A model of spatiotemporal tactile sensitivity linking psychophysics to tissue mechanics

Sensitivities were measured for tangible spatiotemporal sinusoids applied to the index fingertip. The sinusoids had temporal frequencies of 8 and 128 Hz, in order to selectively activate the non-Pacinian I (NP I) and Pacinian (P) cutaneous mechanoreceptor systems, respectively, and had spatial frequencies from 0.00-1.03 cycles/mm. The sensitivity of the NP I system increased as the spatial frequency increased, whereas the sensitivity of the P system generally decreased as the spatial frequency increased. A mechanical model of the fingertip was used to calculate the normal and shear strains in the tissue, and a psychophysical linking hypothesis was introduced to predict tactile sensitivities based on the calculated strains. Specifically, the fingertip was modeled as a slab of a linear, isotropic, homogeneous, viscoelastic material. The boundary conditions were imposed by the spatiotemporal sinusoid at the top of the slab and the rigidly attached bone at the bottom of the slab. It was then assumed that the detection threshold was equal to the stimulus amplitude, which produced a constant, criterion strain at the location of the receptor. For both the P and NP I responses, the agreement between the predicted and measured sensitivities was best for calculations based on the normal strain, and for spatial frequencies below 0.5 cycles/mm. At higher spatial frequencies, the measured sensitivities were higher than predicted. The model also predicted the location of the P and NP I receptors in the tissue, the thickness of the tissue, and the value of the threshold strain for both receptor types. The predicted values agreed reasonably well with independent anatomical and physiological measurements.