Reduction in perceived intensity of cutaneous stimuli during movement: a quantitative study

The effect of movement of the index finger on the perceived intensity of trains of electrical stimuli to the digital nerves of the same finger was studied quantitatively using a novel intramodality stimulus-matching task. Subjects consistently were able to match reliably the perceived intensity of stimuli delivered on the 'reference' side with that of stimuli delivered simultaneously to the digital nerves of the index finger on the other ('matching') side. Both active and passive movement of the index finger (on the reference side) in the palmar plane reduced the matched stimulus voltage by about 10% of its control value for stimuli at twice the sensory threshold. This reduction in perceived intensity did not persist beyond the period of stimulation. An isometric contraction of first dorsal interosseous muscle produced a smaller, but statistically significant, reduction in perceived intensity. Non-noxious electrical stimulation of the digital nerves of the ipsilateral thumb or little finger also reduced the perceived intensity of stimuli to the index finger. Perceived intensity of stimuli during movement was also reduced, but to a lesser extent, when the index finger was stimulated at painful levels. Psychophysical studies using open magnitude scaling indicated that the relationship between stimulus intensity and perceived magnitude of electrical stimuli could be described by a power law with an exponent close to 1.0. Therefore, the percentage reduction in matching voltage accurately represents the percentage decline in perceived intensity. These results suggest that the perceived intensity of cutaneous stimuli to the index finger over a range of intensities can be reduced by afferent activity from the hand. The motor command appears to play a relatively minor role in modulating the perceived intensity.