Abstract
The double-drift illusion, also known as the infinite regress and curveball illusion, is a particularly dramatic example of a motion-induced position shift; its combination of internal and external motion vectors of a drifting gabor produces large misperceptions of both position and direction of motion. Here, we measure the size of the illusion at 7 internal speeds combined with 6 external speeds in order to determine how internal and external velocities influence the perceived direction of the illusion. While fixating on a black fixation point, participants were instructed to report the perceived direction of the gabor. A separate VonMises distribution was fit to the distribution of directions for each of the 42 internal/external speed combinations for each session and participant individually. Our results confirm previous work (Cavanagh & Tse, 2019; Tse & Hsieh, 2006), showing that the magnitude of the illusion, as measured by the displacement of the perceived trajectory in the direction of internal motion, increases as the speed of the internal motion increases, and decreases as the speed of the external motion increases. Moreover, our results also reveal that the best fitting weights in the combination of the internal and external motions uniquely depends on the speed of the external motion. We compare vector combination models (AIC=61.9) and optimal object tracking models (AIC=268.8) and find that the former does a better job accounting for our results.