Summary: Using hydroxy-α-sanshoo, a bioactive compound of Szechuan pepper, researchers gain new insight into how the brain detects and perceives touch.
New insight into how human brains detect and perceive different types of touch, such as fluttery vibrations and steady pressures, has been revealed by UCL scientists with the help of the ancient Chinese cooking ingredient, Szechuan pepper.
Humans have many different types of receptor cells in the skin that allow us to perceive different types of touch. For more than a century, scientists have puzzled over whether touch signals from each type of receptor are processed independently by the brain, or whether these different signals interact before reaching conscious perception.
For the study, published in Proceedings of the Royal Society B, UCL researchers took a novel approach to this question by stimulating one type of touch receptor chemically, and another type mechanically. This bypasses the problem of different mechanical touch stimuli potentially interacting within the skin, with unknown effects on the receptors.
Instead, the UCL team used hydroxy-α-sanshool, a bioactive compound of Szechuan pepper responsible for the characteristic tingling quality of Szechuan cuisine, to stimulate the touch receptors responsible for the sensation of fluttery vibration.
In the study, consisting of 42 participants, hydroxy-α-sanshool was applied to a small skin area on the lip. Once participants started to experience a tingling sensation, they were asked to note the strength of the tingling sensation.
Next researchers applied a steady pressure stimulus to different locations on the upper and lower lips. Participants reported their subjective perception of the intensity of the tingling sensation, by rating it relative to the initial sensation before pressure was applied.
Across several tests, the tingling sensation caused by hydroxy-α-sanshool, was dramatically reduced by steady pressure. The intensity of tingling sensation caused by hydroxy-α-sanshool decreased as the steady pressure increased, and also decreased as the site of steady pressure was moved closer to the site where sanshool was applied.
Lead author Professor Patrick Haggard (UCL Institute of Cognitive Neuroscience), said: “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.
“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.”
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, we show that inputs from two sub-modalities 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. We tested whether this tingling sensation on the lips was modulated by sustained mechanical pressure. Across four experiments, we show that sustained touch inhibits sanshool tingling sensations in a location-specific, pressure-level and time-dependent manner. Additional experiments ruled out the 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.