How to Calculate and Validate Inter-brain Synchronization in a fNIRS Hyperscanning Study

Summary

The dynamics between coupled brains of individuals have been increasingly represented by inter-brain synchronization (IBS) when they coordinate with each other, mostly using simultaneous-recording signals of brains (namely hyperscanning) with fNIRS. In fNIRS hyperscanning studies, IBS has been commonly assessed through the wavelet transform coherence (WTC) method because of its advantage on expanding time series into time-frequency space where oscillations can be seen in a highly intuitive way. The observed IBS can be further validated via the permutation-based random pairing of the trial, partner, and condition. Here, a protocol is presented to describe how to obtain brain signals via fNIRS technology, calculate IBS through the WTC method, and validate IBS by permutation in a hyperscanning study. Further, we discuss the critical issues when using the above methods, including the choice of fNIRS signals, methods of data preprocessing, and optional parameters of computations. In summary, using the WTC method and permutation is a potentially standard pipeline for analyzing IBS in fNIRS hyperscanning studies, contributing to both the reproducibility and reliability of IBS.

Abstract

The dynamics between coupled brains of individuals have been increasingly represented by inter-brain synchronization (IBS) when they coordinate with each other, mostly using simultaneous-recording signals of brains (namely hyperscanning) with fNIRS. In fNIRS hyperscanning studies, IBS has been commonly assessed through the wavelet transform coherence (WTC) method because of its advantage on expanding time series into time-frequency space where oscillations can be seen in a highly intuitive way. The observed IBS can be further validated via the permutation-based random pairing of the trial, partner, and condition. Here, a protocol is presented to describe how to obtain brain signals via fNIRS technology, calculate IBS through the WTC method, and validate IBS by permutation in a hyperscanning study. Further, we discuss the critical issues when using the above methods, including the choice of fNIRS signals, methods of data preprocessing, and optional parameters of computations. In summary, using the WTC method and permutation is a potentially standard pipeline for analyzing IBS in fNIRS hyperscanning studies, contributing to both the reproducibility and reliability of IBS.

Introduction

When people coordinate with others, their brains and bodies become a coupled unit through continuous mutual adaption. The coupling between brains can be represented by inter-brain synchronization (IBS) through the hyperscanning approach, which simultaneously records two or more individuals' brain signals¹. Indeed, a growing body of fNIRS/EEG hyperscanning studies has found IBS in various collaboration contexts, including finger tapping², group walking³, playing drums⁴, guitar playing⁵, and singing/humming⁶. fNIRS is widely....

Protocol

The protocol presented here was approved by the University Committee on Human Research Protection of East China Normal University.

1. Preparation for the experiment

1. Participants
   1. Recruit a group of undergraduate and graduate students with monetary compensation by the campus advertising.
   2. Ensure that the participants are right-handed and have normal or corrected-to-normal vision and hearing. Ensure that they have not studied music or have studied it for fewer than 3 years before.
   3. Randomly match the students in dyads. To control the potential effect of partner familiarity on social coordination

Results

The results showed that there was IBS at channel 5 in the meter coordination condition, whereas no IBS existed in other conditions (i.e., meter independence, non-meter coordination, non-meter independence; Figure 2A). At channel 5, the IBS in the meter coordination condition was significantly higher than the coherence values in the non-meter coordination and meter independence condition (Figure 2B). Channel 5 approximately belonged to the left dorsolateral prefr.......

Discussion

This protocol provides a step-by-step procedure to calculate and validate IBS, using the fNIRS hyperscanning approach to simultaneously collect two participants' brain signals. Some critical issues involved in fNIRS data preprocessing, IBS calculation, statistics, and IBS validation are discussed below.

Data Preprocessing
It is necessary to preprocess fNIRS data in hyperscanning studies to extract real signals from the possible noise (i.e., motion artifacts, systemic.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Computer	Hewlett-Packard Development Company, L.P.	HP S01-pF157mcn
Earphone	Royal Philips Electronics, Eindhoven, The Netherlands	SHE2405BK/00
EEG cap	Compumedics Neuroscan, Charlotte, USA	64-channel Quik-Cap
E-Prime software	Psychology Software Tools, Inc., Pittsburgh, USA	E-Prime 3
fNIRS system	Hitachi Medical Corporation, Tokyo, Japan	ETG-7100 Optical Topography System
MATLAB 2014b	The MathWorks, Inc., Natick, MA	MATLAB 2014b
MuseScore	Musescore Company, Belgium	MuseScore 3.6.2.548021803
Swimming cap	Decathlon Group, Villeneuve-d'Ascq, France	1681552