An Entrainment Oscillator Mechanism underlies Human Beat Matching Performance

Humans possess an innate ability to effortlessly entrain to auditory rhythms, which previous theories have linked to the supplementary motor area (SMA). Yet, whether entrainment, as measured by electrophysiological (EEG) recordings reflects actual processing of rhythms or merely a reflection of their periodical nature, is unknown. Here we conducted tested human participants on a novel beat matching task, in which they listened to two simultaneous tempos and asked to modulate the rate of a variable tempo (1.67-2.34 Hz between trials) to match a constant target tempo (2 Hz). EEG recordings exhibited entrainment to both 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; further, tempo judgments were shifted away from the variable tempo on the previous trial. A model of phase-coupled oscillators, in which both tempos were pulled towards one another, replicated both effects. This model further predicted that by strengthening the coupling strength of the constant tempo oscillator, both bias effects could be eliminated. To test this, a second group of subjects underwent transcranial alternating current stimulation (tACS) to the SMA phase-locked to the target tempo at 2 Hz. Consistent with model predictions, tACS attenuated both behavioral effects. Overall, these results provide causal support to the role of the SMA in entrainment during human beat matching.