THE MOVEMENT-INDUCED MODULATION IN DISCRIMINABILITY BETWEEN CUTANEOUSNONPAINFUL 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 condit ions. The effect of active and passive finger movement on cutaneous se nsitivity to nonpainful electrical stimulation of threshold and suprat hreshold intensity was studied in human subjects. The detection thresh old and the just-noticeable amplitude difference (discrimination thres hold) at two suprathreshold intensities (3 x and 10 x detection thresh old) 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 passiv ely moved at the frequency of 1.5 Hz or 3 Hz, and the test stimuli wer e applied to the moving finger, except in one condition to the contral ateral finger. The contralateral condition was used to rule out vigila nce- and attention-related mechanisms as a cause of sensitivity change s. Active as well as passive movement of the finger produced a signifi cant increase in the detection threshold during the movement. Also jus t prior to the movement the detection threshold was increased. Suprath reshold discrimination thresholds in the moving finger were not signif icantly changed during or just prior to the active movement, whereas d uring 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, nei ther the detection threshold nor the discrimination between stimuli of suprathreshold intensities were significantly changed. The results in dicate that both active and passive movement can regionally suppress c utaneous sensitivity to threshold level stimuli, whereas discriminatio n between cutaneous stimuli of suprathreshold intensities can be impro ved or be unchanged during passive or active movement, respectively. T he suppression of threshold level stimuli may be explained by both the activation of afferent-induced inhibition and corollary efferent barr age from motor to sensory areas of the brain, whereas afferent-induced mechanisms seem to have a predominant role in the improvement of disc riminability between suprathreshold stimuli. The suppression of thresh old sensitivity and the enhancement (or lack of suppression) of suprat hreshold discriminability may be beneficial during movement allowing s uppression of ''noise'' (e.g., skin receptor activation induced by mov ing finger) and allowing better discrimination of behaviorally relevan t (suprathreshold) stimuli (e.g.. discrimination of environmental stim uli during movement).