Something in the Sway: Effects of the Shepard-Risset Glissando on Postural Activity and Vection

This study investigated claims of disrupted equilibrium when listening to the Shepard-Risset glissando (which creates an auditory illusion of perpetually ascending/descending pitch). During each trial, 23 participants stood quietly on a force plate for 90 s with their eyes either open or closed (30 s pre-sound, 30 s of sound and 30 s post-sound). Their centre of foot pressure (CoP) was continuously recorded during the trial and a verbal measure of illusory self-motion (i.e., vection) was obtained directly afterwards. As expected, vection was stronger during Shepard-Risset glissandi than during white noise or phase-scrambled auditory control stimuli. Individual differences in auditorily evoked postural sway (observed during sound) were also found to predict the strength of this vection. Importantly, the patterns of sway induced by Shepard-Risset glissandi differed significantly from those during our auditory control stimuli - but only in terms of their temporal dynamics. Since significant sound type differences were not seen in terms of sway magnitude, this stresses the importance of investigating the temporal dynamics of sound-posture interactions.