Segregated audio-tactile events destabilize the bimanual coordination of distinct rhythms

We examined to what extent the CNS can efficiently bind together the perception of non-coincident multimodal events with coordinated movements. To do so, we selected a bimanual coordination with left-right asymmetry, which was, achieving 3:2 polyrhythmic movements. We asked participants to synchronize left and right fingers' movements to events presented, respectively, to the left and to the right side. In two segregated conditions, sound was presented on one side at one frequency while touch was presented on the other side at the other frequency; thus, the left and right rhythms were paced via a distinct sensory modality. In the three control conditions, the stimuli on both sides were presented via the same sensory modality: sound, touch, or coincident sound and touch. Our aim was to contrast two opposing hypotheses: Sensory segregated pacing (1) stabilizes polyrhythmic coordination because it favors the distinction between the fast and the slow rhythm versus (2) destabilizes polyrhythmic coordination because it introduces a very strong asymmetry. We performed a parametric study in which the ability to maintain the polyrhythmic coordination was explored over a broad range of pacing rates. We found that switches from the polyrhythmic coordination to an isofrequency pattern took place only in the sensory segregated conditions, at the highest frequencies. Moreover, transitions were preceded by an increase in the variability of the synchronization of movement to stimuli. We therefore propose that the destabilization originating from the asymmetry between sensory modalities overrides the assumed segregation effect. We discuss the possible neuronal underpinnings of this failure of binding of movement to segregated sound and touch.