Computational Modeling Reinforces that Proprioceptive Cues May Augment Compliance Discrimination When Elasticity Is Decoupled from Radius of Curvature

Our capability to discriminate object compliance is based on cues both tactile and proprioceptive, in addition to visual. To understand how the mechanics of the fingertip skin and bone might encode such information, we used finite element models to simulate the task of differentiating spherical indenters of radii (4, 6 and 8 mm) and elasticity (initial shear modulus of 10, 50 and 90 kPa). In particular, we considered two response variables, the strain energy density (SED) at the epidermal-dermal interface where Merkel cell end-organs of slowly adapting type I afferents reside, and the displacement of the fingertip bone necessary to achieve certain surface contact force. The former variable ties to tactile cues while the latter ties to proprioceptive cues. The results indicate that distributions of SED are clearly distinct for most combinations of object radii and elasticity. However, for certain combinations – e.g., between 4 mm spheres of 10 kPa and 8 mm of 90 kPa – spatial distributions of SED are nearly identical. In such cases where tactile-only cues are non-differentiable, we may rely on proprioceptive cues to discriminate compliance.