A poke, a pat, a nudge, and a stroke may seem like similar forms of touch, but the brain does not seem to perceive them the same way. Recent studies of a patient with a unique disability show that the differences may be hardwired into the body. One sensation—a soft caress—is processed by a set of nerves totally separate from those that mediate all other types of touch.
Neuroscientist Håkan Olausson of Sahlgrenska University Hospital in Göteborg, Sweden, made this discovery while examining a patient whose rare autoimmune disease left her with minimal bodily sensation below the nose. Although she could still feel heat, cold, and pain, she had lost her sense of touch and could no longer perceive the angle of her limbs. Doctors discovered that the disease had destroyed the patient's fast-conducting neurons, which had been thought to transmit all forms of touch to the brain, leaving only the slow-conducting fibers, which communicate pain and temperature. Yet when Olausson stroked the back of the woman's hand with a soft paintbrush, she reported feeling a faint and agreeable pressure. That unexpected response indicates that some of the slow fibers were also signaling the caress.
Magnetic resonance imaging showed that these nerves stimulate the insula, a deep-brain region that processes emotion. "The areas activated in this patient are also activated if you look at someone that you're deeply in love with," Olausson says. He theorizes that the caressing nerves play a key role in sexual arousal, the release of hormones, and even infant development. "Others have shown that if you caress newborns, they get increased levels of oxytocin. This hormone makes the child calmer, improves breast-feeding, and promotes growth," Olausson says. "It emphasizes how important skin is as an emotional organ."