Scientists got strangers to paint together — and six weeks later, they discovered a strange effect

Among young people of the same age, brain activity gradually became more synchronised. In pairs consisting of a young person and an older person, however, the opposite occurred: the longer they spent time together, the less synchronised certain areas of the brain became — even though the individuals themselves felt an ever-increasing sense of closeness. The result casts doubt on the simple notion that good relationships necessarily cause two brains to operate increasingly ‘in sync’.

Details

The experiment involved 122 people, from whom the researchers formed 61 pairs.

In 31 pairs, the participants were from different generations: one was a young adult, the other an older adult. The younger participants were aged between 18 and 35, whilst the older ones were aged between 70 and 85. A further 30 pairs consisted of two young people. The partners did not know each other before the study began.

The experiment lasted six weeks. The participants met six times and carried out a creative task on each occasion: first they drew on their own, and then they created a drawing together with their partner.

The researchers did not specify any particular themes. Participants were free to draw whatever they liked and decide for themselves how to work on the joint picture. During these tasks, the brain activity of both participants was recorded simultaneously using portable fNIRS systems.

fNIRS — functional near-infrared spectroscopy — allows changes in the concentration of oxygenated and deoxygenated haemoglobin in the cerebral cortex to be monitored.

Put simply, the device records changes in blood flow associated with activity in specific areas of the brain.

The researchers were interested in what is known as ‘interpersonal neural synchrony’. They investigated the extent to which the brain signals of two interacting people change in a similar way over time. The analysis covered regions of the inferior frontal gyrus and the temporo-parietal junction — areas involved in social cognition, the perception of others’ actions and social interaction.

The first finding was fairly predictable.

When two people drew together, the synchronisation between certain regions of their brains under investigation was greater than when the same participants carried out the task independently. Moreover, this pattern was observed in both same-age pairs and intergenerational pairs.

This is consistent with the results of many previous experiments: when people work on a shared task, monitor one another and coordinate their actions, their brain activity can become more synchronised.

But the researchers were far more interested in something else — what would happen after a few weeks of working together.

The second finding turned out to be unexpected.

The closer the people became, the stranger their brains behaved.

The researchers had initially envisaged a fairly straightforward scenario: strangers gradually get to know one another, it becomes easier for them to interact, and brain synchronisation increases over time.

And indeed, the relationships between the participants did develop in the direction of greater closeness.

With each subsequent meeting, measures of subjective social closeness increased by an average of around 4 per cent, with this trend observed in both groups. At the same time, measures of loneliness decreased by approximately 1 per cent per session.

In other words, the repeated meetings worked: the strangers gradually felt a stronger connection with their partner.

However, brain synchronisation did not follow the same pattern at all.

In pairs consisting of two young adults, certain measures of inter-brain synchronisation increased with each meeting.

In contrast, in ‘young + older’ couples, the opposite occurred in several brain regions: synchronisation decreased. This was particularly evident in connections with the right temporo-parietal junction.

This created a paradox.

The young and older participants got to know each other better and better and reported a growing sense of closeness — but this did not cause their brain activity to become increasingly similar.

It is precisely this result that prevents us from viewing inter-brain synchronisation as a simple neurobiological ‘friendship meter’: greater synchronisation does not necessarily mean a closer relationship, and a decrease in synchronisation does not necessarily mean that people are drifting apart.

Why did the brains of people from different generations initially synchronise more strongly, only to begin diverging later on?

The authors do not have a definitive answer.

One of the explanations they propose relates not to similar ways of thinking, but to the ability to predict another person’s behaviour.

When a young and an older stranger meet for the first time, they may have fewer shared topics, habits and social contexts. To understand their partner and coordinate their actions, each person has to observe the other more closely and adapt more fully.

Over time, the other person’s behaviour becomes more predictable. If the participants already know how their partner usually acts and reacts, such intense coordination of the two brains’ activity may simply not be necessary. ETH Zurich refers to this possible mechanism as ‘mutual prediction’.

But this is still merely an explanatory hypothesis. The study does not directly show that an improvement in the ability to predict one’s partner was the cause of the reduction in synchronisation.

A separate study by the same researchers, based on these sessions, allowed them to examine the experiment from a different angle.

Instead of looking at average synchronisation, the researchers analysed the constantly changing combinations of connections within each brain and between the two brains simultaneously. The algorithm identified seven recurring ‘two-brain states’.

One of these occurred for longer during joint drawing in intergenerational pairs. It was characterised by a comparatively weak connection between the two brains, but pronounced connectivity between regions within the brain of one of the partners.

The authors cautiously link this pattern to asymmetric interaction, in which one person may adapt more strongly to the actions of the other — something akin to a ‘leader–follower’ dynamic.

However, this cannot be called proof that one person was actually ‘in charge’ of the other. Moreover, in the second study, none of the seven states showed a convincing systematic change across all six sessions.

Why this is important

The study has several important limitations.

First and foremost, the researchers measured social closeness, not friendship. The participants did indeed feel closer to their partners, but the study does not establish whether they became friends after six weeks or whether they continued to socialise after the experiment ended.

There is also a particular feature of the control group. Intergenerational pairs were compared with pairs consisting of two young people. There was no separate ‘older + older’ group in the experiment.

The authors themselves note that such a comparison needs to be carried out in future. Without it, it is impossible to fully distinguish the specific characteristics of intergenerational interaction from the usual age-related differences between participants.

Finally, fNIRS does not directly record the activity of individual neurons. The method measures changes in blood flow associated with brain activity, which may also depend on age-related physiology.

Therefore, for the time being, the result is best understood not as the discovery of a universal ‘mechanism of friendship’, but as a warning against an overly simplistic formula: if two people become close, their brains do not necessarily become increasingly similar.

Source

Study:“Social interactions between people of the same and different generations shape longitudinal changes in interpersonal neural synchrony, loneliness, and social connection”.

Authors: Ryssa Moffat, Guillaume Dumas, Emily S. Cross.

Journal: PLOS Biology, 24(8), e3003899.