Spatial summation of vibrotactile sensations at the foot

Thresholds for the perception of vibration on the hand reduce with increasing area of excitation when the thresholds are mediated by the Pacinian channel (a phenomenon known as spatial summation) but thresholds are generally independent of the area of excitation when they are mediated by non-Pacinian channels. The effect of the area of excitation on vibrotactile thresholds at the sole of the foot has not been thoroughly investigated. In the study reported in this paper, thresholds for the perception of 20 Hz vibration and 160 Hz vibration were determined on the foot (at the big toe (hallux), the medial (inside) ball, the lateral (outside) ball, and the heel) and on the hand (at the thenar eminence and at the fingertip) in 12 male subjects using four probe diameters: 1mm (0.19 cm(2) excitation area), 3mm (0.38 cm(2)), 6 mm (0.78 mm(2)) and 10 mm (1.53 cm(2)) with a 2mm gap between the vibrating probe and a fixed surround. On both the hand and the foot, thresholds for the perception of 160 Hz vibration decreased as the probe diameter increased. There was no overall consistent change in thresholds for the perception of 20 Hz vibration. Thresholds for the perception of 160 Hz vibration were lowest at the fingertip and highest at the big toe. Thresholds for 20 Hz vibration were also lowest at the fingertip. It is concluded that on the sole of the foot there is evidence of spatial summation in the perception of 160 Hz vibration, mediated by the Pacinian channel, but not in the perception of 20 Hz vibration, mediated by a non-Pacinian channel. The findings show that vibrotactile thresholds at the foot obtained with different areas of excitation, or an unknown area of excitation, should not be compared. It is concluded that there is a need to standardise methods of measuring the vibrotactile thresholds at the foot that are obtained for clinical applications.