AN INVESTIGATION OF THE MECHANICS OF TACTILE SENSE USING 2-DIMENSIONAL MODELS OF THE PRIMATE FINGERTIP

Tactile information about an object in contact with the skin surface i s contained in the spatio-temporal load distribution on the skin, the corresponding stresses and strains at mechanosensitive receptor locati ons within the skin, and the associated pattern of electrical impulses produced by the receptor population. At present, although the respons es of the receptors to known stimuli can be recorded, no experimental techniques exist to observe either the load distribution on the skin o r the corresponding stress-state at the receptor locations. In this pa per, the role of mechanics in the neural coding of tactile information is investigated using simple models of the primate fingertip. Four mo dels that range in geometry from a semi-infinite medium to a cylindric al finger with a rigid bone, and composed of linear elastic media, are analyzed under plane strain conditions using the finite element metho d. The results show that the model geometry has a significant influenc e on the surface load distribution as well as the subsurface stress an d strain fields for a given mechanical stimulus. The elastic medium ac ts like a spatial low pass filter with the property that deeper the re ceptor location, the more blurred the tactile information. None of the models predicted the experimentally observed surface deflection profi les under line lends as closely as a simple heterogeneous waterbed mod el that treated the fingerpad as a membrane enclosing an incompressibl e fluid (Srinivasan, 1989). This waterbed model, however, predicted a uniform state of stress inside the fingertip and thus failed to explai n the spatial variations observed in the neural response. For the cyli ndrical model indented by rectangular gratings, the maximum compressiv e strain and strain energy density at typical receptor locations emerg ed as the two strain measures that were directly related to the electr ophysiologically recorded response rate of slowly adapting type I(SAI) mechanoreceptors. Strain energy density is a better candidate to be t he relevant stimulus for SAIs, since it is a scalar that is invariant with respect to receptor orientations and is a direct measure of the d istortion of the receptor caused by the loads imposed on the skin.