Tempo-dependent selective enhancement of neural responses at the beat frequency can be explained by both an oscillator and an evoked model

Abstract

The synchronization of neural oscillations with an external regularity such as a musical beat has been regarded as an important mechanism for the brain to make sense of our auditory environment. Such synchronization is often quantified as phase locking of neural oscillations to a stimulus, but this method has been criticized for not differentiating between entrainment – the rate-dependent adjustment of an ongoing endogenous oscillation to an external regularity – and evoked neural responses to the rhythmic stimulus. Here, we aimed to differentiate between these two accounts by measuring EEG responses to non-isochronous rhythmic sequences played at five different rates. Behaviorally, participants shifted the perceived level of regularity depending on the tempo, towards the preferred beat rate (∼2 Hz). We found a similar shift in the EEG data, with strongest neural phase locking at the level of the note rate for slow tempi, and at the level of a hierarchical beat for faster tempi, independent of active attention to the sounds. While this pattern of results is in line with entrainment accounts of beat perception and could indeed be mimicked by an oscillator model, it was explained equally well using a model simulating evoked responses. An additional phase concentration metric of the EEG data fell in between the predictions of these two models. In conclusion, we show that neural responses to rhythm are selectively enhanced at the beat rate in a tempo-dependent manner, but that this selective neural enhancement can be explained by successive evoked responses as well as by assuming the presence of oscillatory entrainment.