Investigating internal representation of musical rhythm using fMRI and frequency-tagging

Battal, Ceren [UCL] Lenc, Tomas [UCL] Nozaradan, Sylvie [UCL]

When listening to musical rhythm, humans perceive and spontaneously move to a periodic pulse-like meter. It has been proposed that internal representation of a meter is induced by simple rhythms where sounds are arranged in a way that emphasizes a periodic pulse, compared to complex rhythms with a less regular arrangement of sounds. Previous neuroimaging studies revealed a functional sensory-motor network preferentially activated by simple rhythms, suggesting its special role in meter processing. However, meter processing might not solely depend on the complexity of the rhythmic input, but also on context. To test this, we used functional MRI to characterize brain activity of participants as they listened to stimuli where simple and complex rhythmic sequences were presented either continuously or separated by silent gaps (i.e., block design). To achieve high sensitivity, we analyzed the data using a model-free frequency-tagging approach. The whole-brain conventional analysis of the block design revealed activity in the premotor and supplementary motor areas similar for both simple and complex rhythms. Simple rhythms evoked similar activity to complex rhythms as shown by the model-free analysis of both continuous and block designs. This lack of difference was not due to limited sensitivity of the model-free approach, as (i) the same approach yielded powerful results in a control experiment contrasting low-level acoustic features (pitch), (ii) this null result was also confirmed by a conventional analysis. Importantly, these results also aligned well with a behavioral experiment where participants showed uniform response while tapping their finger along with the same simple and complex rhythms. Together, these findings validate the frequency-tagging approach to analyze auditory responses captured with fMRI. Our results also challenge the view that complex rhythms fail to induce internal representation of meter.