Why touchy-touchy can also be feely

Scientists speculate that the nerves may be designed to guide humans toward tenderness and nurture—a theory bolstered by the fact that the nerves are wired to the same brain areas activated by romantic love and sexual arousal.

While these special nerves, which have thin fibres and send relatively slow signals to the brain, have been previously identified in animals and humans, their role had been unclear. The new research, published in the current issue of Nature Neuroscience, indicates that while the thick fibres rapidly shoot electrical signals to the somatosensory cortex of the brain near the crown of the head and convey information about contact and pressure, the slow fibres are connected to the insular cortex deep inside the brain and convey the emotional context of the touching.

Both sets of fibres fire together, and the brain combines information about physical contact with information about emotional context, melding them together into the richness of physical experience.

A crucial reason nature may have endowed people with two different sets of nerves is that the slow fibres function from the earliest hours of life, perhaps even in the womb, while the fast fibres develop slowly after birth.

Wondrously, newborn infants may be able to feel the love in a parent’s touch before they can feel the touch itself.

Referring to studies showing that babies need physical contact and nurture, the group of scientists from Sweden and Canada who made the discovery wrote, ‘‘The profound importance of such a system for human well-being has long been suggested, at least since the classical study of baby monkeys who show affection for a surrogate mother in response to tactile comfort.’’

The nerve system continues to function throughout life, underscoring the importance , of such comfort. While the thicker nerve fibres that communicate contact information are more densely packed into areas like the palm, the thinner nerves are found on hairy areas of the skin, like the forearm.

Lead scientist Hakan Olausson, a neurophysiologist at the Sahlgrenska University Hospital in Goteborg, Sweden, said, ‘‘The fast fibres indicate when we are touched and how strong the touch is; these signal the fine aspects of touch.’’

Evidence about the functioning of the slow fibre nerve system was difficult to obtain because gentle touches also trigger the parallel nerve system. Researchers knew how to trigger just the thin nerve system in animals, but that didn’t reveal much about pleasure: ‘‘The cat cannot say, ’It’s good and this is what I feel,’ ’’ said Yves Lamarre, a professor of neurophysiology at the University of Montreal and one of the authors of the research.

Olausson, Lamarre and a team of scientists from Sweden and Canada based their new report on studies of a 54 year-old woman from Montreal, dubbed GL, who suffers from a disease that destroyed the nerve system that responds to the rougher touches. Stricken since she was 31, GL reported being unable to feel touch below the level of her nose. While GL reported that her ability to perceive temperature, pain and itching were intact, researchers found that she could feel only a dull burning sensation when pinched or when exposed to cold. She could not feel a vibrating sensation at all.

‘‘Not only is this patient lacking skin sensation, she lacks information about movement,’’ said Lamarre. ‘‘When she closes her eyes, she has no idea where she is in space. When she wakes up in the night, she doesn’t know if she has blankets on her.’’

Olausson and Lamarre realised that the patient might be able to tell them of the working of the thin nerve system. Tests showed that that nerve system had not been affected by her disorder. In an experiment where GL could not see what the scientists were doing, they ran a soft watercolour brush up her forearm. When GL concentrated, she reported feeling a faint, diffuse sensation. ‘‘Without knowing what kind of stimulus we delivered, she reported the (touch) was pleasant, with no sensation of pain, temperature, itch or tickle.’’

The researchers compared GL’s response with 24 neurologically intact individuals. They found that GL found the movement of the brush to be as pleasant as normal volunteers. Interestingly, GL could not tell the direction in which the brush was moving—information presumably relayed by the thick nerve fibres—meaning that when people with uncompromised nervous systems perceive pressure, temperature and pleasure, different nerves may be involved for different sensations. After conducting sophisticated brain scans of the volunteers, Olausson and Lamarre also found that the thin nerve system hooked into the anterior parts of the insular cortex, the same area that was activated ‘‘during visually evoked romantic love and sexual arousal.’’ These nerves ‘‘are an important component in the construction of the sense of self.’’(LA Times-Washington Post)