Fingertip microstructures aid tactile edge sensation

One way that people interact with their environment is through the sense of touch. The sensation and perception of edge stimuli at our fingertips is a fundamental human ability whose underlying process is not fully understood. Mechanoreceptive transduction is the process with which receptors in the skin measure and report the deflection of skin to the central nervous system, where this information is ultimately interpreted. One of several cooperating mechanoreceptive systems, the slowly adapting type I (SA-I) system is particularly adept at detecting edges. Although many researchers have modeled SA-I sensation as stress and strain sensors embedded within smooth layers of skin, other researchers have suggested that the fingerprint lines (papillary ridges) and other skin microstructures play an important role in the SA-I system function. This dissertation considers whether the material properties and geometric arrangement of the papillary and intermediate ridge skin microstructures significantly affects the patterns of stress and strain that are sensed and discriminated by SA-I mechanoreceptors. Three specific aims are pursued using a solid mechanics modeling approach to simulate fingerpad tissues and correlating stress and strain distributions to actual neural responses to represent those same stimuli. The first aim demonstrates that currently accepted models are inconsistent with recent laboratory findings and that papillary ridges play little to no role in edge sensation. The second aim reconsiders the stress/strain measure used with current solid mechanics models for situations in which detailed skin microstructure is modeled. The third aim proposes that a relatively unexplored skin microstructure, the intermediate ridges, may play an important role in tactile perception. Together these aims suggest that the role of skin microstructures has been misunderstood and underappreciated and that edge discontinuities may be more accurately distinguished by adding intermediate ridge microstructure.