Opinion – Suzana Herculano-Houzel: The difference between being poked and caressed

One of the most beautiful sequences of discoveries in neuroscience is that touch is not just one thing. We have the sense that “serves” to know where something touches the body, yes, and what shape it has. This is touch, a spatial sense, which allows the brain to match the representation of what we see and hear around us with what touches our body. The sense of touch is very precise, and causes alertness, especially when unexpected: a poke on the shoulder produces a movement of the head in that direction, a fact exploited by various pranksters and thieves.

But there is the other sense of touch of the skin, without any power of spatial discrimination, useless for us to represent what touches us, but powerful in its result: feeling of comfort and pleasure, when touch comes with the feeling of control of being desired, caused by someone we accept in our social space.

This is the social touch, the best indication that we are not alone in the world, which explains the preference of baby monkeys for soft and warm “mothers” to cold hard metal “mothers” provided with bottles, in the absence of the mother herself. , and also the powerful effect on newborns of being caressed by human skin. Social touch, with some pressure and always some movement—not too fast, not too slow—activates circuits in the brain that soothe body and soul and, in children, encourage growth.

A team of researchers at the Washington University School of Medicine in Saint Louis, USA, recently showed in the journal Science that social touch is mediated by a particular class of sensory neurons, located in ganglia along the spinal cord of mice, and connected to others within the medulla.

Using genetic techniques that allow the identification, visualization, activation and even ablation of specific neurons in mice, the researchers demonstrated that the neurons responsible for social touch are a population of sensory neurons that produce the peptide prokineticin, and spinal cord neurons that produce the sensitive receptor. to prokineticin.

The neurons that produce and respond to prokineticin are central to the various behavioral changes that ensue when animals previously kept in isolation, which makes even mice greedy for social contact, have the opportunity to be gently brushed when they are in one but not the other. , chamber of a cage: they become more and more in that chamber, where brush strokes lower their heart rate and calm them down.

Social touch also appears to be a powerful mediator of interactions between mice. Normal animals spend much of their time licking and petting each other—but animals lacking prokinetic or its receptor, and therefore insensitive to social touch, no longer participate in social grooming sessions. It is quite possible that this is yet another cause of variation in the autistic spectrum: without prokineticin, caresses, even if well-intentioned, are perceived as just another nudge.