Two-point discrimination of vibratory perception on the sole of the human foot.

The capability for discrimination of vibratory stimuli of the sole of the foot was tested on 12 subjects (6 men, 6 women). Two vibratory stimuli (sinusoidal impulse: frequency, 200 Hz; amplitude, 0.1 mm; duration, 2 sec) were applied at different sites to the sole of the foot. Subjects were asked whether they detected one or two vibratory sensations. The discrimination threshold was defined as the shortest distance at which two vibrations can be differentiated at repeated trials. In addition, the absolute threshold was measured by one plunger (sinusoidal vibration: frequency, 200 Hz; variable amplitude). The values of the absolute threshold (average value, 1.2 microns) were in accordance with the results of other authors. A decrease of discrimination capability was observed along the longitudinal axis of the foot from distal to proximal parts and was about 15 mm at the big toe and 34 mm at the heel. Discrimination at the medial longitudinal arch was inferior to that at the lateral longitudinal arch. This was also true for inverse vibration in which all subjects showed lower thresholds. No relationship was found with respect to thickness of the skin (measured by ultrasound), side of foot, or gender. Determining factors for vibratory threshold might be the density of the Pacinian corpuscles per cm2 as well as their peripheral and central representation. Anatomical or physical factors (e.g., thickness of the skin, impedance of the deeper tissue) can probably be disregarded.