Figure 1. Experimental design.

In an adaptation period, observers were exposed to apparent visual motion for 18 min, during which two white circles placed side by side were presented in alternation. The onset of the left circle was accompanied by tone A1 and the right circle by tone A2. In the test phase, a white circle was presented twice. The circle was perceived as lateral motion in the same direction as the previously presented apparent motion when the onset of the circle was synchronized to tones of alternating frequencies. When the onset of the first circle was synchronized with tone A1 and the second circle with tone A2, the circle appeared to move from left to right. The strength of this illusory motion was quantified by a motion-nulling procedure.