Scientists at the University of California, Los Angeles, have revealed new insights into how the human brain detects and perceives different types of touch, such as vibrations and steady pressures, aided by an ancient Chinese cooking spice, Sichuan pepper. In fact, humans possess different types of receptor cells in the skin that allow us to perceive various forms of touch.
For over a century, scientists have been puzzled over whether the touch signals from each type of skin receptor are processed independently by the brain, or if these different signals somehow interact before reaching conscious perception and analysis. In the study published in the journal Proceedings of the Royal Society B, researchers from the University of California approached this question anew by chemically stimulating one type of touch receptor and mechanically stimulating another. This approach allowed them to bypass the problem of different mechanical touch stimuli that could potentially interact within the skin with unknown effects on the skin receptors. The UC team used the compound hydroxy-α-sanshool, a biologically active natural compound extracted from Sichuan pepper responsible for the characteristic spicy flavor in hot Chinese foods, to stimulate touch receptors responsible for sensing vibrations known as 'flutery' vibrations.
During the study, which involved 42 participants, the hydroxy-α-sanshool compound was applied to a small area of skin on the lip. Once the participants began to experience a tingling sensation, they were asked to note the intensity of the tingling and its sharp taste. The researchers then applied a steady pressure stimulus to various locations on the upper and lower lips to compare the two sensations. Participants reported their self-perceived intensity of the tingling sensation by classifying it relative to their initial sensation before pressure application. Through several tests, it was found that the tingling sensation caused by hydroxy-α-sanshool was significantly reduced by the constant pressure, showing that the intensity of the tingling sensation decreased with increased steady pressure and also diminished when the pressure was applied near the site where hydroxy-α-sanshool was administered.
The lead author of the study, Professor Patrick Haggard, stated, "Scientists have previously described how touch can inhibit pain, but our work provides new evidence that a certain type of touch can inhibit another type of touch." He added, "Our results suggest that the pressure-responsive touch system must suppress the flutery vibration-responsive touch system at some level in the nervous system. This inhibition between these signals may explain how the brain produces a unified perception of touch, despite the wide range of signals transmitted by different sensory receptor types in the skin continuously and without interruption."
