Exploring Interbrain Synchronization Dynamics in Challenging Duo Piano Performances
by Viktor Müller1*, Anja Lender1,2, Dionysios Perdikis1,3, Walter Gruber4, and Ulman Lindenberger1,5,6
1Max Planck Institute for Human Development, Center for Lifespan Psychology, Berlin, Germany
2Centre for Cognitive Neuroscience, University of Salzburg, Salzburg, Austria
3Charité, Universitätsmedizin Berlin, Berlin, Germany
4Department of Physiological Psychology, University of Salzburg, Salzburg, Austria
5Max Planck UCL Centre for Computational Psychiatry and Ageing Research, London, England
6Max Planck UCL Centre for Computational Psychiatry and Ageing Research, Berlin, Germany
*Corresponding author: Dr. Viktor Müller, Center for Lifespan Psychology, Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, Germany. Email: vmueller@mpib-berlin.mpg.de
This user research article summarizes the publication: Lender, A., Perdikis, D., Gruber, W., Lindenberger, U., & Müller, V. (2023). Dynamics in interbrain synchronization while playing a piano duet. Ann NY Acad Sci., 1530, 124–137. https://doi.org/10.1111/nyas.15072
Introduction
Social interaction and coordination require interpersonal synchrony, and cognitive neuroscience suggests that this coordination often involves interbrain synchrony (IBS) and hyperbrain activity (Keller et al., 2014; Müller et al., 2021; Müller, 2022). However, the precise mechanisms linking IBS to coordinated behavior are not fully understood. This study investigates the role of IBS in musical duos, focusing on how IBS supports behavioral synchrony, particularly during challenging coordination periods, such as tempo changes. While shared sensory and motor inputs contribute to IBS, emerging evidence indicates that endogenous cognitive processes may also play a significant role (Gugnowska et al., 2022). By examining perturbations in synchronized music performance, the study aims to reveal the dynamic relationship between IBS and social interaction.
Methods
Participants
Twenty amateur pianists (12 female, 8 male) with at least three years of experience participated in the study. Four confederates induced standardized temporal perturbations, forming 16 pairs with regular participants. After excluding three pairs due to errors or EEG artifacts, data from 17 participants (11 female, 6 male, mean age 26.29, SD = 4.75) in 13 piano duos were analyzed. The Ethics Committee of the Max Planck Institute for Human Development approved the study.
Experimental Procedure
The experiment took place in an acoustically and electromagnetically shielded cabin, with participants seated back-to-back at separate digital pianos. They were briefed on the procedure and instructed to minimize movements and maintain a tempo of 120 bpm. Regular participants were informed that their partner would make deliberate mistakes but were unaware of the timing. After a metronome-guided start, they played a pre-memorized, simple two-voice piece (see Fig. 1A for details). Confederates shortened pauses during duets to induce a phase shift, forcing the partner to accelerate and resynchronize. Each experimental run consisted of 80 trials, split into 4 blocks, with participants rating perceived harmony and coordination demands after each trial.
Data Acquisition
Behavioral Data: The pianists played on Yamaha NP-11 digital pianos, both set to the “Grand Piano” timbre. Sound was delivered through speakers, allowing both players to hear each other clearly. MIDI data, capturing key event details such as pitch and timing, were recorded via a Roland UM-2G MIDI-USB interface using Cubase 7 Artist workstation (Steinberg Media Technologies, Germany). The MIDI data were analyzed using MATLAB’s MIDI toolbox.
EEG Data: EEGs from both pianists were recorded simultaneously using 64-electrode caps (actiCAP, Brain Products GmbH, Germany) and referenced to TP10 (right mastoid). Two separate EEG amplifiers (BrainAmp DC, Brain Products GmbH, Germany) for each participant were optically coupled to the recording computer in order to guarantee synchrony between the two EEG recordings. Data were recorded at 5000 Hz using BrainVision Recorder software, with an antialiasing band pass filter, ranging from 0.016 to 1000 Hz. Data were then preprocessed and filtered to 0.5–70 Hz using BrainVision Analyzer 2. Eye movements were corrected with FastICA, and remaining artifacts were removed through manual inspection. The final EEG data consisted of artifact-free epochs time-locked to the confederate’s first tap, focusing on 21 selected electrodes for each participant to minimize volume conduction.
To synchronize MIDI and EEG data, audio signals from the pianos were recorded alongside EEG signals using BrainAmp ExG (Brain Products GmbH, Germany).
Data Analysis
Behavioral Data: Two measures were compared between perturbed and control conditions to assess the influence on interpersonal coordination. These were based on inter-tap intervals (ITIs), reflecting the time difference between the participant’s and confederate’s key taps. First, behavioral asynchrony was calculated by averaging ITI lengths across trials. Second, variability was measured using the standard deviation of ITIs. Differences in asynchrony and variability for taps 8, 9, and 10 (post-perturbation) were analyzed with paired t-tests (Bonferroni corrected).
EEG Data: Artifact-free EEG data, from 1000 ms before to 1000 ms after the confederate’s taps, were transformed into time-frequency signals using Gabor expansion function. This method ensures consistent time and frequency resolution (0.5 Hz frequency resolution, 4 ms time resolution). Interbrain phase coherence (IPC) was calculated as the phase synchronization across trials between two electrodes on different brains (Lindenberger et al., 2009). For further analyses, the IPC values were averaged within the two time-frequency windows: 100-300 ms post-tap in delta (0.5–4 Hz) and theta (4.5–8 Hz) frequency bands. Partial Least Squares (PLS) analysis was used to determine IPC differences between perturbed and non-perturbed conditions. In addition to IPC, the angle of phase differences was calculated and compared across conditions using the Watson-Williams test.
To quantitatively assess the relation between behavioral and brain synchrony, angular-linear correlations (ral) between phase and time differences were calculated to assess the relationship between behavioral and brain synchrony (cf. Müller and Lindenberger, 2022).
Next, using information about the phase angle, we computed the phase alignment across all trials and pianist pairs for each tap and frequency of interest (2 and 6 Hz). The corresponding phase angles were sorted as a function of the behavioral asynchrony in play onsets (ITIs) between the two piano players, focusing on taps 8, 9, and 10, which were most affected by the perturbation. The results were compared between conditions through visual inspection
Results
Behavioral Results
As expected, asynchrony (mean ITI) and variability (SD of ITIs) were significantly greater in perturbed trials compared to non-perturbed ones. In perturbed trials, the confederate was faster, resulting in larger negative asynchronies: tap 8 (ITI Diff = 260 ms, p < 0.001), tap 9 (ITI Diff = 137 ms, p < 0.001), and tap 10 (ITI Diff = 40 ms, p < 0.001). Similarly, the variability of asynchrony was higher in perturbed trials: tap 8 (SD Diff = 0.029, p < 0.01), tap 9 (SD Diff = 0.053, p < 0.001), and tap 10 (SD Diff = 0.034, p < 0.01) (see Fig. 1B and 1C).
EEG Results
The mean-centered task partial least squares (PLS) analysis revealed significant differences in IPC between perturbed and non-perturbed conditions for taps 8, 9, and 10. In the delta band, IPC increased during tap 8 under the perturbed condition, with strong couplings particularly at electrodes P8 and Fz. However, IPC decreased at taps 9 and 10 in the perturbed condition, except for a few connections in parieto-occipital areas. In the theta band, IPC was consistently higher in the perturbed condition across all taps (see Fig. 1D).
Figure 1. Score of the musical piece and related behavioral and brain data during perturbation. (A) Excerpt of the music, composed by Sabine Pendl, in a 4/4 time signature. The confederate played the upper voice and initiated a premature entry after the duet pause in half of the trials. The regular participant played the lower voice and had to adapt during perturbed trials. (B) Asynchrony between the players across the 15 taps, measured by the average ITI differences (confederate − regular participant) across trials. (C) Consistency of the performance across the 15 taps, measured by the standard deviation (SD) of ITIs across trials. (D) Connectivity maps showing IPC values with a bootstrap ratio greater than 2.576 for delta (upper row) and theta (lower row) frequency bands, depicting the electrodes of the confederate (left) and the regular participant (right). The three taps following perturbation (taps 8, 9, and 10) are highlighted. Red lines indicate stronger coupling under perturbed conditions, while blue lines indicate inverse contrasts. **, p < 0.01; ***, p < 0.001.
Phase difference angles between homologous electrodes in the confederate’s and regular participant’s brains (Fz-Fz and Cz-Cz) generally oscillated around zero at delta frequencies, indicating in-phase synchronization across taps. However, during taps 8 and 9, these angles significantly shifted toward anti-phase synchronization under perturbation (p < 0.01, see Fig. 2A). The Watson-William test revealed significant differences in the phase difference angles between the two conditions (Fz-Fz: tap 8, F1,24 = 8.49, p < 0.01 and tap 9, F1,24 = 37.46, p < 0.001; Cz-Cz: tap 8, F1,24 = 34.46, p < 0.001, and tap 10, F1,24 = 4.97, p < 0.05).
Angular-linear correlations showed a significant relationship between time and phase differences, particularly in the theta band. Significant correlations were found at taps 8, 10-15 (non-perturbed) and taps 11-13 (perturbed), indicating that time and phase differences are more closely related in the faster theta frequency (see Fig. 2B).
Figure 2. Phase differences and angular-linear correlation for two electrode pairs in the delta and theta frequency bands. (A) Phase differences (confederate − regular participant) for the Fz-Fz (upper panels) and Cz-Cz (lower panels) electrode pairs in delta (left) and theta (right) frequency bands across 15 taps for non-perturbed (red line) and perturbed (blue line) trials. Phase difference angles were averaged across trials for each participant, and the Watson-William test was used to compare mean phase difference angles between conditions. *, p < 0.05; **, p < 0.01; ***, p < 0.001. (B) Angular-linear correlation between phase and time differences across all trials and pianist pairs for Fz-Fz (upper panels) and Cz-Cz (lower panels) in delta (left) and theta (right) frequency bands across the 15 taps, comparing non-perturbed (red line) and perturbed (blue line) trials. SL, significance level.
Phase alignment, measured in relation to the behavioral asynchrony in play onsets (ITIs), was strongest at tap 8 under perturbed conditions, especially at 2 Hz in the regular participant (see Fig. 3). The results indicate generally strong phase alignment that closely follows the behavioral onset synchrony across all trials and pianists in the pairs.
Figure 3. Phase alignment of phase angles related to behavioral play-onset asynchrony. Phase alignment at delta (2 Hz) and theta (6 Hz) frequencies in relation to behavioral play-onset asynchrony across all trials and pianist pairs at tap 8, shown separately for the confederate and regular participant. Phase angles were sorted based on the behavioral play-onset asynchrony (difference in onset times between the regular participant and confederate). The black curve represents the behavioral asynchrony. Phase alignment was calculated for mid-frontal (Fz) and mid-central (Cz) electrodes under both non-perturbed and perturbed conditions.
Phenomenological Results
Subjectively, participants perceived less harmony and required more effort to synchronize during perturbed trials (p < 0.05 and p < 0.01, respectively).
Figure 4. Harmony and effort rating scores. Box plots displaying harmony and effort ratings for non-perturbed and perturbed conditions. Both phenomenological ratings show significant differences between the two conditions, indicating that participants perceived less harmony and exerted more effort during perturbed trials. *, p < 0.05; **, p < 0.01.
Discussion
This study aimed to explore how behavioral and neural synchronization is affected during duet piano playing under perturbed and non-perturbed conditions. The key findings are: (1) Behavioral asynchrony and variability were significantly higher in perturbed trials; (2) Neural synchronization, measured by IPC, was higher in the delta and theta bands during perturbation; (3) Phase angles were closely aligned with behavioral asynchrony, particularly in the perturbed condition; (4) Delta-band synchronization tended to shift from in-phase to anti-phase during perturbation, and (5) Angular-linear correlations between phase and time-onset asynchronies were significant mainly in the theta band.
These results suggest that IBS reflects both sensory-motor alignment and the cognitive demands of coordination, particularly in response to perturbation. Increased IPC during perturbation likely corresponds to heightened attention and adaptive behaviors as the musicians attempted to resynchronize. The phase alignment analysis revealed that during perturbation, the pianists’ brain activity became tightly linked to the behavioral asynchrony in play onsets, highlighting the neural mechanisms behind real-time coordination during joint musical performance (Lindenberger et al., 2009; Gugnowska et al., 2022; Müller, 2022; Müller and Lindenberger, 2022). Overall, these findings emphasize the role of IBS in managing the cognitive and motor challenges of synchronous duo performance.
References
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