This research was supported by the following funding sources: A NARSAD Young Investigator Award Grant from the Brain and Behavior Research Foundation (Grant #29736) (SSA), a Global Grand Challenges Grant from the Bill and Melinda Gates Foundation (Grant #INV-005792) (RNA) and a Discovery Fund Grant from the Department of Psychology at Yale University (RNA). The authors also wish to acknowledge Richard Watts (Yale Brain Imaging Center) for his support during data collection and Adam Eggebrecht, Ari Segel and Emma Speh (Washington University in St Louis) for their assistance in data analysis.

Publication fees for this article are sponsored by NIRx. The authors have nothing else to disclose.

This protocol for simultaneous data collection of fMRI&#160;and fNIRS signals uses a whole-head fNIRS optode array and short-distance channels for measuring and regressing out the systemic non-cortical physiological signals. Critical steps in this protocol include modification and development of the fNIRS equipment for collecting fNIRS signals in the MRI environment. To the best of our knowledge, there is no turn-key commercial system that is fully optimized for capturing simultaneous fMRI&#160;and fNIRS measurements usi...

Cognitive function has been studied in the adult human brain via functional magnetic resonance imaging (fMRI) for nearly three decades. Although fMRI provides high spatial resolution and both functional and structural images, it is often not practical for studies conducted in naturalistic contexts or for use with infants and clinical populations. These constraints substantially limit our understanding of brain function. An alternative to fMRI is the use of portable methodologies that are more cost-effective and robust to motion, such as functional near-infrared spectroscopy (fNIRS)1,2,3. fNIRS has been used with infants and young children to assess brain function across a range of cognitive domains, such as language development, processing of socially relevant information and object processing 4,5,6. fNIRS is also a neuroimaging modality especially suitable for testing clinical populations due to its potential for repeated testing and monitoring across ages7,8,9. Despite its wide applicability, there are no studies quantitatively comparing fMRI and fNIRS signals collected simultaneously from the same subjects with whole-head coverage. This comparison is necessary to comprehensively validate area-level activations and functional connectivity between regions of interest (ROIs) against the fMRI gold standard. Furthermore, establishing this inter-modality correspondence has the potential to enhance the interpretation of fNIRS when it is the only collected signal across both typical and atypical development.
Both fMRI and fNIRS signals detect changes in cerebral blood oxygenation (CBO) during functional brain activation10,11. fMRI relies on changes in electromagnetic fields and provides a high spatial resolution of CBO changes12. fNIRS, in contrast, measures absorption levels of near-infrared light using a series of light-emitting and light-detecting optodes2. Since fNIRS measures changes in absorption at different wavelengths, it can assess concentration changes in both oxy- and deoxyhemoglobin. Prior studies using simultaneous recordings of fMRI and fNIRS signals with a small number of optodes have shown that the two signals have high spatial and temporal correspondence10. There are strong correlations between blood-oxygen-level-dependent (BOLD) fMRI and optical measures11,13, with deoxyhemoglobin showing the highest correlation with the BOLD response, as reported by prior work comparing the temporal dynamics of the fNIRS and fMRI hemodynamic response functions (HRFs)14. These early studies implemented motor response paradigms (i.e., finger tapping) and used a limited number of optodes covering primary motor and premotor cortex areas. In the last decade, studies have expanded the focus to include a larger battery of cognitive tasks and resting-state sessions, although still using a limited number of optodes covering specific ROIs. These studies have shown that variability in fNIRS/fMRI correlations is dependent on the optode&#39;s distance from the scalp and the brain15. Furthermore, fNIRS can provide resting-state functional connectivity measures comparable to fMRI16,17.
The current protocol builds on prior work and addresses key limitations by i) providing whole-head fNIRS coverage, ii) including short-distance measurements for regression of non-cortical physiological signals, iii) implementing two different methods for optode-to-scalp co-registration of fNIRS measurements and iv) enabling assessment of the test-retest reliability of the signal across two independent sessions. This protocol for simultaneous data collection of fMRI&#160;and fNIRS signals was initially developed for testing young adults. However, one of the goals of the study was to create an experimental setup for collecting simultaneous fMRI/fNIRS signals that can be subsequently adapted for testing developmental populations. Therefore, the current protocol can also be used as a starting point for developing a protocol to test young children. In addition to using whole-head fNIRS coverage, the protocol also aims to incorporate recent advances in the field of fNIRS hardware, such as the inclusion of short-distance channels to measure the systemic physiological signal (i.e., vascular changes arising from noncortical sources, such as blood pressure, respiratory and cardiac signals)18,19 ;and the use of a 3D structure sensor for optode-to-scalp co-registration20. Although the focus of the present protocol is on the results of a visual flashing checkerboard task, the entire experiment includes two sessions with a mix of traditional block-task designs, resting-state sessions, and naturalistic movie-viewing paradigms.
The protocol describes the steps needed to adapt the fNIRS equipment for use in the MRI environment, including cap design, temporal alignment via trigger synchronization and phantom tests required before the start of data collection. As noted, the focus here is on the results of the flashing checkerboard task, but the overall procedure is not task-specific and can be appropriate for any number of experimental paradigms. The protocol further outlines the steps required during data collection, which include fNIRS cap placement and signal calibration, participant&#160;and experimental equipment setup, as well as post-experiment clean up and data storage. The protocol ends by providing an overview of the analytic pipelines specific for preprocessing fNIRS and fMRI data.

<table><tbody><tr><td>280 low-profile MRI-compatible grommets for NIRs caps</td><td>NIRx</td><td>GRM-LOP</td><td></td></tr><tr><td>4 128-position NIRS caps with 128x unpopulated slits in 10-5 layout</td><td>NIRx</td><td>CP-128-128S</td><td>Sizes: 52, 54, 56, 60</td></tr><tr><td>8 bundles of 4x detector fibers with low-profile tip; MRI-, MEG-, and TMS-compatible.&amp;nbsp;</td><td>NIRx</td><td>DET-FBO- LOW</td><td>10 m long</td></tr><tr><td>8 bundles of 4x laser source fibers with MRI-compatible low-profile tip</td><td>NIRx</td><td>SRC-FBO- LAS-LOW</td><td>10 m long</td></tr><tr><td>Bundle set of 8 short-channel detectors with specialized ring grommets that fit to low-profile grommets</td><td>NIRx</td><td>DET-SHRT-SET</td><td>Splits a single detector into 8 short channels that may be placed anywhere on a single NIRS cap</td></tr><tr><td>Magnetom 3T PRISMA</td><td>Siemens</td><td>N/A</td><td>128 channel capacity, 64/32/20 channel head coils, 80 mT/m max gradient amplitude, 200 T/m/s slew rate, full neuro sequences</td></tr><tr><td>NIRScout XP Core System Unit</td><td>NIRx</td><td>NSXP- CHS</td><td>Up to 64x Laser-2 (or 32x laser-4) illuminators or 64 LED-2 illuminators; up to 32x detectors; capable of tandem (multi-system) and hyperscanning (multi-subject) measurements; compatible with EEG, tDCS, eye-tracking, and other modalities; modules available for fMRI, TMS, MEG compatibility</td></tr><tr><td>NIRStar software</td><td>NIRx</td><td>N/A</td><td>Version 15.3</td></tr><tr><td>NIRx parallel port replicator</td><td>NIRx</td><td>ACC-LPT-REP</td><td>The parallel prot replicator&amp;nbsp; comes with three components: parallel port replicator box, USB power cable and BNC adapter</td></tr><tr><td>Physiological pulse unit</td><td>Siemens</td><td>PPU098</td><td>Optical plethysmography allowing the acquisiton of the cardiac rhythm.</td></tr><tr><td>Respiratory unit</td><td>Siemens</td><td>PERU098&amp;nbsp;</td><td>Unit intended for the acquisition of the respiratory amplitude (by means of a pneumatic system and a restraint belt).</td></tr><tr><td>Structure Sensor Mark II</td><td>Occipital</td><td>101866 (SN)</td><td>3D structure sensor for optode digitization.</td></tr></tbody></table>

simultaneous data collection of fmri and fnirs measurements using a whole-head optode array and short-distance channels

Functional near-infrared spectroscopy (fNIRS) is a portable neuroimaging methodology, more robust to motion and more cost-effective than functional magnetic resonance imaging (fMRI), which makes it highly suitable for conducting naturalistic studies of brain function and for use with developmental and clinical populations. Both fNIRS and fMRI methodologies detect changes in cerebral blood oxygenation during functional brain activation, and prior studies have shown high spatial and temporal correspondence between the two signals. There is, however, no quantitative comparison of the two signals collected simultaneously from the same subjects with whole-head fNIRS coverage. This comparison is necessary to comprehensively validate area-level activations and functional connectivity against the fMRI gold standard, which in turn has the potential to facilitate comparisons of the two signals across the lifespan. We address this gap by describing a protocol for simultaneous data collection of fMRI&#160;and fNIRS signals that: i) provides whole-head fNIRS coverage; ii) includes short-distance measurements for regression of the non-cortical, systemic physiological signal; and iii) implements two different methods for optode-to-scalp co-registration of fNIRS measurements. fMRI and fNIRS data from three subjects are presented, and recommendations for adapting the protocol to test developmental and clinical populations are discussed. The current setup with adults allows scanning sessions for an average of approximately 40 min, which includes both functional and structural scans. The protocol outlines the steps required to adapt the fNIRS equipment for use in the magnetic resonance (MR) environment, provides recommendations for both data recording and optode-to-scalp co-registration, and discusses potential modifications of the protocol to fit the specifics of the available MR-safe fNIRS system. Representative subject-specific responses from a flashing-checkerboard task illustrate the feasibility of the protocol to measure whole-head fNIRS signals in the MR environment. This protocol will be particularly relevant for researchers interested in validating fNIRS signals against fMRI across the lifespan.

This section presents representative subject-specific responses for the flashing checkerboard task for both fMRI and fNIRS signals. First, representative raw fNIRS data and quality assessments are shown in Figure 6 and Figure 7 to illustrate the feasibility of the experimental setup to measure fNIRS signals in the MRI environment. A diagram of the whole head optode array and sensitivity profile is shown in Figure 8....

Watch this Scientific Journal Video about Simultaneous Data Collection of fMRI and fNIRS Measurements Using a Whole-Head Optode Array and Short-Distance Channels at JoVE.com

Simultaneous Data Collection of fMRI and fNIRS Measurements Using a Whole-Head Optode Array and Short-Distance Channels

We present a method for simultaneously collecting fMRI and fNIRS signals from the same subjects with whole-head fNIRS coverage. The protocol has been tested with three young adults and can be adapted for data collection for developmental studies and clinical populations.

Functional near-infrared spectroscopy (fNIRS) is a portable neuroimaging methodology, more robust to motion and more cost-effective than functional ...

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Neuroscience

945 Views.  Yale University School of Medicine. We present a method for simultaneously collecting fMRI and fNIRS signals from the same subjects with whole-head fNIRS coverage. The protocol has been tested with three young adults and can be adapted for data collection for developmental studies and clinical populations. 

Video: Simultaneous Data Collection of fMRI and fNIRS Measurements Using a Whole-Head Optode Array and Short-Distance Channels

Best Current Practice for Obtaining High Quality EEG Data During Simultaneous fMRI

Simultaneous EEG-fMRI allows the excellent temporal resolution of EEG to be combined with the high spatial accuracy of fMRI. The data from these two modalities can be combined in a number of ways, but all rely on the acquisition of high quality EEG and fMRI data. EEG data acquired during simultaneous fMRI are affected by several artifacts, including the gradient artefact (due to the changing magnetic field gradients required for fMRI), the pulse artefact (linked to the cardiac cycle) and movement artifacts (resulting from movements in the strong magnetic field of the scanner, and muscle activity). Post-processing methods for successfully correcting the gradient and pulse artifacts require a number of criteria to be satisfied during data acquisition. Minimizing head motion during EEG-fMRI is also imperative for limiting the generation of artifacts.
Interactions between the radio frequency (RF) pulses required for MRI and the EEG hardware may occur and can cause heating. This is only a significant risk if safety guidelines are not satisfied. Hardware design and set-up, as well as careful selection of which MR sequences are run with the EEG hardware present must therefore be considered.
The above issues highlight the importance of the choice of the experimental protocol employed when performing a simultaneous EEG-fMRI experiment. Based on previous research we describe an optimal experimental set-up. This provides high quality EEG data during simultaneous fMRI when using commercial EEG and fMRI systems, with safety risks to the subject minimized. We demonstrate this set-up in an EEG-fMRI experiment using a simple visual stimulus. However, much more complex stimuli can be used. Here we show the EEG-fMRI set-up using a Brain Products GmbH (Gilching, Germany) MRplus, 32 channel EEG system in conjunction with a Philips Achieva (Best, Netherlands) 3T MR scanner, although many of the techniques are transferable to other systems.

Simultaneous electroencephalography (EEG) and functional Magnetic Resonance imaging (fMRI) is a powerful neuroimaging tool. However, the inside of an MRI scanner forms a difficult environment for EEG data recording and safety must be considered whenever operating EEG equipment inside a scanner. Here, we present an optimised EEG-fMRI data acquisition protocol.

Simultaneous  EEG-fMRI allows the excellent temporal resolution of EEG to be combined with the  high spatial accuracy of fMRI. The data  from these two ...

Behavior

fMRI Validation of fNIRS Measurements During a Naturalistic Task

We present a method to compare brain activity recorded with near-infrared spectroscopy (fNIRS) in a dance video game task to that recorded in a reduced version of the task using fMRI (functional magnetic resonance imaging). Recently, it has been shown that fNIRS can accurately record functional brain activities equivalent to those concurrently recorded with functional magnetic resonance imaging for classic psychophysical tasks and simple finger tapping paradigms. However, an often quoted benefit of fNIRS is that the technique allows for studying neural mechanisms of complex, naturalistic behaviors that are not possible using the constrained environment of fMRI. Our goal was to extend the findings of previous studies that have shown high correlation between concurrently recorded fNIRS and fMRI signals to compare neural recordings obtained in fMRI procedures to those separately obtained in naturalistic fNIRS experiments. Specifically, we developed a modified version of the dance video game Dance Dance Revolution (DDR) to be compatible with both fMRI and fNIRS imaging procedures. In this methodology we explain the modifications to the software and hardware for compatibility with each technique as well as the scanning and calibration procedures used to obtain representative results. The results of the study show a task-related increase in oxyhemoglobin in both modalities and demonstrate that it is possible to replicate the findings of fMRI using fNIRS in a naturalistic task. This technique represents a methodology to compare fMRI imaging paradigms which utilize a reduced-world environment to fNIRS in closer approximation to naturalistic, full-body activities and behaviors. Further development of this technique may apply to neurodegenerative diseases, such as Parkinson&#8217;s disease, late states of dementia, or those with magnetic susceptibility which are contraindicated for fMRI scanning.

We present a method to compare functional brain activity recorded during a naturalistic task using fNIRS with activity recorded during fMRI.

We present a method to compare brain activity recorded with near-infrared spectroscopy (fNIRS) in a dance video game task to that recorded in a reduced ...

Functional Near Infrared Spectroscopy of the Sensory and Motor Brain Regions with Simultaneous Kinematic and EMG Monitoring During Motor Tasks

There are several advantages that functional near-infrared spectroscopy (fNIRS) presents in the study of the neural control of human movement. It is relatively flexible with respect to participant positioning and allows for some head movements during tasks. Additionally, it is inexpensive, light weight, and portable, with very few contraindications to its use. This presents a unique opportunity to study functional brain activity during motor tasks in individuals who are typically developing, as well as those with movement disorders, such as cerebral palsy. An additional consideration when studying movement disorders, however, is the quality of actual movements performed and the potential for additional, unintended movements. Therefore, concurrent monitoring of both blood flow changes in the brain and actual movements of the body during testing is required for appropriate interpretation of fNIRS results. Here, we show a protocol for the combination of fNIRS with muscle and kinematic monitoring during motor tasks. We explore gait, a unilateral multi-joint movement (cycling), and two unilateral single-joint movements (isolated ankle dorsiflexion, and isolated hand squeezing). The techniques presented can be useful in studying both typical and atypical motor control, and can be modified to investigate a broad range of tasks and scientific questions.

Monitoring brain activity during upright motor tasks is of great value when investigating the neural source of movement disorders. Here, we demonstrate a protocol that combines functional near infrared spectroscopy with continuous monitoring of muscle and kinematic activity during 4 types of motor tasks.

There are several advantages that functional near-infrared spectroscopy (fNIRS) presents in the study of the neural control of human movement. It is ...

Using Fiberless, Wearable fNIRS to Monitor Brain Activity in Real-world Cognitive Tasks

Functional Near Infrared Spectroscopy (fNIRS) is a neuroimaging technique that uses near-infrared light to monitor brain activity. Based on neurovascular coupling, fNIRS is able to measure the haemoglobin concentration changes secondary to neuronal activity. Compared to other neuroimaging techniques, fNIRS represents a good compromise in terms of spatial and temporal resolution. Moreover, it is portable, lightweight, less sensitive to motion artifacts and does not impose significant physical restraints. It is therefore appropriate to monitor a wide range of cognitive tasks (e.g., auditory, gait analysis, social interaction) and different age populations (e.g., new-borns, adults, elderly people). The recent development of fiberless fNIRS devices has opened the way to new applications in neuroscience research. This represents a unique opportunity to study functional activity during real-world tests, which can be more sensitive and accurate in assessing cognitive function and dysfunction than lab-based tests. This study explored the use of fiberless fNIRS to monitor brain activity during a real-world prospective memory task. This protocol is performed outside the lab and brain haemoglobin concentration changes are continuously measured over the prefrontal cortex while the subject walks around in order to accomplish several different tasks.

Monitoring brain activity outside the lab without physical constraints presents methodological challenges. A fiberless, wearable functional Near Infrared Spectroscopy (fNIRS) system was used to measure brain activity during an ecological prospective memory task. It was demonstrated that this system could be used to monitor brain activity during non-lab based experiments.

Functional Near Infrared Spectroscopy (fNIRS) is a neuroimaging technique that uses near-infrared light to monitor brain activity. Based on neurovascular ...

Cerebral Blood Flow-Based Resting State Functional Connectivity of the Human Brain using Optical Diffuse Correlation Spectroscopy

To obtain a comprehensive understanding of the human brain, utilization of cerebral blood flow (CBF) as a source of contrast is desired because it is a key hemodynamic parameter related to cerebral oxygen supply. Resting state low frequency fluctuations based on oxygenation contrast have been shown to provide correlations between functionally connected regions. The presented protocol uses optical diffuse correlation spectroscopy (DCS) to assess blood flow-based resting state functional connectivity (RSFC) in the human brain. Results of CBF-based RSFC in human frontal cortex indicate that intra-regional RSFC is significantly higher in the left and right cortices compared to inter-regional RSFC in both cortices. This protocol should be of interest to researchers who employ multi-modal imaging techniques to study human brain function, especially in the pediatric population.

This protocol demonstrates how to measure resting state functional connectivity in the human prefrontal cortex using a custom-made diffuse correlation spectroscopy instrument. The report also discuss practical aspects of the experiment as well as detailed steps for analyzing the data.

To obtain a comprehensive understanding of the human brain, utilization of cerebral blood flow (CBF) as a source of contrast is desired because it is a ...

Bioengineering

Conducting Hyperscanning Experiments with Functional Near-Infrared Spectroscopy

Concurrent brain recordings of two or more interacting persons, an approach termed hyperscanning, are gaining increasing importance for our understanding of the neurobiological underpinnings of social interactions, and possibly interpersonal relationships. Functional near-infrared spectroscopy (fNIRS) is well suited for conducting hyperscanning experiments because it measures local hemodynamic effects with a high sampling rate and, importantly, it can be applied in natural settings, not requiring strict motion restrictions. In this article, we present a protocol for conducting fNIRS hyperscanning experiments with parent-child dyads and for analyzing brain-to-brain synchrony. Furthermore, we discuss critical issues and future directions, regarding the experimental design, spatial registration of the fNIRS channels, physiological influences and data analysis methods. The described protocol is not specific to parent-child dyads, but can be applied to a variety of different dyadic constellations, such as adult strangers, romantic partners or siblings. To conclude, fNIRS hyperscanning has the potential to yield new insights into the dynamics of the ongoing social interaction, which possibly go beyond what can be studied by examining the activities of individual brains.

The present protocol describes how to conduct fNIRS hyperscanning experiments and analyze brain-to-brain synchrony. Further, we discuss challenges and possible solutions.

Concurrent brain recordings of two or more interacting persons, an approach termed hyperscanning, are gaining increasing importance for our understanding ...

Conducting Concurrent Electroencephalography and Functional Near-Infrared Spectroscopy Recordings with a Flanker Task

Concurrent EEG and fNIRS recordings offer an excellent opportunity to gain a full understanding of the neural mechanism of cognitive processing by inspecting the relationship between the neural and hemodynamic signals. EEG is an electrophysiological technology that can measure the rapid neuronal activity of the cortex, whereas fNIRS relies on the hemodynamic responses to infer brain activation. The combination of EEG and fNIRS neuroimaging techniques can identify more features and reveal more information associated with the functioning of the brain. In this protocol, fused EEG-fNIRS measurements were performed for concurrent recordings of evoked-electrical potentials and hemodynamic responses during a Flanker task. In addition, the critical steps for setting up the hardware and software system as well as the procedures for data acquisition and analysis were provided and discussed in detail. It is expected that the present protocol can pave a new avenue for improving the understanding of the neural mechanisms underlying various cognitive processes by using the EEG and fNIRS signals.

The present protocol describes how to perform concurrent EEG and fNIRS recordings and how to inspect the relationship between the EEG and fNIRS data.&#160;

Concurrent EEG and fNIRS recordings offer an excellent opportunity to gain a full understanding of the neural mechanism of cognitive processing by ...

Qualitative and Comparative Cortical Activity Data Analyses from a Functional Near-Infrared Spectroscopy Experiment Applying Block Design

Neuroimaging studies play a pivotal role in the evaluation of pre- vs. post-interventional neurological conditions such as in rehabilitation and surgical treatment. Among the many neuroimaging technologies used to measure brain activity, functional near-infrared spectroscopy (fNIRS) enables the evaluation of dynamic cortical activities by measuring the local hemoglobin levels similar to functional magnetic resonance imaging (fMRI). Also, due to lesser physical restriction in fNIRS, multiple variants of sensorimotor tasks can be evaluated. Many laboratories have developed several methods for fNIRS data analysis; however, despite the fact that the general principles are the same, there is no universally standardized method. Here, we present the qualitative and comparative analytic methods of data obtained from a multi-channel fNIRS experiment using a block design. For qualitative analysis, we used a software for NIRS as a mass-univariate approach based on the generalized linear model. The NIRS-SPM analysis shows qualitative results for each session by visualizing the activated area during the task. In addition, the non-invasive three-dimensional digitizer can be used to estimate the fNIRS channel locations relative to the brain. To corroborate the NIRS-SPM findings, the amplitude of the changes in hemoglobin levels induced by the sensorimotor task can be statistically analyzed by comparing the data obtained from two different sessions (before and after intervention) of the same study subject using a multi-channel hierarchical mixed model. Our methods can be used to measure the pre- vs. post-intervention analysis in a variety of neurological disorders such as movement disorders, cerebrovascular diseases, and neuropsychiatric disorders.

We describe the analysis of continuous-wave functional near-infrared spectroscopy experiment using a block design with a sensorimotor task. To increase the reliability of the data analysis, we used the qualitative general linear model-based statistical parametric mapping and the comparative hierarchical mixed models for multi-channels.

Neuroimaging studies play a pivotal role in the evaluation of pre- vs. post-interventional neurological conditions such as in rehabilitation and surgical ...

Inter-Brain Synchrony in Open-Ended Collaborative Learning: An fNIRS-Hyperscanning Study

fNIRS hyperscanning is widely used to detect the neurobiological underpinnings of social interaction. With this technique, researchers qualify the concurrent brain activity of two or more interactive individuals with a novel index called inter-brain synchrony (IBS) (i.e., phase and/or amplitude alignment of the neuronal or hemodynamic signals across time). A protocol for conducting fNIRS hyperscanning experiments on collaborative learning dyads in a naturalistic learning environment is presented here. Further, a pipeline of analyzing IBS of oxygenated hemoglobin (Oxy-Hb) signal is explained. Specifically, the experimental design, the process of NIRS data recording, data analysis methods, and future directions are all discussed. Overall, implementing a standardized fNIRS hyperscanning pipeline is a fundamental part of second-person neuroscience. Also, this is in line with the call for open-science to aid the reproducibility of research.

The protocol for conducting fNIRS hyperscanning experiments on collaborative learning dyads in a naturalistic learning environment is outlined. Further, a pipeline to analyze the Inter-Brain Synchrony (IBS) of oxygenated hemoglobin (Oxy-Hb) signals is presented.

fNIRS hyperscanning is widely used to detect the neurobiological underpinnings of social interaction. With this technique, researchers qualify the ...

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

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.&#160;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.

The dynamics between coupled brains of individuals have been increasingly represented by inter-brain synchronization (IBS) when they coordinate with each ...