Neural tracking of the musical beat is enhanced by low-frequency sounds

Abstract

Music makes us move, and using bass instruments to build the rhythmic foundations of music is especially effective at inducing people to dance to periodic pulse-like beats. Here, we show that this culturally widespread practice may exploit a neurophysiological mechanism whereby low-frequency sounds shape the neural representations of rhythmic input by boosting selective locking to the beat. Cortical activity was captured using electroencephalography (EEG) while participants listened to a regular rhythm or to a relatively complex syncopated rhythm conveyed either by low tones (130 Hz) or high tones (1236.8 Hz). We found that cortical activity at the frequency of the perceived beat is selectively enhanced compared with other frequencies in the EEG spectrum when rhythms are conveyed by bass sounds. This effect is unlikely to arise from early cochlear processes, as revealed by auditory physiological modeling, and was particularly pronounced for the complex rhythm requiring endogenous generation of the beat. The effect is likewise not attributable to differences in perceived loudness between low and high tones, as a control experiment manipulating sound intensity alone did not yield similar results. Finally, the privileged role of bass sounds is contingent on allocation of attentional resources to the temporal properties of the stimulus, as revealed by a further control experiment examining the role of a behavioral task. Together, our results provide a neurobiological basis for the convention of using bass instruments to carry the rhythmic foundations of music and to drive people to move to the beat.

Keywords: EEG; frequency tagging; low-frequency sound; rhythm; sensory-motor synchronization.

Fig. 1.

Spectra of the acoustic stimuli (processed through the cochlear model) and EEG responses (averaged across all channels and participants; n = 14; shaded regions indicate SEMs; see ref. 30). The waveform of one cycle for each rhythm (2.4-s duration) is depicted in black (Left) with the beat period indicated. The rhythms were continuously repeated to form 60-s sequences, and these sequences were presented eight times per condition. The cochlear model spectrum contains peaks at frequencies related to the beat (1.25 Hz; dark-gray vertical stripes) and meter (1.25 Hz/3, ×2, ×4; light-gray vertical stripes), and also at frequencies unrelated to the beat and meter. The EEG response includes peaks at the frequencies contained in the cochlear model output; however, the difference between the average amplitude of peaks at frequencies related vs. unrelated to the beat and meter is increased in the low-tone compared with high-tone conditions (see Relative Enhancement at Beat and Meter Frequencies and Fig. 2). Note the scaling difference in plots of EEG responses for unsyncopated and syncopated rhythms.

Fig. 2.

Grand average topographies (n = 14) of neural activity measured at meter-related (Left column) and meter-unrelated (Right column) frequencies for the unsyncopated and syncopated rhythm conveyed by low or high tones.

Fig. 3.

Effect of tone frequency on the selective enhancement of EEG activity at beat- and meter-related frequencies. Shown separately are z scores for the beat frequency (Top) and mean z scores for meter-related frequencies (Bottom) averaged across participants for the unsyncopated (Left) and syncopated (Right) rhythms. Error bars indicate SEMs (30). Asterisks indicate significant differences (P < 0.05). Responses from individual participants are shown as gray points linked by lines. The horizontal lines represent z score values obtained from the cochlear model. The low tone led to significant neural enhancement of the beat frequency in both rhythms. The low tone also elicited an enhanced EEG response at meter frequencies, but only in the syncopated rhythm. There was no significant modulation of meter-related responses by tone frequency for the unsyncopated rhythm.