Auditory self-motion illusions ("circular vection") can be facilitated by vibrations and the potential for actual motion

It has long been known that sound fields rotating around a stationary, blindfolded observer can elicit self-motion illusions ("circular vection") in 20--60% of participants. Here, we investigated whether auditory circular vection might depend on whether participants sense and know that actual motion is possible or impossible. Although participants in auditory vection studies are often seated on moveable seats to suspend the disbelief of self-motion, it has never been investigated whether this does, in fact, facilitate vection. To this end, participants were seated on a hammock chair with their feet either on solid ground ("movement impossible" condition) or suspended ("movement possible" condition) while listening to individualized binaural recordings of two sound sources rotating synchronously at 60°/s. In addition, hardly noticeable vibrations were applied in half of the trials. Auditory circular vection was elicited in 8/16 participants. For those, adding vibrations enhanced vection in all dependent measures. Not touching solid ground increased the intensity of self-motion and the feeling of actually rotating in the physical room. Vection onset latency and the percentage of trials where vection was elicited were only marginally significantly (p<.10) affected, though. Together, this suggests that auditory self-motion illusions can be stronger when one senses and knows that physical motion might, in fact, be possible (even though participants always remained stationary). Furthermore, there was a benefit both of adding vibrations and having one's feet suspended. These results have important implications both for our theoretical understanding of self-motion perception and for the applied field of self-motion simulations, where both vibrations and the cognitive/perceptual framework that actual motion is possible can typically be provided at minimal cost and effort.