The double-drift illusion is dominated by the first-order motion energy of the internal texture

In the double drift (also infinite regress and curve ball) illusion, a Gabor patch moves in one direction (external motion) while its internal texture moves orthogonally to the external motion. Instead of appearing to move in its physical direction, the Gabor patch appears to move in a direction midway between the external and internal motions. However, because of the effect of the Gaussian aperture on the carrier, the texture’s first-order motion also moves in the illusory direction. It is not yet known whether the illusory motion is produced by a vector combination of two motion sources or simply tracks the apertured, first-order motion. Here, we test whether the illusion is driven by first-order motion or a combination of motion sources. To overcome the aperture problem, we used 1/f noise as the texture that moved back and forth along a path at 12° eccentricity. We varied the angular difference between the internal and external direction from 0° to 90° in steps of 15°. Pilot observations revealed that larger angle differences produced complex, non-linear percepts. Therefore, participants had two response options. If they perceived a linear trajectory, they matched its angle with a rotating bar. If they perceived a non-linear trajectory, participants pressed a key and moved on. For smaller angle differences (<45°), the illusory motion closely followed the direction of the internal motion. As the angle increased, the responses increasingly tended toward the direction of the external motion. These findings suggest that the illusion is dominated by first-order motion up to the 45° direction that characterizes the typical double drift Gabor stimuli. Beyond that, the internal and external motions begin to interact and the percepts become increasingly non-linear.