Scientists have identified neurones that transform touch into social bonds

What could be warm­er and more com­fort­ing than the touch of someone we care about? Touch­ing a per­son to whom we are attached gives us pleas­ure and brings us closer to that per­son. But is it the pleas­ure that strengthens the attach­ment, or is it the attach­ment that causes the pleasure?

Attach­ment the­ory has a long his­tory in psy­cho­logy, and Harry Har­low can be con­sidered one of its pre­curs­ors. Through exper­i­ments¹ that were already con­tro­ver­sial at the time, he man­aged to refute a rather per­sist­ent pre­con­cep­tion of the era: that infants devel­op attach­ment to their moth­ers through breast­feed­ing. By sep­ar­at­ing baby mon­keys from their moth­ers at birth, he dis­covered that between two mater­nal sub­sti­tutes – a stiff wire doll with a bottle of milk, and a softer cloth doll that was arti­fi­cially heated – the mon­keys chose the one that was com­fort­able and warm to the touch, rather than the one that would sat­is­fy their vital need for food.

From this exper­i­ence, the Eng­lish psy­cho­lo­gist John Bowlby developed “attach­ment the­ory”². This the­ory states that an infant attaches itself to its moth­er not for the nour­ish­ment she provides, but for the com­fort­ing touch she gives. This the­ory may date back to 1969, but psy­cho­lo­gists have found it to be increas­ingly cred­ible in light of numer­ous exper­i­ments. The upshot is that for a child to devel­op socially and emo­tion­ally, he or she must have at least one fig­ure who cares for him or her con­tinu­ously and con­sist­ently – a fig­ure for whom he or she will devel­op an attachment. 

Amaury François, a research­er at the Insti­tute of Func­tion­al Gen­om­ics in Mont­pel­li­er, and his team decided to go bey­ond the psy­cho­lo­gic­al bound­ar­ies of this the­ory by delving into the bio­lo­gic­al com­pon­ent³. To do this, they focused on the pos­sible influ­ence of the sen­sa­tion of pleas­ant touch in strength­en­ing our social rela­tion­ships. “A net­work of neur­ons in humans, dis­covered by the Swedish neur­o­lo­gist Åke Vallbo⁴, which is respons­ible for this sen­sa­tion of pleas­ant touch,” he explains. “These are called C‑Tactiles. Our exper­i­ment con­sisted of dis­cov­er­ing its equi­val­ent in mice to test its effects on the devel­op­ment of social rela­tion­ships. In doing so, we were there­fore able to val­id­ate the influ­ence of the C‑LTMR net­work (the equi­val­ent of C‑Tactiles in rodents) on the social­isa­tion of mice.”

“What we dis­covered is that, in both mice and humans, there is a net­work of neur­ons spe­cif­ic to social touch, innerv­at­ing the skin and send­ing inform­a­tion to the cent­ral nervous sys­tem (not itself part of it) when it is activ­ated,” explains Amaury François. “It’s activ­a­tion only occurs under very spe­cif­ic con­di­tions. In gen­er­al, in humans, these touches are made at a cer­tain speed (between 3 and 10 cm/sec), from a heat source that is close to the tem­per­at­ure of the human body. These are stim­uli that almost all of us find pleas­ant, con­sciously, or uncon­sciously.” No won­der infants feel so good in their mother­’s arms.

So, in mice the equi­val­ent of this net­work (called C‑LMTR) has been iden­ti­fied. Using this, the research team designed a paradigm to test its influ­ence on the devel­op­ment of social rela­tion­ships. “For a group of mice genet­ic­ally mod­i­fied to have a defi­ciency in this net­work, the res­ults are clear,” says the research­er. “The group of mice in ques­tion no longer seems to inter­act nor­mally with their fel­low mice, they favour isol­a­tion.” One ele­ment of this res­ult intrigued the research­er: the anim­al with the defect­ive net­work does not flee from oth­ers, it simply finds no interest in com­ing into con­tact with them.

The pleas­ant sen­sa­tion of touch is thought to be a motiv­at­ing factor in socialisation.

It is import­ant to note that this net­work of neur­ons is not the only one to be activ­ated dur­ing dir­ect con­tact. “The C‑LMTRs are present for the emo­tion­al value, and their activ­a­tion is enough,” says Amaury François. “The ques­tion of the influ­ence of oth­ers may still arise. We believe that this net­work func­tions some­what like the reward sys­tem. This pleas­ant sen­sa­tion may be a motiv­at­ing factor in socialisation.”

We can there­fore estab­lish a dir­ect link between con­tact with oth­ers and our attach­ment to them. “It is this com­fort­ing aspect of the warm, gentle touch that we feel that gives us the motiv­a­tion, and the desire to repro­duce it. Which is, in the end, pretty much what Harry Har­low had observed. Since all these exper­i­ments on attach­ment the­ory, noth­ing sig­ni­fic­ant had been done,” recalls the research­er. Today, we have addi­tion­al answers to these ques­tions, which allow us to bet­ter ensure the prop­er social devel­op­ment of a child.

How­ever, this exper­i­ment does not answer all the ques­tions that this the­ory raises. “We stud­ied a neur­al net­work that only sends inform­a­tion to the brain,” adds Amaury François. “The point is to under­stand how this inform­a­tion is trans­lated in the brain so that it is per­ceived as pleas­ant, but also to under­stand why cer­tain con­tacts, although they respect the con­di­tions of pleas­ant touch, cause an almost oppos­ite effect.” 

After all, if a stranger comes up to you in the street and caresses you, your reac­tion is likely to be one of aver­sion. For the research­er, the explan­a­tion would come from oth­er stim­uli, which are not neces­sar­ily per­cept­ible. “In the mice’s cages, one factor that we couldn’t influ­ence was odours,” he says, “because you don’t see them and per­ceive them in the same way as mice. Smell can be asso­ci­ated with pleasure/comfort with a place or an indi­vidu­al, also help­ing with attachment.” 

This net­work is nor­mally innate in humans. How­ever, it is not fully formed at birth, it must devel­op. As the genet­ic­ally mod­i­fied mice show us, if this net­work does not func­tion as it should, the indi­vidu­al will tend to favour isol­a­tion. But the effect can also be observed in mice with hyper­sens­it­iv­ity. “In a neut­ral con­text, the activ­a­tion of this net­work is pleas­ant. In a patho­lo­gic­al con­text, its over­activ­a­tion will pro­voke con­tact adverse beha­viour,” says Amaury François. The reac­tion is there­fore even stronger than for a lack of activ­a­tion. This time, the mouse will avoid social con­tact and will have an interest in isolation.

In a patho­lo­gic­al con­text, hyper­sens­it­iv­ity will pro­voke con­tact-averse behaviour.

“This reac­tion is par­tic­u­larly evid­ent in people with aut­ism, which makes the situ­ation all the harder for the indi­vidu­al and their par­ents. As con­tact is rejec­ted by the child, the par­ents are unsure of how to inter­act with the child. And all this will have a ser­i­ous impact on the child’s social devel­op­ment,” he admits. 

This dis­cov­ery there­fore opens the way for a mul­ti­tude of dif­fer­ent research pro­jects that will one day allow us to bet­ter under­stand how and why we devel­op social bonds with oth­ers. And, giv­en their proven import­ance, per­haps it will also allow us to bet­ter sup­port people who have dif­fi­culties in this area. “When the dif­fer­ence is at the devel­op­ment­al level, and there is a prob­lem with neur­al net­work, there will cer­tainly be gaps. But it is not too late to devel­op and adapt to them,” con­cludes the researcher.

Pablo Andres

Fur­ther reading 

For more details on the research: https://​doi​.org/​1​0​.​1​1​2​6​/​s​c​i​a​d​v​.​a​b​o7566