Neural representation of a rhythm depends on its interval ratio

Rhythm is determined solely by the relationship between the time intervals of a series of events. Psychological studies have proposed two types of rhy thm representation depending on the interval ratio of the rhythm: metrical and nonmetrical representation for rhythms formed with small integer ratios and noninteger ratios, respectively. We used functional magnetic resonance imaging to test whether there are two neural representations of rhythm dep ending on the interval ratio. The subjects performed a short-term memory ta sk for a seven-tone rhythm sequence, which was formed with 1:2:4, 1:2:3, or 1:2.5: 3.5 ratios. The brain activities during the memory delay period wer e measured and compared with those during the retention of a control tone s equence, which had constant intertone intervals. The results showed two pat terns of brain activations; the left premotor and parietal areas and right cerebellar anterior lobe were active for 1:2:4 and 1:2:3 rhythms, whereas t he right prefrontal, premotor, and parietal areas together with the bilater al cerebellar posterior lobe were active for 1:2.5:3.5 rhythm. Analysis on individual subjects revealed that these activation patterns depended on the ratio of the rhythms that were produced by the subjects rather than the ra tio of the presented rhythms, suggesting that the observed activations refl ected the internal representation of rhythm. These results suggested that t here are two neural representations for rhythm depending on the interval ra tio, which correspond to metrical and nonmetrical representations.