Endogenous Expectations for Sequence Continuation after Auditory Beat Accelerations and Decelerations Revealed by P3a...

People commonly synchronize taps to rhythmic sounds and can continue tapping after the sounds stop, indicating that time intervals between sounds can be internalized. Here, we investigate what happens in the brain after simply listening to auditory beats in order to understand more about the automatic internalization of temporal intervals without tapping. Electroencephalograms were recorded while musicians attended to accelerating, decelerating, or steady click sequences. Evoked responses and induced beta power modulations (13-30 Hz) were examined for one beat following the last physical beat of each sequence (termed the silent beat) and compared to responses obtained during physical beats near the sequence endings. In response to the silent beat, P3a was observed with the largest amplitude occurring after accelerations and the smallest after decelerations. Late beta power modulations were also found after the silent beat, and the magnitude of the beta-power suppressions was significantly correlated with the concurrent P3a amplitudes. In contrast, physical beats elicited P2 responses and early beta suppressions, likely reflecting a combination of stimulus-related processing and temporal prediction. These results suggest that the activities observed after the silent beat were not produced via sustained entrainment after the physical beats, but via automatically-formed expectation for an additional beat. Therefore, beta modulations may be generated endogenously by expectation violation, while P3a amplitudes may relate to strength of expectation, with acceleration endings causing the strongest expectations for sequence continuation.