Electrophysiological activity associated with a cross-modal anapaest rhythm: evidence for the vestibular syncopation hypothesis

We report an experiment that tested the vestibular syncopation rhythm hypothesis, which holds that the rhythmic effect of syncopation is a form of vestibular reflexive/automated response to a postural perturbation, for example during locomotion. Electrophysiological signals were recorded from the cerebral cortex and cerebellum during processing of rhythmic sequences in a sample of experienced participants. Recordings were made using four different stimulus modalities, auditory, axial, vestibular and visual, under different rhythmic timing conditions, irregular, regular and syncopated/uncertain. Brain current activity was measured using a model with10-dipole source regions of interest in each of the participants, each modality, each timing condition, and for each beat within the bar of the rhythm. The cross-modal spectral power in the frontal electroencephalogram (EEG) and the cerebellar electrocerebellogram (ECeG) was also analysed. The results show that the brain activity increases from the irregular to the regular and then from the regular to the uncertain timing conditions. However, the vestibular modality induces the greatest total brain activity across the regions of interest and exhibits the highest sensitivity to the interaction of beat structure with the timing conditions in both source currents and spectral power. These data provide further evidence to support the primal role of the vestibular system in human processing of rhythm.