A coupled oscillator model predicts the effect of neuromodulation and a novel human tempo-matching bias.

Humans are known to exhibit endogenous neural oscillations in response to rhythmic stimuli that are phase-locked and frequency matched to those stimuli, a process known as entrainment. Yet, whether entrainment, as measured by electrophysiological recordings, reflects actual processing of rhythms or merely a reflection of the periodic nature of the stimulus is debated. Prior evidence for entrainment as a perceptual phenomenon comes from studies requiring subjects to listen to, compare sequentially, or detect features in rhythmic stimuli. However, one paradigm so far not used is one where subjects must listen to two simultaneous rhythms at different frequencies and adjust them to match. Here, human participants performed this task during EEG recordings (experiment 1), demonstrating spectral peaks at both tempo frequencies at frontocentral electrodes that shifted into alignment over the course of each trial. Behaviorally, participants tended to anchor the matched tempo to the starting comparison frequency, such that they underestimated the tempo for slower initial conditions and overestimated for faster initial conditions. A model of phase-coupled oscillators, in which both tempos were pulled toward one another, replicated both effects. This model further predicted that by enhancing the coupling strength of the constant tempo oscillator, both bias effects could be reduced. To test this, a second group of subjects performed the task while receiving 2 Hz transcranial alternating current stimulation (tACS) to the frontocentral region. Consistent with model predictions, tACS attenuated both behavioral effects, particularly for initially slower conditions. These results support entrainment as an endogenous process that mediates beat perception.NEW & NOTEWORTHY This work proposes how humans perceive the difference between two simultaneously presented tempos and bring them into perceived synchrony. EEG data provide evidence of entrainment to both tempos that move into alignment, and transcranial alternating current stimulation (tACS) data provide causal evidence that strengthening one tempo improves performance.