The movement-induced modulation in discriminability between cutaneous nonpainful stimuli depends on test stimulus intensity

The purpose of this study was to find out whether the finger movement-induced modulation of cutaneous discrimination thresholds varies with the intensity level of the test stimulation in various movement conditions. The effect of active and passive finger movement on cutaneous sensitivity to nonpainful electrical stimulation of threshold and suprathreshold intensity was studied in human subjects. The detection threshold and the just-noticeable amplitude difference (discrimination threshold) at two suprathreshold intensities (3× and 10× detection threshold) were determined using a forced-choice paradigm before and after (controls) or during finger movement. In one condition the stimuli were applied just prior to the movement. The finger was actively or passively moved at the frequency of 1.5 Hz or 3 Hz, and the test stimuli were applied to the moving finger, except in one condition to the contralateral finger. The contralateral condition was used to rule out vigilance- and attention-related mechanisms as a cause of sensitivity changes. Active as well as passive movement of the finger produced a significant increase in the detection threshold during the movement. Also just prior to the movement the detection threshold was increased. Suprathreshold discrimination thresholds in the moving finger were not significantly changed during or just prior to the active movement, whereas during passive movement the discrimination threshold to suprathreshold level stimulation was significantly decreased. When test stimuli were applied to the finger contralateral to the actively moving finger, neither the detection threshold nor the discrimination between stimuli of suprathreshold intensities were significantly changed. The results indicate that both active and passive movement can regionally suppress cutaneous sensitivity to threshold level stimuli, whereas discrimination between cutaneous stimuli of suprathreshold intensities can be improved or be unchanged during passive or active movement, respectively. The suppression of threshold level stimuli may be explained by both the activation of afferent-induced inhibition and corollary efferent barrage from motor to sensory areas of the brain, whereas afferent-induced mechanisms seem to have a predominant role in the improvement of discriminability between suprathreshold stimuli. The suppression of threshold sensitivity and the enhancement (or lack of suppression) of suprathreshold discriminability may be beneficial during movement allowing suppression of “noise” (e.g., skin receptor activation induced by moving finger) and allowing better discrimination of behaviorally relevant (suprathreshold) stimuli (e.g. discrimination of environmental stimuli during movement).