion. In this activity, you can explore how the correspondence problem works to create the wagon wheel effect illusion.


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

On the Illustration tab, you can start, stop and alter the motion of squares to experience optic flow.


Below is a list of the ways that you can alter the illustration. The settings include the following:

Move: Start and stop the motion of the wheel. If the wheel is set to flicker it will only flicker when moving
Speed of Wheel: change how fast the wheel rotates in degree/each time the wheel is drawn.
Show Dot: select to show a red dot on the outside of the wheel (default: hide).
Show Flicker: flicker or not the wheel to make the intermittent nature of the stimuli clear (default: show flicker).
Size of Wheel: change the size of the wheel.
Number of Spokes: change the number of spokes in the wheel.


Pressing this button restores the settings to their default values.