Touch is usually the first sense that mammals develop. Through touch, they feel vibrations on the surface of their skin and this helps them to interpret the different stimuli in the environment. Scientists supported by the EU-funded H-Reality project have used this principle to develop a universal scaling law of mammalian touch that could pave the way for new advances in virtual reality. The H-Reality team’s findings have been published in the journal ‘Science Advances’.
How mammals perceive vibrations on the skin’s surface
When you run your hand along a wall, vibrations travel through the skin, exciting nerve endings called mechanoreceptors. Mechanoreceptors convert mechanical vibrations to electrical signals that are then transmitted to the brain. The brain, in turn, interprets this as a tactile experience. This dynamic response of the nerve endings in your skin to vibration is called vibrotaction. Using mathematical modelling and touch receptors, the H-Reality researchers demonstrated that vibrotaction is governed by Rayleigh waves that are mainly associated with the study of earthquakes. They further showed that Rayleigh waves don’t only travel along the surface of the skin but also traverse all the skin layers and bone to reach the mechanoreceptors. “Touch is a primordial sense, as important to our ancient ancestors as it is to modern day mammals, but it’s also one of the most complex and therefore least understood,” stated Dr Tom Montenegro-Johnson of project coordinator University of Birmingham in a news item posted on the university website. “While we have universal laws to explain sight and hearing, for example, this is the first time that we’ve been able to explain touch in this way.” The scientists demonstrated that differences in skin hydration – and hence in the stiffness of the outermost layer of skin – didn’t significantly affect Rayleigh wave and receptor interaction. In other words, the mechanoreceptors’ response to Rayleigh waves remained constant, irrespective of the variations in the skin’s outer layer owing to age, profession, gender and other factors. By applying their model to experimental data, the project team found that there exists a universal scaling law for the depth of touch receptors across multiple mammalian species, except small rodents. The ratio of the wavelength of a Rayleigh wave in the skin to the depth of mechanoreceptors was calculated to be about 5/2. This points to an evolutionarily conserved constant in how mammals sense vibrations. “The principles we’ve defined enable us to better understand the different experiences of touch among a wide range of species,” noted co-author Dr James Andrews of the University of Birmingham in the same news item. “For example, if you indent the skin of a rhinoceros by 5 mm, they would have the same sensation as a human with a similar indentation – it’s just that the forces required to produce the indentation would be different. This makes a lot of sense in evolutionary terms, since it’s connected to relative danger and potential damage.” The mathematical model the H-Reality (Mixed Haptic Feedback for Mid-Air Interactions in Virtual and Augmented Realities) researchers developed is based on Nobel laureate Georg von Békésy’s idea that studying earthquakes might shed light on the properties of Rayleigh and Love waves in the skin. Through this research, the project team aspires to create virtual objects with a physical presence, thereby revolutionising virtual reality. For more information, please see: H-Reality project website
H-Reality, skin, touch, virtual reality, Rayleigh wave, mammal