Periodic Reporting for period 4 - ARTTOUCH (Generating artificial touch: from the contribution of single tactile afferents to the encoding of complex percepts, and their implications for clinical innovation)
Reporting period: 2023-07-01 to 2024-12-31
How do the receptors in your skin respond to the touch of silk or a drop of rain? Our touch system plays an essential role in how we discriminate between different surfaces and objects, which allows us to interact effortlessly with our surroundings. We readily distinguish between a vast range of solid textures, from fine hairs to polished surfaces, as well as between different liquids, via complex percepts like wetness, stickiness, and oiliness. This is enabled by highly precise input from different types of mechanoreceptors in the skin that respond in specific ways to touch. ARTTOUCH aims to better understand how such complex sensory sensations are encoded directly in the skin and how these relate to our perception of them. Further, if this somatosensory system is interrupted, such as through a loss of peripheral nerve fibers, interacting with the world becomes problematic. In somatosensory disease or body injury, such as peripheral neuropathy or amputation, the consequences are dramatic on the quality of life. ARTTOUCH address these issues, by understanding the fundamental signals coming from the skin to deliver artificial touch feedback for those who have sensory losses. Overall, the main aim of ARTTOUCH is to understand how touch is encoded in humans: from fundamental signals in single touch receptors in the skin, to the generation and perception of complex sensations. These insights will help in elucidating the causes and treating the symptoms of somatosensory disorders. Furthermore, applying the knowledge for the provision of realistic sensory feedback in prosthetics represents a great advancement in terms of individual and social usage of a prosthetic device, allowing far more naturalistic interactions and increased embodiment. Project ARTTOUCH has two main objectives: (i) to use electrical stimulation in human nerves to activate a single nerve fiber and produce artificial sensations that can be quantified perceptually and through using neuroimaging, and (ii) to investigate how touch of complex surfaces is encoded and perceived, such as when interacting with different textures and liquids. The results have provided exquisite insights into the human touch system and how it works in everyday life, where we easily feel differences in textures, surfaces, and even small drops of water, via few impulses sent in a single mechanoreceptor. We have shown how these precise sensations are generated and we have been able to then re-inject this input artificially, via electrical stimulation down a small needle electrode, which can be applied into nerves in amputees with a prosthetic device, to provide real-time, sensory feedback during prosthetic touch.
The ARTTOUCH project aims to uncover how basic touch is encoded by the skin and processed in the brain, as well as how more complex sensations are generated, such as wetness and tactile pleasantness. To achieve this, the project uses a specialist technique that is only conducted few groups in the world, namely, microneurography. This allows us to access human peripheral nerves, such as the nerves in the hand, and record from a single nerve fiber that receives information from a receptor in the skin. In healthy humans, we insert a small needle electrode into the skin and into a nerve, to record the responses to touch. We ask the human participant to touch different textures and surfaces, then we record the responses from different touch receptors (mechanoreceptors). We have found that when a person actively touches a surface, distinct properties of the surface and movement are encoded by different types of mechanoreceptor, from the initial contact, to sliding movements, to the offset of contact. Further, when we apply drops of water to the skin, we find that only the most sensitive mechanoreceptors respond, and only one type in the glabrous skin of the hand, and these encode the vibration of the drop on the skin. We also use an extension of the technique of microneurography, where we record from a single touch fiber and then artificially stimulate it via re-injecting a small electrical current. This produces an artificial sensation, a very small point of touch, which differs depending on the mechanoreceptor stimulated. For example, one type produces vibration and another the sensation of pressure. We have found that human participants can discriminate between small differences in trains of electrical stimulation injected into a single fiber and we have also mapped the responses generated from this in the brain. We have presented and published this work in a number of ways to disseminate the information to the scientific community, through invited talks, conferences symposia and posters, videos, and journal papers. Further, we have communicated this to a very large public audience through the media, internet, social media, popular press, and talks.
The ARTTOUCH project offers a new and novel approach to studying how touch is encoded and perceived in humans. It goes beyond state-of-the-art, where cutting-edge equipment has been developed and used to probe the workings of the human touch system. This allows us to gain unrivalled insights into how humans sense different things they come into contact with, including solids and liquids, as well as providing new technology to help in diagnosing somatosensory disorders and to stimulate human nerves to provide sensory feedback in prosthetics for amputees. We have published new results on the precise workings of the human touch system, where we show how single touch receptors in the skin (mechanoreceptors) respond to different types of textures, surfaces, and liquids. We have demonstrated how humans encode and perceive drops of water, which has been little-explored, yet we encounter liquids all the time in our everyday lives. Humans do not have any receptor in the skin for wetness (hygroreceptor), yet we have shown that individuals can detect and quantify very small differences in the quantity of water applied to the skin, and that this is directly encoded via the activation of mechanoreceptors sensing vibrational signals. Further novel investigations through peripheral recordings from human nerves (microneurography) and nerve stimulation, combined with advanced brain imaging and behavioral experiments, have shown that single mechanoreceptors in the skin can signal much information, including that the brain interprets single impulses from one mechanoreceptor. These ambitious and ground-breaking approaches have provided unrivalled and detailed information about human touch that can be applied to medicine and prosthetics.