Rhythm and motor function are intrinsically linked to each other and to music, but evidence on the rhythm-motor interplay during music training, and the brain- related mechanisms, is scarce. Here, we asked if rhythm skills play a role in the fine-motor improvements resulting from music training, and which brain regions would be implicated. We conducted a longitudinal study in natural school settings and compared the effects of a 6-month music training to those of a sports training and to a passive control. Fifty-seven third graders were pseudorandomly assigned to music (n = 21), sports (n = 18), and control (n = 18) groups. They completed a structural scan, and rhythm and fine-motor tasks before and after training. Better pre-training rhythm perception was related to less gray matter volume in regions of the cerebellum, fusiform gyrus, supramarginal gyrus, ventral diencephalon, inferior/middle temporal gyri, and superior frontal gyrus and motor area. Music training improved motor performance, and greater improvements were associated with better pre-training rhythm discrimination. Music training led to a significant loss of gray matter volume in the left cerebellum and fusiform gyrus cluster, and volume loss correlated to higher motor gains. No such effects were found for sports and control groups. Summing up, children with fine-tuned rhythm perception are more prone to improve fine-motor skills through music training, and this rhythm-motor interplay is to some extent handled by the left cerebellum and fusiform gyrus. These findings have implications for models of music-related plasticity and rhythm cognition, and for educational/clinical programs targeting motor function.

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