Interpersonal body and neural synchronization as a marker of implicit social interaction

Abstract

One may have experienced his or her footsteps unconsciously synchronize with the footsteps of a friend while walking together, or heard an audience's clapping hands naturally synchronize into a steady rhythm. However, the mechanisms of body movement synchrony and the role of this phenomenon in implicit interpersonal interactions remain unclear. We aimed to evaluate unconscious body movement synchrony changes as an index of implicit interpersonal interaction between the participants, and also to assess the underlying neural correlates and functional connectivity among and within the brain regions. We found that synchrony of both fingertip movement and neural activity between the two participants increased after cooperative interaction. These results suggest that the increase of interpersonal body movement synchrony via interpersonal interaction can be a measurable basis of implicit social interaction. The paradigm provides a tool for identifying the behavioral and the neural correlates of implicit social interaction.

Figure 1. Experimental setup and behavioral results.

(A) Session 1: Participants were asked to straighten their arms, point and hold their index fingers toward each other, and look at the other participant's fingertip. They were instructed to look at the other participant's finger while holding their own finger as stationary as possible. One participant was instructed to use left arm and the other was instructed to use right arm. Session 2: Same as the session 1, except participants changed the arm from left to right and from right to left respectively. Session 3: One participant (leader, who was randomly selected from the naïve participant pair) was instructed to randomly move his finger (in the approximate area of 20×20 cm square) and the other (follower) was instructed to follow. Session 4: Same as the session 3, except participants changed the arm from left to right and from right to left respectively. Session 5 and 6: Same as the session 3 and 4, Session 7 and 8: Same as the session 1 and 2. We call sessions 1–2 the pre-training sessions, the sessions 3–6 the training sessions, and the 7–8 the post-training sessions. (B) Hyperscanning-EEG setup. The EEG data was passed through a client to a EEG server and database, which was regulated by an experiment controller. Client computers received fingertip movement information from the two participants. Two EEG recording systems were synchronized using a pulse signal from the control server computer delivered to both EEG recording systems. (C) Average cross correlation coefficients of fingertip movements in each condition (pre-training, post-training, and crosscheck validation) with its standard errors (gray). The training significantly increased finger movement correlation between the two participants (p<0.03). No significant correlation was found in crosscheck condition (i.e. cross correlation results after random shuffling of participants, p = 0.62). Results are shown as means ± s.e.m. Statistical analyses performed using a two-tailed student's t-test.

Figure 2. sLORETA source localization.

Source localization contrasting between the post- and pre-training in (A) theta (4~7.5 Hz) and (B) beta (12~30 Hz) frequency range. The training significantly increased the theta (4~7.5 Hz) activity in the precuneus (PrC) (BA7, X = −15, Y = −75, Z = 50; MNI coordinates; corrected for multiple comparisons using nonparametric permutation test, red: p<0.01, yellow: p<0.001) and the beta (12~30 Hz) activity in the posterior middle temporal gyrus (MTG) (BA39, X = 50, Y = −74, Z = 24; MNI coordinates; corrected for multiple comparisons using nonparametric permutation test, red: p<0.05, yellow: p<0.01). (C) Regression analysis. Significant positive correlation between the fingertip synchrony change and ventromedial prefrontal cortex beta frequency power change between post- and pre-training sessions (BA11, X = 15, Y = 65, Z = −15; MNI coordinates; regression with nonparametric permutation test, p<0.05, n = 20).

Figure 3. Phase synchrony.

(A) The total number of functional connections that showed significant phase synchrony (phase randomization surrogate statistics, p<0.000001) of inter- and intra-brain in theta (4~7.5 Hz) and beta (12~30 Hz) frequency range (chi-square test, *p<0.005, **p<0.0001). The overall number of significant phase synchrony increased after training in inter-brain connections, but not in intra. (B) Topography of the phase synchrony connections between all 168 cortical ROIs of the two participants (Left brain: leader, right brain: follower) when contrasting post- against pre-training (phase randomization surrogate statistics, p<0.000001) in theta (4~7.5 Hz) and (C) beta (12~30 Hz). Inter-brain connections were found mainly in the inferior frontal gyrus (IFG), anterior cingulate (AC), parahippocampal gyrus (PHG), and postcentral gyrus (PoCG).