Summary. The ability of subjects to perceive innocuous stimuli in the presence and absence of movement was evaluated using electrical stimulation of the skin. The subjective intensity of suprathreshold stimuli was unchanged during movement. Discrimination of small differences in the intensity of suprathreshold stimuli (difference thresholds) was also not altered by movement while, in the same subjects, detection thresholds were increased during movement of the stimulated arm. These results suggest that the elevation of detection thresholds during movement can be explained by masking. Both active and passive movement of the stimulated limb increased detection thresholds, with active movement having a slightly greater and more consistent effect than passive movement. Thus, both central and peripheral feedback factors appear to play a role in diminishing one's ability to detect weak stimuli during movement. Attention was also shown to influence performance of the detection task.
The time course and spatial extent of movement-related suppression of the detection of weak electrical stimuli (intensity, 90% detected at rest) was determined in 118 experiments carried out in 47 human subjects. Subjects were trained to perform a rapid abduction of the right index finger (D2) in response to a visual cue. Stimulus timing was calculated relative to the onset of movement and the onset of electromyographic (EMG) activity. Electrical stimulation was delivered to 10 different sites on the body, including sites on the limb performing the movement (D2, D5, hand, forearm and arm) as well as several distant sites (contralateral arm, ipsilateral leg). Detection of stimuli applied to the moving digit diminished significantly and in a time-dependent manner, with the first significant decrease occurring 120 ms before movement onset and 70 ms before the onset of EMG activity. Movement-related and time-dependent effects were obtained at all stimulation sites on the homolateral arm as well as the adjacent trunk. A pronounced spatiotemporal gradient was observed: the magnitude of the movement-related decrease in detectability was greatest and earliest at sites closest to the moving finger and progressively weaker and later at more proximal sites. When stimuli were applied to the distant sites, only a small (approximately 10%), non-time-dependent decrease was observed during movement trials. A simple model of perceptual performance adequately described the results, providing insight into the distribution of movement-related inhibitory controls within the CNS.
The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger. The surfaces were 7.5x2.4-cm polymer strips embossed with truncated cones 1.8 mm high with a spatial period of 2.0 mm in the transverse direction and 1.5-8.5 mm in the longitudinal, scanning direction. The surfaces were mounted on a six-axis force and torque sensor that measured the perpendicular, contact force (normal to the skin surface) and the tangential force along the axis of stroking. The results confirmed the findings of an earlier study that magnitude estimates of perceived roughness increase approximately linearly up to a longitudinal spatial period of 8.5 mm. Across subjects, no consistent correlations were found between perceived roughness and either the mean normal or tangential force alone. Although significant positive correlations were found between roughness and mean kinetic friction for all subjects, they were not as consistently robust as one might have expected. Furthermore, instantaneous kinetic friction varied widely over the course of a single stroke because of within trial oscillations in the tangential force. The amplitude of these oscillations increased with the longitudinal spatial period and their frequency was determined by a combination of the spatial period and the stroking velocity. These oscillations were even more conspicuous in the first derivative or rate of change of the tangential force (dFx/d t), which was quantified as the root mean square (RMS) of the tangential force rate. The mean normalized RMS proved to be strongly correlated with subjective roughness, averaging 0.88 for all subjects. In order to dissociate the fluctuations in tangential force from both the surface structure and the mean kinetic friction, a second experiment was performed on six additional subjects who estimated the roughness of identical lubricated and unlubricated (dry) surfaces. Lubrication with liquid soap reduced the mean kinetic friction by approximately 40%, the RMS of the tangential force rate by slightly more than 21% and the subjective estimates of roughness by 16.4%. Taken together, the results suggest that in tactile exploration, the RMS of the tangential force rate may be an important determinant of subjective roughness.
Active and passive touch, respectively with and without voluntary movement on the part of the subject, are frequently reported to be equivalent in terms of the resultant perceptual abilities. This review reexamines the notion of perceptual equivalence in the light of growing evidence that the transmission of tactile inputs is diminished, or "gated," during the course of active movement. It is concluded that there is indeed gating of cutaneous inputs during active touch. In most experiments, the paradoxical observation of perceptual equivalence between active and passive touch can partly be explained by the choice of task, namely, tactile discriminations that depend on relative, and not absolute, differences in inputs. The surprising lack of evidence for any superiority of passive touch over active touch can likely be explained by several factors. First, performance with active touch may be enhanced by the motor strategy, e.g., by reducing the speed of movement at critical points, and so reducing the degree of gating, and (or) by optimally orienting the exploring digits so as to bring the most sensitive skin areas into contact with the object in question. Second, central influences, including attention and motor set, may be specifically activated during voluntary movement and contribute to enhancing performance during active touch. Thus, the gating influences associated with active touch may be offset, partly or wholly, by the combined influence of these factors to yield (near) perceptual equivalence for active and passive touch.
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