Measurement and modelling of spatiotemporal tactile sensitivity
To test the hypothesis that the NP I and NP III cutaneous receptor systems are spatial differentiators with finite resolution, and that the P and NP II systems are spatial integrators with finite sensitivity, a stimulator array is constructed to delivery tangible spatiotemporal sinusoids to the fingertips of human subjects. Twelve spatial frequencies from 0.00 to 1.03 cycles/mm are used to stimulate the P system at a temporal frequency of 128 Hz, and to stimulate the NP I system at a temporal frequency of 8 Hz. The measured sensitivities are only qualitatively similar to the hypothesized sensitivities, with the greatest disagreement at high spatial frequencies. To discover the mechanism for the measured sensitivities, a continuum-mechanics model is constructed which describes as a slab of a linear, homogeneous, isotropic, viscous, elastic material with finite thickness and infinite horizontal extent. The equations of motion within the tissue are then solved and the normal and shear strains are calculated everywhere in the tissue. A psychophysical linking hypothesis is proposed, asserting that the detection threshold corresponds to a constant value of one strain component at the location of the receptor. With this hypothesis, the calculated strains are used to predict the psychophysically measured sensitivitites. The model parameters (P and NP I receptor locations and threshold strains, and tissue thickness) are adjusted to produce a minimum-$\chi\sp2$ fit to the data. The results indicate that the P and NP I receptors respond to the normal strain in the tissue, but that the criterion threshold strain and receptor depth are different for the two receptors. The predicted values of the receptor depths and tissue thickness agree well with measurements made from an anatomical survey of a cadaver fingertip, and the predicted threshold strains are physiologically reasonable. Finally, the data suggest, though weakly, that the sensitivities measured at the two highest spatial frequencies may be mediated by the NP III system.