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Understanding That Loving Feeling
Posted: Monday, July 29, 2002
In a Study of the Brain, Special Nerves Registered the Emotional Context of a Pleasurable Touch
By Shankar Vedantam, Washington Post, July 29, 2002
Neuroscientists have discovered what romantics have always known: The touch of a lover's hand is special.
Scientists announced a study today that shows humans have a special set of nerves for feeling pleasure at a mother's caress or a lover's embrace.
These nerves are sensitive to the soft touch of fingers gliding over a forearm or a parent's soothing hand, but not to rough touches, jabs or pinches. Scientists speculate that the nerves might be designed to guide humans toward tenderness and nurturing -- a theory bolstered by the fact that the nerves are wired to the same brain areas activated by romantic love and sexual arousal.
Although these special nerves, which have thin fibers and send relatively slow signals to the brain, had been identified in animals and humans, their role had been unclear.
Scientists had wondered about their purpose, especially because they do not work as efficiently as thick nerve fibers, which are also found in skin.
The research, published in the current issue of Nature Neuroscience, indicates that while the thick fibers rapidly shoot electrical signals to the somatosensory cortex of the brain and convey information about contact and pressure, the thin, slow fibers connect to the insular cortex and convey the emotional context of the touching. Both sets of fibers fire together, and the brain combines information about physical contact with information about emotional context, melding them into the richness of physical experience.
A crucial reason nature might have endowed people with two different sets of nerves is that the slow fibers function from the earliest hours of life, perhaps even in the womb, while the fast fibers develop slowly after birth.
Newborn infants might 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 nurturing, the group of scientists 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 fibers that communicate contact information are more densely packed into areas such as the palm, the thinner nerves are found on hairy areas of the skin such as the forearm.
"Their functional role is below the level of consciousness and has to do with the emotional aspects of touch -- like the pleasure of touch," lead scientist Hakan Olausson, a neurophysiologist at the Sahlgrenska University Hospital in Goteborg, Sweden, said in a telephone interview. "The fast fibers indicate when we are touched and how strong the touch is. [The slow fibers] signal the fine aspects of touch."
Evidence about the functioning of the slow-fiber nerve system was difficult to obtain because gentle touches also trigger the parallel nerve system.
Researchers knew how to trigger only 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. "To study these fibers in humans was theoretically impossible. If you stimulate the skin, you stimulate the large fibers that provide most of the sensation you feel. It would mask the small fibers."
Olausson, Lamarre and a team of scientists from Sweden and Canada based their report on studies of an unusual patient -- a 54-year-old woman from Montreal, referred to as G.L., who suffered from a disease that destroyed the nerve system that responds to the rougher touches. Since she was 31, G.L. has reported being unable to feel touch below the level of her nose.
While G.L. said her abilities 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," Lamarre said. "When she closes her eyes, she has no idea where she is in space. She moves but doesn't know she is moving. When she wakes up in the night, she doesn't know if she has blankets on her."
Olausson and Lamarre realized that the patient might be able to tell them about the working of the thin nerve system. Tests showed that nerve system had not been affected by her disorder.
In an experiment where G.L. could not see what the scientists were doing, they ran a soft watercolor brush up her forearm. When G.L. concentrated, she reported feeling a faint and diffuse sensation.
"Without knowing what kind of stimulus we delivered, she reported the [touch] was clearly pleasant, with no sensation of pain, temperature, itch or tickle," the researchers wrote. The researchers compared G.L.'s response with 24 neurologically intact individuals. They found that G.L. found the movement of the brush to be as pleasant as normal volunteers did.
Interestingly, G.L. could not tell the direction in which the brush was moving -- information presumably relayed by the thick nerve fibers -- meaning that when people with uncompromised nervous systems perceive pressure, temperature and pleasure, different nerves might be involved for different sensations.
After conducting sophisticated brain scans of the volunteers, Olausson and Lamarre found that the thin nerve system was hooked into the front parts of the insular cortex, the same area that was activated, "intriguingly, during visually evoked romantic love and sexual arousal."
These nerves, the researchers concluded, "are an important component in the construction of the sense of self."
Olausson said areas such as the palm might have fewer thin nerves and more thick nerves because the primary role of the palm is to pick up information about objects.
That said, there were still aspects of pleasure and emotion that could be transmitted through the thick fibers in the palm. Healthy volunteers in the experiment were divided about whether a caress on the palm was pleasurable.
"The next step is to examine patients who have no [slow] fibers -- patients with just the fast fibers," Olausson said. "If the hypothesis is correct, they should lack the emotional sense of touch, the sense of emotional well-being."
© 2002 The Washington Post Company
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http://www.washingtonpost.com/wp-dyn/
articles/A12587-2002Jul28.html
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