Changing pitch induced visual motion illusion

We often associate moving objects and changing pitch, e.g., falling stones with descending, and launching rockets with ascending pitch, even when these sounds do not happen in the real-world. The reason for this is unknown. Here we report an illusion in which auditory stimuli with no apparent spatial and motion information [[1–3]] alter human visual motion perception. Subjects made a two alternative forced choice (upward (Vup) or downward (Vdown) visual motion perception) while presented with two superimposed, oppositely moving gratings (experiment 1), accompanied by either an ascending or a descending pitch of pure tone, or broad-band noise (Figure 1A). Gratings with ambiguous motion accompanied by ascending pitch were more likely to be perceived as an upward motion, those accompanied by descending pitch as a downward motion, whereas noise caused no directional bias.