Fingertip skin as a linear medium for wave propagation

Tactile information arising from mechanical interaction with the environment is first conveyed by the skin. Prior studies have shown that mechanical waves propagating in tissues carry significant tactile information far away from the region of contact. It is therefore important to determine whether it is appropriate to consider the skin to be a linear wave propagation medium since linearity would considerably simplify any analysis related to this phenomenon. For example, linearity would enable the application of the superposition principle, of the reciprocity principle, inter alia, even if the skin is considered to be anisotropic. The linearity assumption is important for the scientist interested in the physics of the tactile perception and it can also be surmised to be taken into account by the neural circuits processing tactile information. Such property would be important even if much of the other tactile physics, such as friction, are predominantly nonlinear. We found that indeed, the human fingertip skin could be considered to be a linear propagation medium, except in irregular regions such as the folds near the joints where linearity breaks down.