High phi as a probe of global motion processing

Abstract

In 2013 Wexler and his colleagues demonstrated a visually remarkable phenomenon they called high phi. When a rotating motion pattern (the inducer) is abruptly replaced by a non-coherent motion, an illusory jump is perceived. The illusory jump is characterized by its large amplitude, its brevity, and its direction which depends on the inducer duration (short inducer durations leading to jumps in the same direction as the inducer, while longer durations leading to jumps in the opposite direction). Here, across a series of experiments, we investigated the potential relationship of this effect with 3D global motion (optic flow) processing. We first established that the high-phi effect works with random dots (smooth textures were used in the original demonstration), and that it works with radial motion, a different form of global motion or optic flow. Next, we investigated the global nature of the phenomenon. In a first series of experiments, we established that the high-phi effect is maintained when only a small part of the stimulus underwent inducing motion (2 opposite sectors out of 16). In a second series, by removing a sector of the inducing stimulus and using a probe to test for a high-phi effect in the empty region we established that the effect occurs in the empty, unstimulated region. These results demonstrated the global nature of the high-phi effect. However, in a final experiment we demonstrated that when inducing motion is spatially distributed, the threshold inducer duration at which the effect switches from a positive to a negative jump shifts to longer durations indicating that the perceived jump direction depends on the motion duration in a locality. These results suggested that the high-phi effect is not due to global motion processing but rather it results from earlier, local effects that lead to global effects.