Self-paced alternating nose-poke movements in rats did not spontaneously synchronize with external rhythmic stimuli

Humans can synchronize their movements with external rhythms. This ability has also been observed in other vocal learning species, supporting the “vocal learning and rhythmic synchronization hypothesis,” which posits that only high vocal learning species can flexibly synchronize their movements with external rhythms. Meanwhile, studies reporting rhythmic synchronization in limited/non-vocal learning species have been increasing. In rats, considered to be not a vocal learning species, a previous study reported that peak jerks of head movements spontaneously synchronized with musical beats. However, those movements were small and lacked sustainment. To investigate whether spontaneous rhythmic synchronization could be robustly observed in rats, we utilized a paradigm commonly applied to other species. In the experiment, rats were trained to poke their noses into two holes alternately. During testing, isochronous rhythmic stimuli at various tempi were played to determine if the rats’ movements would spontaneously synchronize with the rhythm. Results indicated that while rhythmic stimuli significantly altered the rats’ response tempo compared to no-stimulus conditions, no actual spontaneous rhythmic synchronization was observed in our experimental setup; there was no increase in responses at specific timings within a rhythm cycle, nor was there a tendency for changes in response tempo to correspond to changes in stimulus tempo. Therefore, our findings do not contradict the vocal learning and rhythmic synchronization hypothesis. It is suggested that spontaneous rhythmic synchronization in rats is not a robust phenomenon observable under experimental paradigms used in other species; rather, it remains an extremely difficult behavior to capture.