Sabrina Richards

 This passage is adapted from Sabrina Richards’ article, 

“Pleasant to the Touch,” which was originally published in 

September 2012 by The Scientist magazine. ©2012 by Sabrina 

Richards and The Scientist. 

 In the early 1990s, textbooks acknowledged that 

humans had slow-conducting nerves, but asserted that 

those nerves only responded to two types of stimuli: 

pain and temperature. Sensations of pressure and 

vibration were believed to travel only along myelinated, 

fast-signaling nerve fbers, which also give information 

about location. Experiments blocking nerve fbers 

supported this notion. Preventing fast fbers from fring 

(either by clamping the relevant nerve or by injecting 

the local anesthetic lidocaine) seemed to eliminate the 

sensation of pressure altogether, but blocking slow fbers 

only seemed to reduce sensitivity to warmth or a small 

painful shock. 

 Håkan Olausson and his Gothenburg University 

colleagues Åke Vallbo and Johan Wessberg wondered 

if slow fbers responsive to gentle pressure might be 

active in humans as well as in other mammals. In 1993, 

they corralled 28 young volunteers and recorded nerve 

signals while gently brushing the subjects’ arms with their 

fngertips. Using a technique called microneurography, 

in which a fne flament is inserted into a single nerve to 

capture its electrical impulses, the scientists were able to 

measure how quickly—or slowly—the nerves fred. Tey 

showed that sof stroking prompted two different signals, 

one immediate and one delayed. Te delay, Olausson 

explains, means that the signal from a gentle touch on 

the forearm will reach the brain about a half second later. 

Tis delay identifed nerve impulses traveling at speeds 

characteristic of slow, unmyelinated fbers—about 1 

meter/second—confrming the presence of these fbers 

in human hairy skin. (In contrast, fast-conducting fbers, 

already known to respond to touch, signal at a rate 

between 35 and 75 m/s.) 

 Ten, in 1999, the group looked more closely at the 

characteristics of the slow fbers. Tey named these 

“low-threshold” nerves “C-tactile,” or CT fbers, said 

Olausson, because of their “exquisite sensitivity” to slow, 

gentle tactile stimulation, but unresponsiveness to 

noxious stimuli like pinpricks. 

 But why exactly humans might have such fbers, 

which respond only to a narrow range of rather subtle 

stimuli, was initially mystifying. Unlike other types of

sensory nerves, CT fbers could be found only in hairy 

human skin—such as the forearm and thigh. No amount 

of gentle stroking of hairless skin, such as the palms and 

soles of the feet, prompted similar activity signatures. 

Olausson and his colleagues decided that these fbers 

must be conveying a different dimension of sensory 

information than fast-conducting fbers. 

 Although microneurography can give information 

about how a single nerve responds to gentle brushing 

and pressure, it cannot tease out what aspect of sensation 

that fber relays, says Olausson. He wanted to know if that 

same slow nerve can distinguish where the brush touches 

the arm, and whether it can discern a difference between 

a goat-hair brush and a feather. Most importantly, could 

that same fber convey a pleasant sensation? 

 To address the question, Olausson’s group sought out 

a patient known as G.L. who had an unusual nerve defect. 

More than 2 decades earlier, she had developed numbness 

across many parts of her body afer taking penicillin to 

treat a cough and fever. Testing showed that she had lost 

responsiveness to pressure, and a nerve biopsy confrmed 

that G.L.’s quick-conducting fbers were gone, resulting in 

an inability to sense any pokes, prods, or pinpricks below 

her nose. But she could still sense warmth, suggesting 

that her slow-conducting unmyelinated fbers were intact. 

 Upon recruiting G.L., Olausson tested her by brushing 

her arm gently at the speed of between 2–10 centimeters 

per second. She had more trouble distinguishing the 

direction or pressure of the brush strokes than most 

subjects, but reported feeling a pleasant sensation. When 

the researchers tried brushing her palm, where CT fbers 

are not found, she felt nothing. 

 Olausson used functional MRI studies to examine 

which areas of the brain lit up when G.L.’s arm was gently 

brushed to activate CT fbers. In normal subjects, both 

the somatosensory and insular cortices were activated, 

but only the insular cortex, which processes emotion, was 

active when researchers brushed G.L.’s arm. Tis solidifed 

the notion that CT fbers convey a more emotional 

quality of touch, rather than the conscious aspect that 

helps us describe what we are sensing. CT fbers, it 

seemed, specifcally provide pleasurable sensations