Szechuan peppers, which are responsible for spicing up a variety of Asian dishes, are considered unique among peppers for their "lemony overtones" and the tingling, almost electric, sensation that they cause in the mouths of consumers. The latter has been attributed to a compound known as hydroxyl alpha sanshool (also known simply as sanshool), but the neurological mechanism responsible for the effects of sanshool has not previously been understood. This inspired a research team from the University College London to perform a suite of experiments characterizing how eaters perceive Szechuan peppers, and exploring the sensory processes influencing these experiences. Their results suggest that the ubiquitously identified tingling sensation is produced by the activity of "rapidly adapting" (RA) sensory nerves that are more generally associated with mechanical, rather than chemical, stimuli.

(Szechuan pepper, also known as Sichuan or Szechwan pepper. There are at least two species responsible for this spice. Although the husk is generally considered the most important part of the plant for culinary purposes, some cooks may also work with leaves. The peppers may also be used in medicine. Image courtesy of Wikimedia.)

In the first portion of their study, the researchers applied a pepper solution to the lower lips of 12 volunteers; additional volunteers were also treated with ethanol and water control solutions. Each volunteer was asked to indicate whether his/her treatment produced a tingling, burning, cooling, warming, and/or numbing sensation. All individuals treated with the pepper solution agreed that it felt tingly. Although some people also described the ethanol treatment in this way, they indicated that the pepper solution resulted in a unique sensation that was both more intense and associated with a regular pulse.

The nature of this temporal effect was explored in more detail in the second part of the study. Volunteers were again treated with a swab of pepper solution applied to their lower lips. Once the tingling sensation began, the researchers applied a mechanical vibration to each volunteer's index finger, then asked the volunteers to indicate whether this vibration was of a higher or lower frequency than the feeling in their lips. Participant responses were used to adjust the frequency after each trial, thus allowing the researchers to gradually narrow in on the exact frequency of the pepper stimulus. Volunteers converged on a mechanical vibration frequency of 50.0 Hz, which happens to be near the midpoint of the range of frequencies (10-80 Hz) to which RA1 afferent fibers are most sensitive.

(Afferent nerves such as the RA1 are responsible for relaying information from sense organs, including skin, back to the brain. Image courtesy of Wikipedia.)

To double-check the legitimacy of comparing vibrations between lips and fingers, the researchers performed a third experiment in which eight volunteers were asked to compare the vibration frequencies of stimuli applied to first their lips and then their fingers, with a 1.5-s delay in between. Volunteers were most sensitive to convergence at a slightly lower frequency than that identified in the second experiment (46.4 Hz vs. 50.0 Hz). However, these two values are quite close, and the results clearly show that people are capable of relating sensations between fingers and mouths--suggesting that the researchers had come up with a suitable method for estimating pepper vibration frequencies. 

In the final portion of the study, volunteers were exposed to both the mechanical and pepper stimuli sequentially. Because nerves need to "reset" between stimulus events, the second stimulus should have a lesser effect than the first--but only if both stimuli are activating the same set of nerves. After being exposed to an initial "adaptation stimulus" to their lips, volunteers were treated again with either a mechanical vibration or a pepper swab. A mechanical vibration was then applied to their fingers, and volunteers were asked to report on whether the frequency of this stimulus was higher or lower than that on their lips. As expected, volunteers had difficulty pinpointing the frequency of the second treatment, indicating that their nerves were still recovering from the adaptation stimulus. This was true regardless of whether the second treatment was a mechanical vibration or a pepper swab. In other words, both types of stimuli probably activate the same nerves.

(Hydroxy alpha sanshool, the compound responsible for the tingling sensation caused by Szechuan pepper. Gourmands might consider adding this chemical to their dishes in order to change the perception of other flavors, or to help diners experience food both chemically and mechanically. Image courtesy of Wikimedia.)

Taken together, results from the four experiments strongly suggest that the "light-touch" RA1 nerves are responsible for creating the tingling sensation that results from eating Szechuan pepper. Although sanshool is known to activate at least four other types of nerve fibers, a variety of factors--including the locations of those fibers and the types of sensation that sanshool produces when interacting with them--suggest that RA1 nerves are the most important player in producing a response to pepper-laden foods.

The authors suggest that these results may help us better understand the "pins and needles" sensations that are associated with some injuries and neural abnormalities. On a more fun note, however, they also point out that their findings may be of interest to gastronomists interested in enhancing their eating experiences. Szechuan pepper may not be the only food that influences taste by mimicking touch; we should be on the lookout for others that might similarly spice up our meals, both literally and figuratively. Further, because of the simultaneous chemical and mechanical effects of Szechuan peppers, chefs may be able to alter other flavors--or diners' perceptions of them--by adding sanshool to their culinary creations.

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Hagura, N., Barber, H., and Haggard, P. 2013. Food vibrations: Asian spice sets lips trembling. Proceedings of the Royal Society B 280(1770): online advance publication.