Mechanotransduction and information coding in the human peripheral tactile system

The human peripheral tactile system is responsible for the initial processing of tactile stimuli and is composed of the skin and various embedded mechanoreceptors innervated by afferents. Spiking models are widely used to characterize this system and infer how populations of afferents shape tactile perception. Leveraging existing models of tactile afferents and moved by their limitations, we present three studies designed to advance these essential tools in the investigation of the human peripheral tactile system. Firstly, reconciling existing evidence, we quantitatively characterize the population of peripheral tactile afferents. We estimate that approximately 230,000 afferents cover the human body, provide innervation densities in different skin areas, and show the relation of these numbers with tactile acuity, hair follicle density, and somatosensory cortical representation. Secondly, we ask how tactile afferents work together to encode information in complex ways. We find that information is spread across classes, and combining information from multiple classes improves transmission. We test the importance of temporal and spatial resolution in the population code, probing that destroying temporal information is more destructive than removing spatial information. Finally, we use Optical Coherence Tomography to image the skin subsurface in vivo and dynamically and quantify the deformation of individual fingerprint ridges down to the type-1 mechanoreceptors' location. When scanning the skin with a flat surface, the ridge deforms as a single unit. Higher strains emerge from the stick-to-slip transition compared to plate movement reversal. When scanning the skin with small features, different ridge sub-units experience different strain patterns. Higher strains occur in the deepest layer imaged. Overall, this research provides a better understanding of coding strategies of tactile afferents on a population level and of the link between skin mechanics and transduction mechanisms underlying tactile perception. Our findings will have implications for developing novel spiking models of the human peripheral tactile system.