Perceptually plausible sounds facilitate visually induced self-motion perception (vection)

We examined whether and how sounds influence visually induced illusory self-motion (vection). Visual stimuli were presented for 40 s. They were made radially, expanding or contracting visual motion field and luminance-defined gratings drifting in a vertical or horizontal direction. Auditory stimuli were presented with the visual stimuli in most conditions; we employed sounds that increased or decreased in intensity, or ascended or descended in frequency. As a result, the sound which increased in intensity facilitated forward vection, and the sound which ascended/descended in frequency facilitated upward/downward vection. The perceptual plausibility of the sound for the corresponding self-motion seemed an important factor of enhancing vection.