Pain threshold changes by skin vibratory stimulation in healthy subjects.

The purpose of this research is to evaluate the changes of cutaneous pain threshold induced by vibratory stimuli applied to the skin. The pain threshold was tested by means of electrical stimuli using the method of limits. Both vibratory stimuli (conditioning stimuli) and electrical stimuli (test stimuli) were driven by a computer. The experiments were carried out on the dorsal surface of the second phalanx of the index finger in the dominant hand of 34 healthy subjects (18 males and 16 females). The conditioning stimuli were sinusoidal waves (250 microm peak to peak) at a rate of 30 and 300 Hz. During the experiment (6 min of vibration and 9 min after) the cutaneous temperature of the stimulated area was recorded. Both vibratory rates induced significant increases of the pain threshold but the 30 Hz vibrations were the more effective. After stimulation the pain threshold lowered to a steady value which was higher than before the start of vibrations. Such threshold changes showed no correlation throughout the experiment with changes in skin temperature. We therefore advance the hypothesis that the main factors which modulate the pain threshold are neuronal connections between tactile and pain pathways.