Brain-to-Brain Synchrony: How Neuroscience Decodes Trust, Rapport and Attachment

two women dual EEG brain-to-brain synchrony while playing chess

Because we’re social creatures, we tend to synchronise our actions and emotions (or at least the ones we choose to express) with the people we’re surrounded by.

Physiological synchrony helps us feel we’re more strongly connected with other people. When people sing in a choir, their heartbeats synchronise. When dancers move together in a troupe, their breathing patterns match (and remarkably, so do the breathing rates of audience members watching the dancers!).

Synchrony may be another of Mother Nature’s tricks to ensure we bond and trust other people — it’s like the social glue that holds groups together and enables entirely genetically unrelated people to cooperate.

Collective Effervescence: The Joy of Togetherness

The energy or ‘vibe’ you feel when you’re singing the words to your favourite song at a concert or celebrating a World Cup win with thousands of strangers is natural and related to group synchrony.

Collective effervescence is a phrase that describes the “joy of gathering” — the high you experience, the positive emotions and the feelings of togetherness we experience in groups.

First described in religious communities, “a sort of electricity is formed by their collecting which quickly transports them to an extraordinary degree of exaltation,” collective effervescence is familiar to anyone who’s lost their sense of self and felt part of something bigger.

“Collective effervescence is the synchrony you feel… We should think of flourishing less as personal euphoria and more as collective effervescence. You can feel depressed and anxious alone, but it’s rare to laugh alone or love alone. Joy shared is joy sustained.”

Adam Grant. New York Times 2021.

What Happens When We Truly Connect?

Human beings are social beings, but rather strangely, other people have largely been absent from social neuroscience. We’d never dream of studying fish out of water, bees living away from their hive, or lions hunting antelope in a cage in the zoo. But we’ve mindlessly studied the human brain in solitary confinement for decades.

The Lonely Study of the Human Brain: A Shift in Neuroscience Perspective

To help understand the social brain, the lone volunteer typically lies motionless inside a brain scanner. Inside the tightly confined space, they’re shown photos, videos, or audio recordings of other people while their brain’s reactions are monitored, or they’re asked to (without moving) report on the mental or emotional state of the people they’re watching on screen.

Other times, the lone figure lies daydreaming while scans of their default mode network are collected to gain a sense of self at rest. This type of traditional neuroscience research has given us plenty of insights into the brain, but it’s very different to studying how human beings and their brains interact ‘in the wild’.

As a brain scanning tool, fMRI is great at showing ‘where’ in the brain something happens, but EEG shows precisely ‘when’ something in the brain occurs.

EEG has several additional benefits. It enables people to move around, have conversations and experience each other in everyday life (or at least, within the everyday research lab). And most excitingly for social neuroscientists, EEG recordings can be made from two brains simultaneously. This is such a new field of research that no one has quite yet landed on a shared vocabulary. Thus, the method goes by various terms such as “dual-EEG”, “hyper-scanning” or “second-person neuroscience”.

The Dance of Dual-EEG: Unveiling Brain-to-Brain Conversations

Neuroscientists are having a lot of fun using dual-EEG to show that when two people move their bodies in time, pay attention to the same book or movie, or have a conversation, their brainwaves also line up.

They’ve made simultaneous recordings from the brains of pairs of jugglers, guitarists performing a duet, two people humming the same tune, people in conversation either face-to-face or back-to-back or when one person lovingly strokes another’s head or holds their hand (what happens if that person is a stranger!).

Rapport and trust are core to the practice of psychotherapy, and two-person neuroscience has also made its way onto the therapist’s couch. Harmonisation of brain signals is observed between patients and their therapists, especially when trust and rapport are high.

But most importantly, EEG is kid-friendly, and a cohort of researchers have turned their attention to recording brain-to-brain synchrony between parents and their offspring.

What directs two brains to synchronise?

In a world full of people, why do we synchronise with some and not others? And what orchestrates the neural harmony between parents and their children?

These are questions that drive Victoria Leong, a cognitive developmental neuroscientist who splits her time between Singapore’s Nanyang Technological University and the University of Cambridge, where she heads up the Baby-LINC (Learning through Interpersonal Neural Communication) Lab.

Whip-smart and fast-talking, Leong tells me she’s interested in the role parents play in building brain architecture — what tunes an infant’s neural circuitry, and in particular, “How we tell a child without using words that ‘I’m here. I’m online, and I’m ready to connect with you?”

“Synchrony is a ubiquitous principle of social group living. It’s how we connect to other human beings at a social level. We see synchrony when we dance, when we chant together, when we march together. And, when I’m speaking, and you’re listening, that’s another form of synchrony.”

Victoria Leong.

How Simple Glimpses Shape Brain Connections

In her Cambridge lab, Leong has created a warm, cosy natural habitat where she observes and gathers data from families.

In one study published in the journal PNAS in 2017, Leong showed eye gaze is key to orchestrating brain-to-brain synchrony.

She figured this out by simultaneously recording EEGs from mother-infant dyads while they sat together singing nursery rhymes such as Wheels on the Bus and Twinkle Twinkle Little Star. Sometimes the mother made direct eye-to-eye contact with the baby, and other times she’d gaze away into the distance.

When Leong analysed the EEG data, the result was clear: when mothers gazed directly into their baby’s eyes, their brain waves aligned. When mothers broke their gaze and looked away, their brain waves fell out of alignment.

But the babies didn’t put up with being ignored — they used their mismatch-repair bag of tricks to return their mother’s attention. And infants who ‘sang’ louder and longer to get their mother’s attention more quickly brought their Mum’s brains back into harmony with their own.  

Leong’s work was the first to show that direct eye contact is the social cue that brings two bodies and brains into synch. As she tells me over a Zoom call from Singapore,

“It’s not ESP!”

Victoria Leong

Synchronised brains are not mirrors; they’re tango-dancing partners.

Leong is quite careful to note that one brain doesn’t simply ‘mirror’ or ‘mimic’ another brain. The analogy she gives is dancing the tango with somebody. “You’re not doing the same thing at the same time,” she says. “That’s line-dancing.” During a tango, someone leads, someone follows, and your actions are not identical; they’re coordinated.

“A conversation is another example of neural synchrony.”

Victoria Leong

Leong is a neuroscientist after my own heart. She tells me that while she’s interested in the “rich narrative descriptions” of the mother-infant ecosystem, she likes gathering, quantifying, and analysing the neurobiological data underlying mother-infant synchrony.

Sometimes this type of research is criticised as reductionist — that the lived experiences of motherhood are about much more than two brains hooking up.

However, as Leong points out, brain-to-brain synchrony is a requirement for social and emotional learning.

She aims to show how and why serve-and-return interactions, warm, sensitive parenting, reading books and telling stories are prerequisites for building little brains. And she’s also curious about what happens to relationships if you tweak or artificially manipulate patterns of activity in one brain or another. Also, can you promote synchrony or feelings of rapport and trust by tweaking brain waves?

“I was awestruck, like never before. And following the awe, I felt an invisible, but strong thread tethering my heart to this little baby – forever – like I was fated to protecting and caring for its future. My sense of time was as if the present and forever were the same.”


The Allure of Direct Eye Contact: Unlocking Brain Synchronisation

The type of experiment that modifies or tweaks a brain can’t be done in a human adult, let alone an infant. Instead, Leong is exploring this paradigm in rodents using a neat biological tool called optogenetics. She’s developing an optogenetic mother-infant mouse dyad where she hopes to, with the “flick of a switch”, see what happens when you turn on or turn off synchrony between the mothers’ and the pups’ brains.

Newborns currently have their hearing checked before they leave the hospital so we can intervene for deafness as soon as possible.

Leong hopes to follow suit and detect alterations or developmental delays in neural synchrony very early in childhood, even before infants start to show problems with social behaviour.

She’s partnered up with machine learning and technology companies in Singapore to develop ‘smart toys’ and wearables that will help her capture moment-by-moment brain synchrony when families are at home instead of those willing or able to visit a research lab. Leong says it could “revolutionise” mother-infant attachment if it works.

“When this research is complete, we will have a better understanding of the specific brain pathways and processes through which responsive caregiving and parental behaviour operate to scaffold early neurodevelopment and cognitive abilities in children.”

Victoria Leong

The Irresistible Aroma of Newborns

Ask any parent, and they’ll tell you there’s nothing quite like the scent of a newborn.

For every parent, the aroma of their baby’s head is nearly addictive. And, just like sheep, 90% of women can identify their newborns by their smell alone within an hour of meeting their birth.

“I remember their smell, like an amazing perfume that I felt I knew before they were even there. I fell in love with them on sight.”


Babies recognise our body odours too.

The scent of their mother’s milk helps newborns nuzzle around and find their way to the nipple. Breastfed babies quickly learn their mother’s distinctive olfactory signature and can thereafter identify her only by smell.

Olfactory Cues: Lingered Memories and Connection

Everyone’s body odour has a unique signature containing information about our genes, emotions, food we’ve eaten and who we live with (I’m sure my dog would be nodding his head vigorously in agreement).

But here’s something else you may not have considered about body odour — it’s the only biological signal that lingers after someone leaves the room.

It’s thought dogs use the fading of body odour in the house to gauge how long their owners have been away.

Dogs smell the passage of time!

Delving into Social Chemosignals: Can We Smell Emotions?

The notion that smell lingers on after someone has gone led Yaara Endevelt-Shapira, a researcher in the Feldman lab, to wonder if scent or ‘olfactory cues’ are involved in establishing brain-to-brain synchrony.

For her PhD research, Yaara Endevelt-Shapira showed that social and emotional cues are transmitted via “social chemosignals”.

To see whether people could “smell fear”, she gathered the armpit sweat of first-time skydivers using cotton swabs she stored. (When I asked her if she also took up the opportunity to skydive, she responded firmly with, “No, no, no!!”).

Later another set of volunteers, who had no idea they were breathing in the social chemosignal for fear (skydiver sweat) had their own stress responses measured while they breathed through a mask (they assumed the mask tracked their breathing rate, when it was actually a stealth BO delivery device).

In response to breathing in the skydiver’s sweat, the people showed increased galvanic skin responses, a reliable indicator of autonomic arousal and anxiety.

In short, humans can detect social chemosignals and “smell fear”.

The Unconscious Sniff: Shaking Hands and Its Aftermath

Incidentally, Endevelt-Shapira has also observed that people unwittingly sniff their hands after shaking hands with a stranger (an observation I find slightly stomach-churning), and that the oral contraceptive pill alters our ability to detect human chemosignals.

But most importantly, she’s shown human body odour helps coordinate brain-to-brain synchrony between mothers and infants.

Mother-Infant Bond Through Odour: A Research Perspective

Working as a post-doc researcher in the Feldman lab, Endevelt-Shapira (no stranger to body-odour gathering techniques!) designed a clever research paradigm to capture maternal body odour.

She instructed mothers to sleep in a new 100% cotton T-shirt for two nights, then store the cotton shirt in a sealed jar in the freezer till their lab visit (thus preserving the maternal chemosignals).

When six-month-old babies visited the lab, dual EEG recordings were made with their mothers or strangers (unknown mothers who lived in the same neighbourhood).

As you’d expect, brain-to-brain synchrony was more robust between mother-infant dyads versus unfamiliar mother-infant dyads.

However, given their mother’s smelly T-shirts to play with, babies’ brains synchronised up with the strangers’ brains. In the absence of their mum, but with her smell lingering close by, the babies synchronised and appeared more social, happier and less anxious with the stranger.

The Role of Body Odour in Establishing Trust and Safety

One idea Endevelt-Shapira has is that body odour may “transfer” a sense of calm and safety.

“It was as if their mother was present in the room when she wasn’t, and the cues from the mother are saying to the baby ‘This is OK.’”

Yarrah Endvelt-Shapira

The practical implications of this finding are exciting. When sending a baby or toddler off to daycare for the first time, the transition will likely be easier with a favourite teddy or blanket that has the smells of home. After all, this is common advice for new puppy owners!

“Infants may benefit from the scent of their mother’s body to ease the transition to new social groups, surroundings, increasing their social repertoire in order to better survive and thrive in their new circumstances.”  

Yaara Endevelt-Shapira.

Endevelt-Shapira’s attention has now turned to measuring brain-to-brain synchrony during breastfeeding and bottle-feeding. Data analysis is “very difficult” says Endevelt-Shapira, because of the movement artefacts caused by the baby’s jaw moving as it feeds.

But the rough data from breastfed babies are striking. So much so that Ruth Feldman has said,

“It’s almost like one brain.”

Ruth Feldman

I can’t help but wonder what parents who, for hundreds of valid reasons, choose to not to breastfeed think about “one brain”. Jodi Pawluski who interviewed Feldman on this issue asked about bottle-feeding parents. “There’s a very high level of synchrony,” Feldman states, “The child is not losing out.”

The Heartbeat Behind the Data: Real Stories of Pregnancy and Parenting

I’ve been fortunate to gather stories from over 270 warm and generous humans from around the world who shared their experiences of pregnancy and parenting with me via online survey. Their honest words and stories have added heart and soul to a story about science. Some of their words are included in this post.

In early parenting I was quite calm, in love with my baby, and simply amazed at everything around me. I think my brain tried to synch with my baby’s brain by seeing everything as though for the first time. I had a beginner’s mind.

This is an edited excerpt from: Baby Brain: The surprising neuroscience of how pregnancy and motherhood sculpt our brains and change our minds (for the better).

You can source ebook versions and Audible versions globally. See here for more information.

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  1. John Mekrut on October 11, 2023 at 6:49 am

    Terrific article! We have a neurofeedback technique called “couples synchrony” which rewards two brains when they move into synchronous patterns. As you say, not mimicry but separate coordinated contributions to a shared reality.

    • Sue Brierly on October 13, 2023 at 7:18 pm

      Really fascinating… Re the gaze and parent child synchrony… Wonder if this may be disrupted or altered with an autistic child who avoids eye connection.

      • Sarah McKay on October 17, 2023 at 9:35 am

        The reference to the Yaara Endevelt-Shapira work on ‘sky diver sweat’ included people with autism. I recommend you start there.

  2. Enrique Margery on October 17, 2023 at 5:08 am

    G’Day from Costa Rica! I would like to know if this eye gaze -key to orchestrating brain-to-brain synchrony- is possible in the context of an online classroom, where we all see each other in the little boxes on the screen? If yes, how can we improve it? If no, what are the implications of this obstacle in our online learning spaces?

    • Sarah McKay on October 17, 2023 at 9:33 am

      Eye gaze is impossible to replicate via screens – you either look into the camera or look at the other person’s face.
      There is a study that looked at brain-to-brain synchrony between a mother and child (so plenty of rapport) that found communicating via a screen attenuates the synchrony.
      Schwartz, L., Levy, J., Endevelt-Shapira, Y., Djalovski, A., Hayut, O., Dumas, G., & Feldman, R. (2022). Technologically-assisted communication attenuates inter-brain synchrony. NeuroImage, 264, 119677.

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