The Surprising, Undeniable Power of Touch

 
Science 19th Nov 2015

A newly recognized nerve system connects us in ways we never realized.
My three sons are nearly all teenagers, but I remember the minutes immediately after each child's birth as sharply as if the boys had entered the world this morning. Receiving my new babies into my arms, I hugged each one to my chest, caressed his back, and kissed the top of his tiny head. And then we stayed there like that for quite awhile, mother and child.

The sense of touch had a lot to do with why those moments were so powerful. Touch has long been understood to be important in nurturing relationships—so much so that babies raised in orphanages without it often died. Those first moments with my children, followed by years of cuddles and hugs, no doubt contributed mightily to the deep bonds between us.

Psychologists have proposed a number of explanations of why that should be so, such as attachment theory and increases in oxytocin. Yet those suggestions do not adequately elucidate the immediacy and power of touch, and leave the underlying biology unexamined. Meanwhile, neuroscientists had until recently focused only on the discriminative nature of tactile perception—how touch allows us to tell a baby's skin from his or her blanket and whether that skin feels feverish. They assumed any emotional aspects of touch came later, after the brain had processed the sensation and had a few hundred milliseconds to add the context of feelings.

That view changed a few years ago, when a small but determined group of neuroscientists proposed that something far more fundamental was going on when I held my babies. Their growing body of research has uncovered another dimension of touch, separate from its discriminative function. This newly recognized system, known as affective or emotional touch, consists of nerve fibers triggered by exactly the kind of loving caress a mother gives her child.

It is possible that these neurobiological foundations of attachment might play a far more significant role in human behavior than has previously been recognized, forging connection and increasing our chance of survival. These fibers may also help our minds construct and integrate a sense of self and other, informing our awareness of our own bodies and our ability to relate to people around us.

"Affective touch is a potential way in to understanding the development of the normal social brain," says Francis McGlone, a neuroscientist at Liverpool John Moores University and a leader in the field. "It's giving the brain knowledge of me and you, and the emotional quality of gentle nurturing touch is a very important feeling that underpins a lot of social interaction."

Touch is the first sense to emerge in utero, and though still far from mature, it is the most strongly developed at birth. Neurons in the skin take in information about everything we contact through a variety of nerve fibers and sensory receptors specialized for touch. One class of fiber, A-beta, does most of the work of discriminating and is all over the body, especially in the palm. Because they are myelinated, A-betas are able to conduct messages rapidly. C fibers are different. They are unmyelinated, and carry information up to fifty times slower than their neighbors. The best-known C fibers are those for pain and itch.

Now, says McGlone, "There's another kid on the block." Present only in hairy skin such as the forearm or the back, the new fiber is known as a C-tactile (CT) afferent and is keenly attuned to the gentle velocity and comfortable skin temperature of a caress. Studies have shown that people interpret this kind of touch as pleasant. The area of the brain that responds to it is not the somatosensory region, which processes the fact that the table you're touching is hard. Rather, it is the insular cortex, a part of the limbic brain researchers believe monitors emotion and a sense of one's own body known as interoception.

CT afferents may not tell the whole story of affective touch. Several types of C fibers could be working together like the ensemble members of a symphony. It's also clear that other, more complex parts of the brain play a role. If a stranger caresses your arm on the subway, you are unlikely to interpret the touch as pleasant. In other words, what we think about how we feel does matter.

Nonetheless, there's a sense among those in the field of being in uncharted and thrilling territory. As we come to understand this sense better, we may be able to identify and develop new treatment solutions based on touch—particularly for conditions as diverse as Rett syndrome, autism, neuropathic pain, and spinal cord damage.

In a society that so often substitutes virtual communication for personal contact, the findings on affective touch remind us to relish every embrace—and hold it just a few seconds longer.

Those moments may be the bedrock of our richest relationships.

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