Synchronized drumming enhances activity in the caudate and facilitates prosocial commitment--if the rhythm comes easily

Abstract

Why does chanting, drumming or dancing together make people feel united? Here we investigate the neural mechanisms underlying interpersonal synchrony and its subsequent effects on prosocial behavior among synchronized individuals. We hypothesized that areas of the brain associated with the processing of reward would be active when individuals experience synchrony during drumming, and that these reward signals would increase prosocial behavior toward this synchronous drum partner. 18 female non-musicians were scanned with functional magnetic resonance imaging while they drummed a rhythm, in alternating blocks, with two different experimenters: one drumming in-synchrony and the other out-of-synchrony relative to the participant. In the last scanning part, which served as the experimental manipulation for the following prosocial behavioral test, one of the experimenters drummed with one half of the participants in-synchrony and with the other out-of-synchrony. After scanning, this experimenter "accidentally" dropped eight pencils, and the number of pencils collected by the participants was used as a measure of prosocial commitment. Results revealed that participants who mastered the novel rhythm easily before scanning showed increased activity in the caudate during synchronous drumming. The same area also responded to monetary reward in a localizer task with the same participants. The activity in the caudate during experiencing synchronous drumming also predicted the number of pencils the participants later collected to help the synchronous experimenter of the manipulation run. In addition, participants collected more pencils to help the experimenter when she had drummed in-synchrony than out-of-synchrony during the manipulation run. By showing an overlap in activated areas during synchronized drumming and monetary reward, our findings suggest that interpersonal synchrony is related to the brain's reward system.

Figure 1. Prosocial commitment test results (A) and the ease of rhythm imitation (B).

(A) Mean (and standard deviation) of the number of pencils picked up in order to help I.K. in the prosocial commitment test after experiencing her as a synchronous (green) or asynchronous drummer (red) in the manipulation run. (B) Correlation between the number of pencils and the ease of rhythm imitation as a function of synchrony (green: I.K. who played in-synch; red: she played out-of-synch during the manipulation run). The lines represent the linear best fit and r refers to the correlation coefficient.

Figure 2. Areas of the brain associated with processing of monetary reward (contrast High Monetary Reward - No Monetary Reward; p<0.005 uncorrected, all voxels also survive p<0.05 FDR correction).

Clusters are superimposed on to the average T1 image derived from all participants.

Figure 3. Correlations between the activity in the caudate and the ease of rhythm imitation.

(A) Bilateral caudate activity correlated with the ease of rhythm imitation during synchronous drumming (synch-baseline, p<0.005 uncorrected, all voxels also survive p<0.05 FDR correction) (B) Right caudate activity correlated with the ease of rhythm imitation for the comparison between synchronous and asynchronous drumming (synch - asynch, p<0.005, uncorrected; all voxels also survive p<0.05 FDR correction) (C) Illustration of the correlation identified in (B) by plotting parameter estimates within the cluster against the ease of rhythm imitation rating. The line represents the linear best fit. Clusters in the caudate are superimposed on coronal views of the average T1 image derived from all participants.

Figure 4. Correlations between the activity in the caudate and the prosocial commitment test.

(A) Right caudate activity during synchronous drumming correlated with the number of pencils collected for the synchronous drummer (synch-baseline, p<0.005, uncorrected; all voxels also survive p<0.05 FDR correction) (B) Illustration of the correlation identified in (A) by plotting average parameter estimates within the cluster against the number of pencils. The line represents the linear best fit. (C) The overlap (green) of the correlation between brain activity during synch drumming and ease of rhythm imitation (red) and brain activity during synch drumming and number of pencils picked up (blue) (p<0.005, uncorrected; all voxels also survive p<0.05 FDR correction). Clusters in the caudate are superimposed on coronal views of the average T1 image derived from all participants.