Polyrhythms in the Brain: Metrical Priming, Acoustic Balance, and Perceptual Biases.

In polyrhythms, two pulse trains at different rates are presented simultaneously, yet listeners perceive only one as the underlying beat. This makes polyrhythms ideal for studying neural correlates of divergent percepts from identical stimuli. Across two electroencephalographic studies, we distinguished acoustic balance from perceptual ambiguity by designing stimuli that controlled for acoustic imbalances within the power spectrum and perceptual biases in polyrhythm beat perception. Using frequency tagging, we assessed metrical priming effects in 3:4 (Study 1: perceptual ambiguity) and 2:3 (Study 2: acoustic balance) polyrhythms under three conditions: no priming, fast-pulse priming, and slow-pulse priming. In Study 1, priming the 3-beat slightly increased neural activity at its periodicity. In Study 2, participants tapped the perceived beat after each trial, enabling trial classification by subjective perception. This revealed close correspondence between perceived beat and neural activity, an effect obscured in the priming-based analysis. Tapping responses revealed differences in priming success across conditions and individuals, highlighting the need for trial-by-trial measures of perceived beat in polyrhythm studies assessing endogenous contributions to beat perception. Together, the two studies show that polyrhythm perception is influenced by metrical priming at both behavioral and neural levels, but is also constrained by perceptual biases.