Interpersonal Synchronization in groups: A Review on Research Techniques and Paradigms

Abstract:

Social interactions are in the base of human cognition processes. It arises when two or more individuals develop joint aleatory actions causing interpersonal synchronization. Studies evidences show physiological, behavioral and neural synchronization underlying these processes. Several studies on real life social interactions and their underlying neural mechanisms use a single person or second person approach. However, naturalistic social interactions are not limited to dyadic/dual exchange. Thus, recent studies have focused on group paradigms. The question remains whether the biological patterns found in dyad interactions will also occur in groups of three or more individuals. Furthermore, will all individuals’ brains react the same way? Is the biological reaction associated with a correspondent psychological and/or behavioral experience? How researchers are developing these studies? We expect to clarify some of these issues in the current review which focus is on IPS studies involving three or more individuals using biological signals such as brain activity, peripheral signs (skin temperature and heart beat), body movement and eye tracking.

Keywords: Interpersonal synchronization; Hyperscanning; Biological signals synchrony; Eye Tracking; Accelerometers; Bio sensing devices.

Sincronização Interpessoal em grupos: Uma Revisão de Técnicas de Pesquisa e Paradigmas

Resumo:As interações sociais estão na base dos processos de cognição humana. Surge quando dois ou mais indivíduos desenvolvem ações aleatórias conjuntas, causando sincronização interpessoal. As evidências dos estudos mostram sincronização fisiológica, comportamental e neural subjacente a esses processos. Vários estudos sobre as interações sociais da vida real e seus mecanismos neurais subjacentes usam uma abordagem de pessoa única ou segunda pessoa. No entanto, as interações sociais naturalistas não se limitam à troca entre apenas dois indivíduos. Assim, estudos recentes se concentraram em paradigmas utilizando grupos de indivíduos. Resta saber se os padrões biológicos encontrados nas interações das díades também ocorrerão em grupos de três ou mais indivíduos. Além disso, o cérebro de todos os indivíduos reagirá da mesma maneira? A reação biológica está associada a uma experiência psicológica e / ou comportamental correspondente? Como os pesquisadores estão desenvolvendo estes estudos? Esperamos esclarecer algumas dessas questões na revisão atual, cujo foco está nos estudos de IPS

envolvendo três ou mais indivíduos usando sinais biológicos, como atividade cerebral, sinais periféricos (temperatura da pele e batimentos cardíacos), movimento corporal e rastreamento ocular.

Keywords: sincronização interpessoal; Hyperscanning; Sincronia de sinais biológicos; Rastreamento Ocular; Acelerômetros; Dispositivos de detecção biológica.

Introduction

The interpersonal interaction (IPI) behavior is a defining feature of the human

bonding process.(1)(2) Actually, as a social species, the quality of our attachments is

determinant for healthy states and well-being.(3)

The IPI behavior arises when two or more individuals are interacting and combine

their own behaviors with others creating joint behaviors.(3)(4) This definition is in line

with recent studies results on human attachment process that show significant

evidence of biobehavioral synchrony among individuals during social interaction.(2)(5)(6)

Although the way humans perceive and interact with each other are in the base of

brain organization, only recently researchers started to study cognition processes

within Interpersonal Synchronization (IPS)(7). IPS, defined as the synchrony among

participants, can occur in different forms and intensity during IPI. It can be interbrain

neural synchrony (INS)(8), as well as biological signals synchrony like skin

temperature, heart rate and movements (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). The interpersonal

attunement is also evident in autonomic and endocrine responses. (6)

Social cognition, largely studied on psychology, focus on how humans process, store

and apply external information (social situations and interactions). Social cognitive

processes are interactive, just like our social lives (2). Through the many possible

experimental designs, social cognitive processes will also differ (3), for instance, using

a third-person approach (social observation using relevant stimuli) instead of a

second-person approach (social interactive context). (19)

Social Neuroscience, at first, used single-brain approaches. The aim of those studies

was to analyze the relationship between social stimulus and neural responses in a

third-person context. This means that the focus of the study was on intra-personal

effects only, neglecting the IPI dynamics (4) .The researches involved in those studies

had a high control of the experiment, data collection and analysis. The paradigms’

tasks included the observation of expressive facial photographs, social context

videos, interaction with a real or a perceived partner. Though they used a constrain

behavioral variance and no real interaction (3), this kind of approach allowed

researchers to understand fundamental interactive social processes and how they

are represented on brain and biological levels.(6)

Sequential dual brain studies are more recent than single brain approaches studies.

This higher complexity method enabled the comparison between two participants’

brain activities, even if they have been collected at different times. This kind of study

allowed researchers to study how influence occurs and eventually to predict one’s

brain activation triggered by the other, how they transfer information between them

and how they build shared representations. Within second-person approaches,

Hyperscanning is a technique that enables data collection from two or more

participants’ brains interacting simultaneously. This technique allows researchers to

compare participants neural activity simultaneously and study the dynamics that

emerge between them in real time. In the first Hyperscanning studies the paradigms

were tasks related to game theory that allowed tight controlled experiments. The aim

was to understand the underlying cognitive processes within an IPS and behavioral

correlates.(19)

The experimental paradigms used in IPS Hyperscanning studies with one or two

participants at a time are mainly divided in six tasks’ categories:(20) Imitation (e.g.

imitation of others’ movements or behaviors); coordination (e.g. movements like

walking or dancing); eye contact (e.g. tasks with and without eye contact); game

theory tasks (e.g. economic games like Trust); cooperation (e.g. cooperative tasks

where you have to achieve a goal with a partner) and competition (e.g. virtual games

with individual scores).

Even with the development of new second-person approach paradigms, many

components of IPI were still not practicable, behavioral components like gaze, for

instance.

Second-person approach studies mostly use IPS as outcome measure. Recently

experimental paradigms focusing on inter-brain coherence search for synchrony

mainly using four different task patterns: a common external stimuli like a movie that

causes synchronization; a unidirectional leader/follower approach; an interactive

leader follower environment; and a group situation. (6)

The studies focus have become increasingly sensitive to the demand for paradigms

that included real social situations. Nowadays we observe the increase of paradigms

that use real-time interactive and ecologically valid research scenarios (that can

duplicate real-life situations) to study cognitive abilities and brain functions in groups. (3)(29)(21)

While researching for this review, we observed that many studies have determined

similar paradigms. But while paradigms may be similar, objectives, methods and

results differ greatly depending on the chosen method and technique. For example,

in the classroom paradigm studies, the focus were on sustained attention processes,

Ko et al (26) used EEG technique while Brockington et al (27) used fNIRS. Although

both are neuroimaging techniques, they differ in terms of advantages, limitations and

therefore, results.

Our main goal is to group INS studies conducted with three or more individuals

simultaneously. Understand the different experimental designs. Understand how

researchers related their objectives to the choice of technique and methodology

employed in their studies. We also aim to study the relationship between paradigms,

outcome measures and results. Finally, our aim is to help new researchers to

compare the limitations and experimental advantages of each methodology and so to

make appropriate choices according to their own study objectives.

Techniques used for group studies

Hyperscanning methods – consist of neuroimaging techniques such as EEG and

fNIRS that can measure simultaneously brain’s activity from two or more subjects.

They allow researchers to study the co-variations of neural activities. Therefore, allow

the study of possible correlations with INS and behavioral/ biological correlations. (28)

fNIRS Hyperscanning -

The functional near-infrared spectroscopy (fNIRS) is a functional neuroimaging

acquisition method. The near-infrared spectroscopy (NIRS) method is grounded on

the fact that human tissues are relatively transparent to light from 650 to 1000 nm

wavelengths in the electromagnetic spectrum. The near infrared (NIR) light is either

absorbed by pigmented compounds (chromophores) or scattered. According to Delpy

and Cope (29) scattering is 100 times more probable than absorption and the relatively

high attenuation of NIR light in the tissue is due to the main chromophore hemoglobin

(the oxygen transport red blood cell protein) located in small vessels of the

microcirculation, such as capillary, arteriolar and venular beds.(30)

Thus, this technique offers the possibility to obtain information on the neurovascular

coupling, neurophysiological fluctuations and cerebral hemodynamics. Over time,

what occurs in the functional NIRS data is the variation of oxyhemoglobin and

deoxyhemoglobin concentrations. Based on the tight coupling of neural activity and

oxygen delivery (31), these variations in concentration are taken as indicators for

cortical activation. The common fNIRS signal observed after neural activation is a

decrease of deoxyhemoglobin accompanied by an increase of oxyhemoglobin

concentrations.(32) Therefore, it allows researchers to make sense of physiological

signals and find neural correlates of cognitive and motor processes.

However, we can only measure the relative variations in oxygenation. Absolute

values are not available, especially taking into account the different brain structures

through which light can cross. For functional hemodynamic measures, it is assumed

that the variations in concentrations are insufficient to disturb the light path through

the human tissue and can be generalized through the Beer-Lambert Law. However,

this equation is only valid in the absence of scattering. As the average light path

depends on the spreading properties of the human tissue, the modified Beer-Lambert

law can be used to take into account the diffusive propagation of light. (33)

In general the major advantages of optical methods are: the biochemical specificity;

high temporal resolution; the potential to measure intracellular/intravascular events

simultaneously; and the ease in which devices can be transported. (30) That allows

participants to have more movement freedom and consequently help researchers to

create ecological settings.(34)(35)

However, it has also limitations such as: the impossibility of investigating deep brain

regions, such as basal ganglia and amygdala; the struggle of separating the NIRS

signals of signals originating from extra-cerebral tissues; the impossibility of providing

information about brain structure for anatomical reference; the attenuation of the NIR

light by the layering and dark color hair.(36)

There are a few hyperscanning studies using fNIRS to evaluate the interaction of

more than two brains throughout different interpersonal situations, such as group

communication (37)(38)(39)(40)(41); coordinated group walking (42); drumming together (43)

and classroom interaction.(27)

Experimental design and implementation of fNIRS studies: fNIRS punctually

measures intracellular/intravascular events simultaneously, so that the researcher

can determine areas of activation over time. However, the exposure time must be

short. The tasks are performed in blocks with maximum intervals of 40 seconds,

normally interspersing rest periods (baseline information), experimental condition and

control condition. Outcome measures are associated with the synchronization of

brain signals between individuals in the group.

EEG Hyperscanning –

Electroencephalography (EEG) is a screening procedure that measures the electric

activity of the brain and so reflects its function. Metal electrodes placed on a cap and

applied to the scalp record the data. (44)

The EEG high temporal resolution allows perceiving the quick changes in neuronal

population electrical dynamics. Therefore it enables an adequate sampling. (45) It is

also a non-expensive and noninvasive technique that is good for ecological settings. (28) Therefore, it is the most commonly used method for Hyperscanning. (45) Besides

that, EEG has a limited spatial resolution which means no precision to perceive the

origin of the neuroelectrial signal. It is also less robust to movement artifacts than the

fNIRS technique.(28)

EEG captures the scalp signals that are propagating electric potential fluctuations

that results mainly from large populations of cortical pyramid cells’ postsynaptic

activity. External stimuli can modulate rhythmic and arrhythmic ongoing brain electric

activities. These oscillations are found in frequency bands (delta, 0.5-4 Hz; theta, 4-8

Hz; alpha, 8-13 Hz; beta, 13-30 HZ and gama, above 30 Hz). Each one of these

bands has its own characteristics in terms of topographic location, stimuli sensibility

and expression in pathologies.(44)

There are also a few Hyperscanning studies using EEG to evaluate the interaction of

more than two brains throughout different social situations, such as classroom

situations (26)(46)(47)(48); musical performance paradigms (17)(20)(49)(50) and more.

Experimental design and implementation of EEG studies: EEG tasks are also

performed in blocks. Initially with a rest period (baseline information), followed by the

experimental condition/ control condition. Exposure time varies from a few minutes to

up to an hour. Outcome measures are associated with synchronizing the oscillation

of brain frequency bands.

Biological markers: Skin temperature, Heart beat, body and eye movement -

Although they are not Hyperscanning techniques, wearable bio-sensing devices

(accelerometers, electrodermal activity (EDA) devices, heart rate (HR) devices) are

used to study physiological synchrony relating it to attentional processes, cognitive

processes, and others during IPI. The advantages are the low cost of the methods,

as well as their high applicability and portability. Although they indicate biological

synchrony and therefore interaction behavior across a group of individuals, they must

be used as complementar instruments to study the cognitive or psychological

processes that underlies social interaction.

Accelerometers are noninvasive and unobtrusive devices or app (installed on

cellphones) that measures body movement.(9)

EDA is the variation of the electrical characteristics of the skin. The sweat gland

activity controls skin’s moisture level that is related to its conductance. Therefore, the

increase in sweat gland activity is related to physiological and psychological arousal.

(16) EDA is also known as Galvanic Skin Response (GSR), Electrodermal Response

(EDR), Psychogalvanic Reflex (PGR), Sympathetic skin Response (SSR) and others.

For measuring EDA researchers use noninvasive unobtrusive wearable devices with

electrodes, like wristbands. They measure the low voltage electrical response of the

skin.

Some of the paradigms where researchers use these devices are: group discussion (14); classroom situation (10)(13)(16)(18)(51)(52)(53); music paradigms(54); movie paradigms(55);

audience paradigms(11); music and dance paradigms.(9)(15)(56)

Figure 1 – Example of paradigms: A- Classroom situation (EEG); B- Club environment

(Accelerometers); C- Group communication (EEG) and D- Musical performance (fNIRS).

Methods

For this review, we searched the following databases: PubMed (2011- 2020), Scopus

(2011- 2020). The following combined terms were searched: IPS, biological signals

synchrony, Galvanic skin response, Accelerometers, Heart beat synchronization,

Hyperscanning, Bio sensing devices, eye tracking. We found no Mesh results and

therefore we researched for text words. We selected 2011 as a cutoff year as it is the

year the first Hyperscanning study involving three or more participants was

published. The selection included only research articles in which the methodological

task included groups of three or more individuals together in collective activities. As

the researched articles presented great differences, regarding the testing protocols

and hypothesis it was not possible to control for severity. We conducted a manual

search on text words, keywords, references and citations in order to identify other

appropriate articles. We also considered review articles for the manual search. For

this review we only considered research articles written in English.

Results

Initially we found 153 studies. We excluded all studies in which the tasks involved

two or less individuals together simultaneously. The six remaining articles were

reviewed and their hypothesis, technical setting, methodology and results were

tabulated ( Table 1). The manual search resulted in other 28 studies. These studies

were also reviewed and their hypothesis, methodology, and results described at

Table 1.

Table – 1 – Studies included in this review – Resume including Paradigms,

Technical Settings, Hypothesis, Outcome measures and main results.

2. List of paradigms employed in group studies:

1) Music Paradigms:

a. Playing Instruments/ performing together

Babiloni et al (49) aimed to develop a methodological approach for acquiring and

storing neurophysiological data, EEG, eletrooculographic (EOG), Electromyographic

(EMG) signals while professional musicians played in ensemble simultaneously as

recording the environmental sounds. They hypothesized that EEG could be used to

identify "on" and "off" states during the performance which would allow the

investigation upon interaction X characteristics of the play. The subjects were a

professional saxophonists’ quartet that played together for 22 years. They played

Allegro (sonata I amoll L 223- from Domenico Sciarrino) that lasted 1.5 min after a 10

minutes training session.

Babiloni et al (50) in this EEG Hyperscanning study hypothesized that activities on

Frontal Brodmann areas were associated with emotional and cognitive empathy in

musicians playing in ensemble. Their sample consisted of 3 quartets of professional

saxophonists. Therefore they created a paradigm that included the execution of a

performance, the observation of their own performance and a control task that

consisted of watching a video in which they turned pages of a score. They played

Allegro (sonata I amoll L 223- from Domenico Sciarrino) that lasted 1.5 min after a 10

minutes training session. The results suggested that the activation of BAs during the

observation task reflects an "emotional" empathic process in musicians.

Duan et al (43) intended to create a “multi brain modeling method”, known as a Cluster

Imaging of multi-brain networks (CIMBN), validated by fNIRS. This CIMBN could

enable the measurement of multiple brains simultaneously in a realistic, face to face

social interaction environment. Therefore, they conducted a nine people drumming

together experiment and recorded the participants brains activities simultaneously.

They were asked to seat in a circle and make their best effort to create a consistent

rhythm, beating all together. The researches target areas were the prefrontal cortex

and the Temporoparietal Junction (TPJ) in a four channels probe fNIRS. The

researches also recorded the drumbeats of all participants using vibration

transducers. They hypothesized that CIMBN would be feasible and this cluster

imaging system would be functional for hyperscanning of multiple participants. The

CIMBN showed 9 brains linked during the drumming task. The results suggested a

diversity of roles played by participants and therefore can be useful to find neural

correlates of group IS and develop affective and social neuroscience.

Muller et al (17) in their experiment recorded EEG signals simultaneously from 4

professional guitarists playing in ensemble. Their goal was to study hyperbrain

networks based on interbrain (lower frequencies e.g.,theta and delta) and intra-brain

(higher frequencies. e.g.,beta) connectivity among them during different musical

situations. They hypothesized that situations that needed a greater musical

coordination would lead to interbrain increased activity, while situations in which the

guitarists would have differential roles would increase the intra-brain connections.

The task was to play two different music pieces that were chosen by a specialist

regarding the hypothesis: Libertango by Astor Piasola and Comme un Tango by

Patrick Roux. The conclusions showed complex interactions between the 4 brains

during the performance and pointed to the existence of mechanisms that support

temporally joint and coordinated actions.

Grecco el al (20), developed and experiment comprising empathy and leadership

towards brain activity using a 4 professional saxophonists quartet playing together

paradigm. Their goal was to study oscillatory brain activity possible synchrony during

the task. The protocol comprised a session where they played a 5 minutes musical

arrangement composed specially for the experiment (with a continuous interchange

of leader roles among the quartet) and a second session where they watched their

own previously recorded performance on a screen while their EEG signals and

electrooculogram were acquired. The results confirmed the hypothesis that the

instrument voice affects the synchrony of oscillatory brain-to-brain activity. Also

showed that Broca’s area is not important to characterize empathy trait and that

leadership can be determined by perceptual rather than empathy or conscious

attention.

b. Singing Situation

Muller et al (25) developed a choral singing paradigm to investigate hyper-frequency

neural network (HFN) arrangement connections using a graph-theoretical approach.

The researchers registered the electrocardiogram (ECG), respiratory movements,

and the vocal audio signals at the same time during a singing situation. Eleven

singers and one conductor engaged in choral singing (aged between 23 and 56

years) volunteered to participate in the experiment that consisted in three

experimental variations. The variations consisted in: singing in unison, singing with

three individual parts at regular intervals with open eyes, and singing with three

individual parts at regular intervals with closed eyes. The chosen song canon was

“Signor Abbate” in B major (by Ludwig van Beethoven) and the task lasted 5 minutes

for each variation type.

The results were reached calculating phase coupling between respiratory, cardiac,

and vocalizing subsystems across ten frequencies of interest. When the entire choir

sang together in unison, graph-theory metrics such as clustering coefficients, local

and global efficiency were lower if compared to singing in parts. However, in the

same comparison, characteristic path lengths were longer. That is an indicator that

the choir network is more separated and, at the same time, more attached when

singing the canon in parts. The results also showed that specific changes in the

network topology of HFN structures happened due to the singing condition.

Moreover, the graph-theory metrics showed up to be related to individual heart rate,

as an indicator of arousal, and to an index of heart rate variability indicated by low

and high ratio frequency. Those metrics also reflected the balance between

sympathetic and parasympathetic activity. Their main conclusion appoints that the

dynamics of the neural connections’ layout provide significant information about

mechanisms of social interaction and is a potential indicator for group social behavior

and group dynamics.

In a previous and similar study, Muller et al (24) made it evident that respiratory and

cardiac subsystems are synchronized to each other while singing and are coupled to

oscillatory vocalizing patterns and to the hand-movement fluctuations of the choir’s

conductor. In addition they found out that the cross-frequency connections of the

singing group is the strongest when singing a canon in parts. Contrariwise, within-

frequency couplings (WFC) are more pronounced when singing the same canon in

unison. To reach these results they analyzed the cross-frequency couplings and the

WFC of respiratory, cardiac, vocalizing, and motor subsystems by describing the

network topography of a singing choir.

In order for these studies to be possible, the same group of research had already

suggested that oscillatory coupling of cardiac and respiratory patterns provide a

physiological basis for interpersonal action coordination.(23) It occurred in an

investigation that was based on ECG and respiration measures of couplings of a

choir. Between their findings at the time were the following topics: phase

synchronization higher when singing in unison than when singing pieces with multiple

voice parts; it increases both in respiration and heart rate variability during singing

relative to a rest condition; directed coupling measures are consistent with the

presence of causal effects of the conductor on the singers at high modulation

frequencies; the individual voices of the choir are reflected in network analyses of

cardiac and respiratory activity based on graph theory.

Both studies were composed of five men and seven women, of whom eleven were

singers and one was a conductor. The canon chosen was also “Signor Abbate” in B

major (by Ludwig van Beethoven) in the same experimental variations as the study

cited earlier in the text.

c. Dancing / Club Environment Paradigm:

Ellamil et al (9) studied group synchrony throughout a dance club paradigm using

accelerometers. Out of a 75 individuals sample they recorded movement data from

46 participants while they listened to the music and danced. (33 female and 13 male

aged 20-49 old). The experiment lasted 31 minutes. The music list included a

sequence of well known discotech tracks. The researchers aim was to study

movement synchrony covariation in according to music, and if the previous

knowledge of the music could improve group synchronization. The researches

collected movement data using an accelerometer app on two mobile phones. They

allocated the phones on a running belt placed at waist level. The results suggest that

well-known music caused greater synchrony of movements and rhythm among the

group.

Solberg and Jensenius (56) also created a club-like setting in order to study social

interactions in relation to body movement and music. The experiment goal was to

investigate how people engage to each other and how musical features affects

individuals and groups. They used a 29 people sample divided in 3 groups. Their

paradigm consisted of dancing 4 different music tracks (electronic music) within a 10-

minutes period. 2 tracks had no structural development, the other 2 were structured

and had a break routine (with and without a bass drum). They hypothesized that

dancing together causes similarity of movements and social closeness; and, also,

that change in music dynamics would affect synchronization. They analyzed the 29

participants horizontal and vertical movement patterns and the overall movement

tendencies, They found out that the use of intensifying sound features and rhythmic

frameworks affects group sensorimotor synchronization. The results suggest that the

musical feature influences our movements but also the way we interact with others.

Chauvigné et al (15) study’s aim was to investigate the mechanisms of group

coordination and cooperation. Therefore, they developed a dancing paradigm and

measured the coordination dynamics of the group towards multisensory coupling -

Auditory, haptic and visual coupling. Their participants were all folk dancers (around

70 years old) and they performed under 4 conditions (as usual, to auditory, holding

hands and with eyes opened) two Greek folk dances with different step level of

complexity. They formed a closed circle with the groups’ teacher in the center. They

hypothesized that sensory coupling plays an important whole on stability of

interpersonal coordination. Their results showed that the group relied mostly on the

haptic information to synchronize movements and that visual and auditory

information are spatio-temporal context-dependent. Their results also suggest that

sensory couplings support multi-person coordination in groups and opens the

possibility of a deep understanding on group dynamics in a naturalistic context.

d. Listening to music

Auditory Paradigm: Jaimovich et al (54) study is about emotional contagion. Therefore,

they investigated the influence of music performers emotional and mental states on

an audience and vice-versa. The hypothesis was that emotional responses can be

triggered during a performance. They carried out the experiment in live performance

environments and recorded data from two different 9 subject groups each and 3

performers. The musical program included a piano performance (12 min) an

interactive electronic piece (12min) and an electroacoustic piece (25min). The

researchers collected HR and GSR data and found that there is a correlation

between the biosignals of the audience and the momentum/music characteristics of

the pieces. They also found a correlation between biosignals of performers and

audience. Although emotional contagion´s mechanisms are still undefined, this study

presented a novel approach to its study.

Rhythm paradigm: Ikeda et al (42) also used rhythm as outcome measure in their

fNIRS study. They hypothesized that INS variation would be dependent on the

modulation of coordination related to cognitive processes, therefore, a steady beat

sound would facilitate INS and coordinate a group walking. They experimented a 4

group of 24 or 25 participants each. The task was to perform a walking in circles

experiment, with and without a metronome´s steady beat sound. Each group

performed two 4 minutes blocks, one walking, one stepping (control condition) and

three 30’ resting blocks. The researches target area was the frontopolar region in a

two channels probe. The researchers found that steady beat sound increased INS

and improved the walking flow what suggests the diffuse nature of group

coordination.

2) Classroom Paradigms :

MacNeal et al (16) investigated the relationship between emotional state and

engagement in a classroom environment. They used the GSR methodology in 17

subjects of a 24 undergraduate students’ introductory environmental geology course.

The course was focused on environmental issues related to climate change and was

composed of different learning approaches such as lectures, movies, students’

presentations and discussions. They expected differences of class engagement

measured by skin conductance response due to the pedagogical approach. The

researchers’ hypothesis was that active learning approaches would cause increase

of in-class engagement reflected on skin conductance data. They also hypothesized

the increase of students’ knowledge about the matter. The study suggests that active

learning such as move viewing and dialogue are more engaging than passive

learning approaches such as lectures.

Wang and Cesar (51) had also investigated students’ engagement. Their aim was to

measure engagement in a distributed learning environment, students on classroom

and remote location students at the same time. The experimental design consisted of

34 students, 17 students in a lecture classroom and 17 in a remote classroom

watching a normal scheduled class on Structured Query Language before an exam.

They hypothesized that students in remote class would be less engaged than

students in real classroom and this would reflect differences in bio-signals. The study

resulted in non-engaging learning experiences with different recording patterns of

other studies with engaged students. The results suggested that GSR sensors can

be used to measure engagement.

Senthil et Wong (18) studied the correlation between students’ heart rate and their

engagement during lectures. They have chosen a 30-student second semester class

in an Indian university. Previously they collected baseline data from each participant

(resting and stimulated heart rate), then they chose a random student to have the bio

signals recorded during a one-hour lecture class. Their hypothesis was that there

would be significant differences between heart rates from students academically

performed and those who did not. Researchers concluded that due to the confirmed

correlation of heart rate and engagement, this study can be useful to pedagogy as an

instrument for developing more engaging teaching methods.

Ko et al (26) developed a classroom environment sustained attention paradigm using

EEG Hyperscanning technique. They wanted to investigate possible relations

between activities that causes impaired behavioral performance and brain activity.

They had an 18 university students’ sample (11 males, 7 females) during lectures on

a regular academic semester. Their outcome measure was the level of visual

alertness. They created a visual targets recognition task in which geometric objects

(triangle, square, circle and star), where shown on screen during the lecture (one

object per minute - randomized intervals). When participants noticed the stimulus

they were asked to press the correspondent smartphone button. The response time

was determined as the time between appearance of stimuli and button pressing.

Their hypothesis was a correlation between variation in EEG spectral dynamics and

performing attention related tasks. The results confirmed their hypothesis.

Dikker et al (46), aim was to study brain synchrony during a face to face interaction.

They leaded an EEG study with 12 high school students over 11 regular classes.

Their outcome measure was brain to brain synchrony of neural activities. The

experiment was developed to investigate whether neural synchrony could be

considered a marker for social dynamics and classroom engagement, both of them

critical for the learning process. They started the semester with a course in

neuroscience, after that, along the regular semester, the students developed

research projects and their brains were recorded. In this study teaching styles,

personality traits, affinity and empathy were considered while investigating synchrony

across the group: student-to-group, student-to-student. The researchers concluded

that groups social dynamics and classroom engagement could be predicted by

students’ neural synchrony.

Brockington’s et al (27) fNIRS study aim was to investigate students learning

processes while attending a classroom lecture. They hypothesized neural synchrony

would occur “whenever similar attentional and arousal states were attained during

the lecture” (p.3), and time would be a key factor for sustained attention process. The

longer the lecture, the harder it would be for the students to sustain attention. They

also hypothesized that visual contact and engagement were related in INS. They

experimented it in a four students’ group, the target area was the frontal cortex and

used a probe 4X4 set (8 channels). The researchers confirmed their hypothesis and

results suggests that teacher-students eye contact during classes are important to

the students attentional and learning process.

Bevilacqua et al (47) EEG technique experiment is about 12 senior students and their

biology teacher during their regular classes in a New York high school. The

researchers aim was to study the relationship between classroom learning and

students-teacher, and students-group relationships. Therefore, they focused on

attentional processes and the underlying neural activity of social interaction to

develop a paradigm. The procedure lasted six classroom sessions (80 minutes each)

on a three-month period. They also applied questionnaires. They developed 3

baseline conditions (2minutes each), facing the wall, facing a partner and facing the

group. They recorded the baseline conditions of all participants before recording the

lesson, in which the data was collected simultaneously. After the lessons, students

were tested about the lesson content. Researchers outcome measure was brain to

brain synchrony. Their results showed that there’s a higher engagement for video

classes compared to lectures. Teaching styles and teacher’s likeability also affects

lesson content retention.

Pijeira-Diaz et al (13) focused on collaborative learning, their study goal was to

investigate the dynamics of this teaching method in the classroom. They used the

synchrony of sympathetic arousal as an outcome measure for engagement in 8

Finnish high school students’ triads. The paradigm was watching an 18-75-minutes

classes Advanced Physics course. The lessons had a theoretical part followed by a

group collaborative practical part. They hypothesized that triad members would have

synchronic moments, that there would be a difference of biosignals among

influencers and influenced students and that Eda can predict arousal contagion. The

results showed that only during small percentage of the lessons the triads were

simultaneously in a state of activation. Contagion occurred not for all, and not at the

same time on triads. They concluded that collaborative work is not an automatic

result of group work and that EDA can be used to study collaborative processes.

Di Lascio et al (10) wanted to measure students’ emotional engagement towards

classroom situation. Therefore, they used EDA sensors to monitor 24 students and 9

teachers’ physiological parameters over 41 lessons in a 3 weeks’ time period. Their

paradigm included a set of features to represent three observable physiological data

phenomena that indicates emotional engagement (general arousal; teacher/student

physiological synchronicity and momentary engagement). They hypothesized that it

is possible to define methods that capture students’ engagement during classes.

Their aim was to allow students to self-monitor themselves and increase their scholar

performance, as well as permitting teachers to benefit from the non-engaged

students’ information.

Gashi et al (52), using previous data from Dilascio et al (10) studied emotional climate in

a classroom paradigm throughout the physiological synchrony (PS) of 24 students

during 42 lectures. Their hypothesis was that PS can be an indicator of emotional

climate among students in classroom. They found an increment on students’ OS that

is reflected on their EDA.

Zhang et al (12) in their study collected EDA and HR signals (with a wristband) from

100 seven graded students during two weeks of math and Chinese classes. Students

were from 3 different classrooms. There was a total of 72 classes (40 minute

sessions) for each group of students, 36 math classes and 36 Chinese classes. They

also collected self-reports from students in each lecture and used the student’s final

exam scores to measure academic performance. Their goal was to verify whether

physiological data could be compared to self-reported data on academic

performance variation matter. They found correlations between students EDA

responses and academic performance.

Davidesco et al (48) hypothesized that brain-to-brain synchrony could predict individual

outcomes such as memory retention. In this EEG study their sample was composed

of 11 groups - 4 students each and a teacher groups in a laboratory classroom. Their

paradigm consisted of 4 teacher led lectures, preceded by a brief pre lecture or no

activity, when they could interact with the group freely. The students were tested

about content knowledge of that session 1 week early, after the lecture and 1 week

following. The student/teacher brain-to-brain synchrony was not considered. They

concluded that INS among students (listeners) can predict delayed memory

retention. They also concluded that synchrony can predict immediate and delayed

memory retention. And the variations could discriminate the achieved and not

achieved information by the students.

3) Communication Paradigms :

Jiang et al (37) intended to investigate the role of INS in human communication.

Therefore, they developed a fNIRS paradigm focused on the emergence of

leadership through INS in eleven triads. They were sat in triangles and the task was

to perform a “leaderless group discussion”. They were asked to discuss about a

theme for five minutes and then elect a leader to talk about their conclusions. Their

target regions were left frontal, temporal and parietal cortices, specifically TJP and

Inferior Frontal Gyrus (IFG). They used a probe of ten channels. All the experiment

was filmed by two cameras and the behavior and nonverbal communication was

analyzed. They hypothesized that there would be differences on interpersonal neural

communication between leader-follower (LF) pairs and between follower-follower

(FF) pairs. INS between LF pairs would be higher than INS between FF pairs and

that the INS in LF pairs would activate the social mentalizing and language related

areas. Actually, differences of activation were found between the LF and FF pairs.

For the LF pairs there was an increase of the INS at the left TJP. The study suggests

a correspondence of the emergence of leadership in people that have the ability of

saying the right thing at the right time.

Nozawa et al (38) created a cooperative fNIRS paradigm to study communication and

neural synchrony. They used twelve groups of four participants that were acquainted

to each other. The task was a cooperative word chain game where the participant

says a word, then the next participant uses the last two syllables of that word to say

another word. The paradigm was developed in two sessions of three blocks, two of

them were the communication task and the other one was resting state. The

researches target area was the frontopolar region in a two channels fNIRS probe.

They hypothesized that cooperative and unstructured verbal communication

enhances frontopolar INS and that there is a timescale dependency in the INS

modulation. Their results confirmed the enhance of frontopolar INS and the timescale

dependency in the INS modulation. This study also set parameters and technical

basis for new hyperscanning studies involving groups and communication.

Zhang et al (53), decided to study leadership in a group communication paradigm

using EEG. Their aim was to evaluate brain activities of the group members

perceiving differences in leader/followers activations during group discussions. For

that they developed a 4 blocks task. In the first block participants received a topic

and had a 20 minutes free discussion. After the discussion they elected a leader to

report the results. The second block was a leader established discussion. The third

block was a building block game task in which participants had to simultaneously

construct the same building and discuss the process. The fourth one consisted on

the same task but the leader was elected by the group. Their results showed that the

left hemisphere activation of the leader was much higher than the followers. They

also concluded that there's an indication that followers summarize consciously and

unconsciously the tasks and leader’s orders, and leaders can understand other’s

thoughts fastly and adapt the speech to satisfy their expectations.

Dai et al (39) study was based on the investigation of INS in a noisy multi speaker

situation in 21 same gender adult triads. Participants were sat in a triangle. One was

randomly named the listener. Two tasks were developed. On the first one, only one

speaker would talk to the listener. Then the two speakers would talk simultaneously,

and the listener should pay attention to only one of them. The task was performed in

a face to face condition and in a back to back condition. The experiment was made in

three blocks, one of them, a resting pause and the other two, different tasks. The

researches target areas were the left frontal, temporal, and parietal cortices in a ten

channels fNIRS probe. Their hypothesis was that in a multi speaker situation the INS

is a neural mechanism for selective processing of the target information. Their results

showed a selective enhancement of TJP-TJP INS for listener-attended pairs in all

conditions and the increase of INS between the listeners’ posterior superior temporal

cortex and the attended speaker’s TJP.

Lu and Hao (40), studied INS during a collaborative fNIRS task using the cooperative

interaction hypothesis and the similar task hypothesis. 44 participants in dyads that

did not know each other before, were randomly assigned to interact with a

confederate (a fake participant, an experimental assistant). They were asked to

discuss a topic for 5 minutes in collaborative brainstorming task. The rules were to

answer one at a time in a clockwise order. The researchers target area was the

prefrontal cortex and they developed a 22 channels probe set. Behavior and

cooperation index were also measured. The researchers hypothesized differences of

neural synchrony between different dyads in the groups due to the use of cooperative

or similar tasks. They found different individual engagement, a high INS among real

participants and low INS among participants and confederate.

Lu et al (41) in this fNIRS study intended to evaluate how different feedbacks should

affect creative performance in a brainstorming group. 59 dyads that did not know

each other before, were randomly assigned to interact with a fake participant (an

experimental assistant that acted as an evaluator). They were randomly assigned for

one of the three conditions: positive feedback (20 dyads), negative feedback (19

dyads) and control condition (20 dyads). They were asked to discuss a topic for 5

minutes in a brainstorming task and the feedback was given by the evaluator. The

researchers target areas were frontopolar and bilateral dorsolateral in the prefrontal

cortex. They developed a 22 channels probe 3 X 5 set. Behavior and cooperation

index were also measured. They hypothesized that social interaction affects group

creative performance and different types of feedback will generate differences on

INS. In their results they found that INS showed up to be higher when feedback was

positive rather than negative. The conclusion was that creativity showed to be

suppressed when the group received a negative feedback.

Capozzi et al (14) used eye tracking methods to study leadership in group interactions.

They hypothesized that social gaze behavior is a marker for leadership during social

interaction. They used a 64 people sample (16 groups of 4 strangers each). In each

group one participant was designated the leader. And within a limited period of time

they had to solve a survival task. The participants sat on equidistant chairs in a circle

and had four video cameras recording the experiment. They created 4 leadership

settings according to democratic and autocratic leadership styles or situational

condition (low and high pressure situation). The results suggest that social gaze

behavior can be used to identify leaders in a group.

4) Audience Paradigms:

Wang and Cesar (55) tested the efficiency of GSR sensors to evaluate audience

experiences towards different commercials. They used a 15 people sample in a video

watching paradigm. The subjects were exposed to audio tracks of a Starbucks'

commercial under three conditions, a mute condition, the video with an up-tempo

music audio and the video with a ballad music audio. The differences found in GSR

data analysis in comparison to surveys showed that the use of GSR can be specific

on delivering audience attentional level data and therefore, GSR can be useful to

publicity market since it can report the users' experience more precisely than

surveys.

Gashi et al (11), developed a field study. They used EDA sensors to collect data from

17 presenters and 6 listeners in an auditory environment during a 2 days conference

on Energy Informatics. They also collected IBI traces and applied questionnaires to

investigate the relation between physiological synchrony and attentional processes

among presenter and audience. Their aim was to compare signals between audience

and presenters and study satisfaction, immersion and engagement. Their results

suggest that physiological synchronicity can be used as a proxy to measure

engagement and satisfaction and therefore is a good method to provide audience

feedback in lectures, conferences, classroom, etc.

Future Directions

The study field of neural and behavioral mechanisms related to group interaction is

still novelty and a promising research area. Hyperscanning techniques such as fNIRS

and EEG, as well as the use of eye tracking and other measuring devices for

biological signals have allowed more ecological settings and therefore more freely

forming group interactions. This improvement enriched researchers’ ability to study

neural mechanisms and behavior within IPI. However, most of Hyperscanning

studies have as an outcome measures of synchronicity and mirroring-based analysis,

which occurs not necessarily due to IPI and are just possible biological signatures of

the dynamic process of interaction.

Due to technical limitations tasks and set-ups are still constrained, so that to fully

understand the emergence of interpersonal dynamics we still have to develop more

portable, affordable and integrated devices. We also have to look forward to more

robust analysis tools and processing algorithms, as well as signals and data

processing and artefacts removal, making them more replicable and scalable for

other studies.

The future signalizes for multidimensional approaches. With the development of

Hyperscanning techniques that are able to capture spatial and temporal data at the

same time, therefore allowing researchers to study similarities and differences across

individual brains during a free social interaction, characterizing inter-brain

organization from interpersonal behavioural, reciprocal influence process and the

construction of joint representations.

Also, the functional implication of synchrony will be better understood and delineated

in an integrative approach of techniques that study inter-brain coherence with

biobehavioral synchronicity devices.

Acknowledgements

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