Summary: Researchers used hydroxy-α-sanshool, the active compound in Szechuan pepper, to reveal how the human brain detects and integrates different types of touch.
Source: UCL
UCL scientists have uncovered new details about how the human brain detects and perceives distinct forms of touch—such as fluttering vibrations versus steady pressure—by using hydroxy-α-sanshool, the compound that gives Szechuan pepper its characteristic tingling sensation.
Human skin contains multiple receptor types that transmit a wide range of touch signals to the brain. For many years scientists have debated whether each receptor type contributes independently to touch perception, or whether signals from different receptors interact before reaching conscious awareness.
To address this question, researchers at UCL took a novel experimental approach: they activated one tactile channel chemically and another mechanically. By using a chemical stimulant for one receptor type and mechanical stimuli for another, the team avoided confounding interactions that can occur when different mechanical stimuli are applied to the same area of skin.
The chemical agent used was hydroxy-α-sanshool, a bioactive molecule from Szechuan pepper known to induce a tingling or vibration-like sensation. This compound selectively evokes sensations associated with the flutter-range vibration channel of touch receptors, allowing the team to isolate that channel’s contribution to perception.
In a set of experiments involving 42 participants, hydroxy-α-sanshool was applied to a small area on the lip. Participants were asked to report the onset and perceived intensity of the tingling sensation. After the tingling was established, researchers applied sustained mechanical pressure to various locations on the upper and lower lips. Participants then rated the strength of the sanshool-induced tingling relative to the initial sensation before pressure was applied.
Across multiple tests, sustained steady pressure reliably reduced the sanshool-induced tingling. The suppression of the tingling was stronger with greater pressure and when the pressure was applied closer to the site where sanshool had been applied. These findings indicate a clear, graded interaction: steady pressure inhibits flutter-range vibration sensations in a location-specific and pressure-dependent way.
Lead author Professor Patrick Haggard (UCL Institute of Cognitive Neuroscience) commented on the significance of the findings: “Scientists had previously described how ‘touch inhibits pain’, but our work provides novel evidence that one kind of touch can inhibit another kind of touch.”
He added: “Our results suggest that the touch system for steady pressure must inhibit the touch system for fluttery vibration at some level in the nervous system. The inhibition between these signals may explain how the brain produces a single perception of touch, despite the wide range of signals transmitted by the different types of sensory receptor in the skin.”
The published study, reported across four experiments, also included additional controls that ruled out alternative explanations for the interaction. Specifically, the researchers showed that the effect is unlikely to be mediated by nociceptive (pain) pathways or by affective C-tactile channels, reinforcing the conclusion that interaction occurs between distinct mechanoreceptive submodalities. The pattern of results is consistent with inhibitory influences occurring at an early stage of somatosensory processing, where inputs from different mechanoreceptors converge.
These findings advance our understanding of tactile perception by demonstrating that tactile submodalities do not act entirely independently. Instead, steady pressure and flutter-range vibration interact in predictable ways that shape conscious touch experience. This insight helps explain how the brain integrates diverse receptor signals into coherent tactile sensations and may inform future research into somatosensory processing, prosthetic sensation, and clinical conditions that affect touch.
About this touch perception research news
Source: UCL
Contact: Henry Killworth – UCL
Image: The image is in the public domain
Original Research: Open access.
“Touch inhibits touch: sanshool-induced paradoxical tingling reveals perceptual interaction between somatosensory submodalities” by Antonio Cataldo, Nobuhiro Hagura†, Yousef Hyder and Patrick Haggard. Proceedings of the Royal Society B (DOI: 10.1098/rspb.2020.2914)
Abstract
Touch inhibits touch: sanshool-induced paradoxical tingling reveals perceptual interaction between somatosensory submodalities
Human perception of touch is mediated by inputs from multiple channels. Classical theories postulate independent contributions of each channel to each tactile feature, with little or no interaction between channels. In contrast to this view, the study shows that inputs from two submodalities of mechanical input channels interact to determine tactile perception. The flutter-range vibration channel was activated anomalously using hydroxy-α-sanshool, a bioactive compound of Szechuan pepper, which chemically induces vibration-like tingling sensations. The researchers tested whether this tingling sensation on the lips was modulated by sustained mechanical pressure. Across four experiments, sustained touch inhibited sanshool tingling sensations in a location-specific, pressure-level and time-dependent manner. Additional experiments ruled out mediation of this interaction by nociceptive or affective (C-tactile) channels. These results reveal novel inhibitory influence from steady pressure onto flutter-range tactile perceptual channels, consistent with early-stage interactions between mechanoreceptor inputs within the somatosensory pathway.