Relationship between vibrotactile detection threshold in the Pacinian channel and complex mechanical modulus of the human glabrous skin.

The goal of this study was to investigate the relationship between the psychophysical vibrotactile thresholds of the Pacinian (P) channel and the mechanical properties of the skin at the fingertip. Seven healthy adult subjects (age: 23-30) participated in the study. The mechanical stimuli were 250-Hz sinusoidal bursts and applied with cylindrical contactor probes of radii 1, 2, and 3.5 mm on three locations at the fingertip. The duration of each burst was 0.5 s (rise and fall time: 50 ms). The subjects performed a two-interval forced-choice task while the stimulus levels changed for tracking the threshold at 75% probability of detection. There were significant main effects of contactor radius and location (two-way ANOVA, values of p < 0.001). The thresholds decreased as the contactor radius increased (i.e., spatial summation effect) at all locations. The thresholds were lowest near the whorl at the fingertip. Additionally, we measured the mechanical impedance (specifically, the storage and loss moduli) at the contact locations. The storage moduli did not change with the contactor location, but the loss moduli were lowest near the whorl. While the loss moduli decreased, the storage moduli increased (e.g., more springiness) as the contactor radius increased. There was moderate and barely significant correlation between the absolute thresholds and the storage moduli (r = 0.650, p = 0.058). However, the correlation between the absolute thresholds and the loss moduli was high and very significant (r = 0.951, p < 0.001). The results suggest that skin mechanics may be important for locally shaping psychophysical detection thresholds, which would otherwise be expected to be constant due to uniform Pacinian innervention density at the fingertip.