Eating something seasoned with Szechuan pepper is a pretty weird experience for the unaccustomed palate. After a few minutes, a tingling feeling comes on in the lips and tongue, often accompanied by a burning or numbing. Now, a group of researchers has described just what that tingling sensation feels like and named a particular kind of nerve fiber they think is involved.
Szechuan pepper is used in certain East Asian cuisines. For example, in Japan, Szechuan pepper is sprinkled on top of eel. “That buzzing sensation with a burning sensation is quite interesting,” says Nobuhiro Hagura, a cognitive neuroscientist at University College London and one of the authors of the new study. “I like it,” says Hagura, a native of Japan.
Hagura is interested in understanding how people perceive information transmitted from the skin. From reading the literature, he knew that Szechuan pepper might activate a particular group of nerve fibers, known as RA1 fibers, which respond to light touch and vibration. Vibration receptors help tell you whether, for example, you’re running your finger across silk or denim. Hagura and his colleagues wanted to know whether Szechuan pepper set off those receptors.
In a series of experiments, the researchers put ground-up Szechuan pepper on the lower lips of volunteers, as they describe this week in the Proceedings of the Royal Society B. All the participants agreed that the pepper created a tingling sensation; most also felt burning or numbing. In one experiment, the researchers put pepper on the lip and at the same time stimulated the person’s finger with a mechanical vibrator that could be slowed down or sped up. The person judged when the frequency of tingling on the lips and in the finger was the same.
The researchers found that Szechuan pepper tingled at an average frequency of 50 cycles per second. That’s the frequency of vibration that RA1 fibers are most sensitive to. The harmonious result suggests that in addition to responding to mechanical stimuli, RA1 fibers may respond to the chemical compound in Szechuan pepper known to trigger nerves, hydroxyl-α-sanshool.
Neuroscientists have used menthol and chili peppers to help them understand how we sense pain and temperature, and Szechuan pepper could be useful for understanding the mysteries of touch, Hagura says. “If you grab a cup, it sounds like a very simple act,” he says. “In reality, what you feel from grabbing a cup is many signals from many fibers.” Szechuan pepper might help neuroscientists by giving them a tool that can create the sense of touch in the lab.
“It’s great that it was done in humans,” says Cheryl Stucky, a neurophysiologist at the Medical College of Wisconsin in Milwaukee who has studied how Szechuan pepper activates fibers in mice. But the researchers haven’t proved that the RA1 fibers are the only fibers set off by Szechuan pepper, she points out. “I think it includes that fiber, definitely,” Stucky says. “But I think there are other things involved”—perhaps, for example, so-called C fibers, which can respond to many kinds of stimuli, including touch and heat.
Whatever nerve fibers are involved, there’s an application already, says Charles Spence of the University of Oxford in the United Kingdom, a self-described neurogastronomist. Knowing the frequency of the tingling sensation might enable cooks and artists to devise resonating multisensory experiences of sound, video, and Szechuan pepper—the kind of mind-blowing trip people might pay for: “It could be the start of a whole new exciting era of in-mouth illusions.”
*Correction, 27 September, 12:26 p.m.: This version corrects a quote from Nobuhiro Hagura in the second paragraph; Hagura discussed the buzzing and burning "sensation" of Szechuan pepper, not the buzzing and burning "situation."