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Scientists have identified neurones that transform touch into social bonds

Amaury François
Research Fellow at Institut de Génomique Fonctionnelle de Montpellier
Key takeaways
  • Attachment theory assumes that physical contact with someone we care about strengthens our social bond with them.
  • Experiments have shown that an infant attaches to its mother not for her nutritive input but rather for the comforting contact she provides.
  • In humans, the force of this "pleasant" physical contact is in the range of 3-10 cm/sec and from a heat source approaching human body temperature.
  • However, some contacts may cause an aversive reaction, due to other stimuli or a pathological condition.
  • A better understanding of how we develop social bonds would allow us to better support people who have difficulties in this respect.

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.”

A pleasant sensation of touch

“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.”

The evidence for attachment theory

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.” 

A faulty network can always be redeveloped

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

1Har­ry F. Har­low, “Love in Infant Mon­keys,” Sci­en­tif­ic Amer­i­can 200 (June 1959):68, 70, 72–73, 74.
2John Bowl­by, Attache­ment et perte : La perte, vol. 3, Paris, Press­es uni­ver­si­taires de France, 1978
3Huzard, D., Mar­tin, M., Main­gret, F., Chemin, J., Jean­neteau, F., Mery, P. — F., Fos­sat, P., Bourinet, E., & François, A. (s. d.). The impact of C‑tactile low-thresh­old mechanore­cep­tors on affec­tive touch and social inter­ac­tions in mice. Sci­ence Advances, 8(26), eabo7566. https://​doi​.org/​1​0​.​1​1​2​6​/​s​c​i​a​d​v​.​a​b​o7566
4Johans­son, R. S., & Vall­bo, A. B.Tactile sen­so­ry cod­ing in the glabrous skin of the human hand (pdf).

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