Memory-paced tapping to auditory rhythms: Effects of rate, speech and motor-engagement

Humans have a near-automatic tendency to entrain their motor actions to rhythms in the environment. Entrainment is hypothesized to play an important role in processing naturalistic stimuli, such as speech and music, which have intrinsically rhythmic properties. Here we studied two facets of entraining ones rhythmic motor actions to an external stimulus: (1) synchronized finger tapping to auditory rhythmic stimuli, and (2) memory-paced reproduction of a previously heard rhythm. Using modifications of the Synchronization-Continuation tapping paradigm, we studied how these two rhythmic behaviours were affected by different stimulus and task features. We tested synchronization and memory-paced tapping for a broad range of rates, from sub-second to supra-second, both for isochronous tone-sequences and for rhythmic speech stimuli (counting from one to ten), which are more ecological yet less strictly isochronous. We also asked what role motor engagement plays in forming a stable internal representation for rhythms and guiding memory-paced tapping. Results show that individuals can flexibly synchronize their motor actions to a very broad range of rhythms. However, this flexibility does not extend to memory-paced tapping, which is accurate only in a narrower range of rates, around ~1.5Hz. This pattern suggests that intrinsic rhythmic-defaults in the auditory/motor systems influence the internal representation of rhythms, in the absence of an external pace-maker. Interestingly, memory-paced tapping for speech rhythms and simple tones shared similar optimal rates, although with reduced accuracy, suggesting that internal constraints on rhythmic entrainment may generalize to more ecological stimuli. Last, active synchronization led to more accurate memory-paced tapping vs. passive listening, which emphasizes the importance of action-perception interactions in forming stable entrainment to external rhythms.