In the double-drift illusion, the combination of the internal and external motion vectors produces large misperceptions of both position and direction of motion. Here, we investigate the role that speed plays in determining how these two sources of motion are combined to produce the double-drift illusion. To address this question, we measure the size of the illusion at seven internal speeds combined with six external speeds. We find that the illusion increases with increasing internal speed and decreases with increasing external speed. We model this by combining the external and internal vectors to produce the resulting, illusory direction (Tse & Hsieh, 2006). The relative effect of the two vectors is specified by a constant K in this model and the data reveal that K decreases linearly as external speed increases. This critical role of external speed in modulating the vector combination uncovers new details about how the visual system combines different sources of motion information to produce a global motion percept.
Effects of internal and external velocity on the perceived direction of the double-drift illusion