From a study by Grahn JA and Brett M:
When we listen to rhythm, we often move spontaneously to the beat. This movement may result from processing of the beat by motor areas. Previous studies have shown that several motor areas respond when attending to rhythms. Here we investigate whether specific motor regions respond to beat in rhythm. We predicted that the basal ganglia and supplementary motor area (SMA) would respond in the presence of a regular beat. To establish what rhythm properties induce a beat, we asked subjects to reproduce different types of rhythmic sequences. Improved reproduction was observed for one rhythm type, which had integer ratio relationships between its intervals and regular perceptual accents. [emphasis added] A subsequent functional magnetic resonance imaging study found that these rhythms also elicited higher activity in the basal ganglia and SMA. This finding was consistent across different levels of musical training, although musicians showed activation increases unrelated to rhythm type in the premotor cortex, cerebellum, and SMAs (pre-SMA and SMA). [emphasis added] We conclude that, in addition to their role in movement production, the basal ganglia and SMAs may mediate beat perception.
This study confirms the results of other studies showing that in musicians, significantly larger areas of the brain are activated when they hear music than in nonmusicians. One interesting fact about the musicians’ brains is that the premotor cortex was activated, which may indicate a “readiness to play”. In any case, the basal ganglia and SMA do seem to be involved in beat perception, but beat perception works best when the rhythm is regular and accented. Just another argument for using the metronome!
See the Neuroanatomy page for an explanation of what the various affected parts of the brain do.