Entrainment Echoes in the Cerebellum

Abstract

Historically, researchers have considered the cerebellum a coordinator of motor programs that ensures precise timing of movements and their adaptation to external events1. However, it has become increasingly clear that this role is not restricted to the motor system2. Rather, the cerebellum seems to play an important role in temporal prediction in general, as shown in its involvement in multiple functions that rely on precise event timing3. Although previous work suggested that the cerebellum exclusively predicts the interval between two events4, rather than tracking a global rhythm, it is also active when a rhythmic stimulus changes in rate5. The latter finding is in line with a cerebellar role in speech processing6 that entails frequent rate changes7. Neural mechanisms underlying the cerebellums involvement in speech processing, however, remain poorly understood. Moreover, there is a lack of studies contrasting speech and non-speech stimuli to establish speech-specificity of the observed effects8. In a re-analysis of magnetoencephalography (MEG) data9, we found that activity in the cerebellum aligned to rhythmic sequences of noise-vocoded speech, irrespective of its intelligibility. We then tested whether these "entrained" responses persist, and how they interact with other brain regions, when the rhythmic stimulus stopped and temporal predictions had to be updated. We found that only intelligible speech produced rhythmic responses in the cerebellum that outlasted the stimulus. During this "entrainment echo", but not during rhythmic speech itself, cerebellar activity was coupled with that in the left inferior frontal gyrus (IFG), and specifically at rates corresponding to the preceding stimulus rhythm. This finding represents unprecedented evidence for specific cerebellum-driven temporal predictions in speech processing and their relay to cortical regions.