Food vibrations: Asian spice sets lips trembling

Abstract

Szechuan pepper, a widely used ingredient in the cuisine of many Asian countries, is known for the tingling sensation it induces on the tongue and lips. While the molecular mechanism by which Szechuan pepper activates tactile afferent fibres has been clarified, the tingling sensation itself has been less studied, and it remains unclear which fibres are responsible. We investigated the somatosensory perception of tingling in humans to identify the characteristic temporal frequency and compare this to the established selectivity of tactile afferents. Szechuan pepper was applied to the lower lip of participants. Participants judged the frequency of the tingling sensation on the lips by comparing this with the frequencies of mechanical vibrations applied to their right index finger. The perceived frequency of the tingling was consistently at around 50 Hz, corresponding to the range of tactile RA1 afferent fibres. Furthermore, adaptation of the RA1 channel by prolonged mechanical vibration reliably reduced the tingling frequency induced by Szechuan pepper, confirming that the frequency-specific tactile channel is shared between Szechuan pepper and mechanical vibration. Combining information about molecular reactions at peripheral receptors with quantitative psychophysical measurement may provide a unique method for characterizing unusual experiences by decomposing them into identifiable minimal units of sensation.

Sanshool on The Fingertip Interferes with Vibration Detection in a Rapidly-Adapting (RA) Tactile Channel

“…Since the PC channel mechanoreceptor is absent in orofacial skin [27–29], our aim of testing multiple channels suggested using the fingertips, rather than the lips. We therefore first confirmed that the tingling sensation by sanshool could be also reliably elicited on the finger, not only on the lips as it has been shown previously [7]. …”

“…Since all of the participants reported the tingling sensation on the lips in the previous study [7], this result may be due to the individual differences in the skin thickness on the finger; for these two participants, the sanshool may have not reached the dermic layer to activate the receptors/fibres. Therefore, these two participants were excluded from the further experiments.…”

“…The concentration is almost 100%, and we apply to the participants’ finger as much as we can to induce reliable tingling sensation on the finger. Sanshool reliably produces “tingling” sensation when applied on the lips [7]. Since the skin of the finger is thicker than that of the lips, we asked participants to scratch the surface skin of their fingers with a lancet (softclix, ACCU-CHEK, USA).…”

“…Furthermore, Hagura et al [7] have previously demonstrated that, in humans, the perceived frequency of the sanshool-induced tingling on the lips was 50 ± 2.4 Hz. It is known that the human sensory system processes mechanical input in a frequency dependent manner.…”

“…It has been shown that the masking/adaptation effect on tactile signal detection reveals the vibro-frequency tunings similar to those of the mechanoreceptor channels responsible for signal detection [8, 13–21]. This indicates that the interference/masking effect dominantly reflects signal interactions within each individual mechanoreceptor channel, and therefore the psychophysical method should provide direct and objective evidence about the contribution of individual peripheral frequency channels during sanshool-induced tingling-like sensation, in addition to the measurement of subjective perceptual frequency similarity [7]. We did not test sanshool effect on SA2 channel, since this channel is not vibration sensitive and therefore is irrelevant for vibratory stimuli [8].…”