Perception of audio-visual synchrony is modulated by walking speed and step-cycle phase

Investigating sensory processes in active human observers is critical for a holistic understanding of perception. Recent research has demonstrated that locomotion can alter visual detection performance in a rhythmic manner, illustrating how a very frequent and natural behaviour can influence sensory performance. Here we extend this line of work to incorporate variations in walking speed, and test whether multi-sensory processing is impacted by the speed and phase of locomotion. Participants made audio-visual synchrony judgements while walking at two speeds over a range of stimulus onset asynchronies (SOAs). We find that sensitivity to multi-sensory synchrony decreases at slow walking speeds and is accompanied by an increase in reaction times, compared to when walking at a natural pace. A further analysis of the shortest SOAs was conducted to test whether subjective synchrony modulated over the step cycle. This revealed that synchrony judgements were quadratically modulated with perceived synchrony being higher in the swing phase of each step and lower when both feet were grounded during stance phase. Together, these results extend an earlier report that walking dynamically modulates visual sensitivity by contributing two new findings: first, that walking speed modulates perceived synchrony of audio-visual stimuli, and second, that modulations within the step-cycle extend to multisensory synchrony judgements which peak in the swing phase of each step.