Movement-related modulation of vibrotactile detection thresholds in the human orofacial system

By virtue of the direct coupling between circumoral skin and the underlying orofacial musculature, mechanosensation associated with precise orofacial force control may contribute significantly to processes associated with perception, proprioception, and sensorimotor control in this region. The purpose of this study was to assess lower lip (LL) vibratory detection thresholds of adult subjects during the simultaneous performance of a visually guided and continuous lip motor control task. Vibrotactile inputs were delivered to the right LL vermilion at test frequencies of 5, 10, 50, 150, 250, and 300 Hz. The psychophysical detection task was performed simultaneously with the three force control conditions: a null-force baseline condition, an active force control task performed with the right index finger, and an active force control task performed with the lip musculature. For the active tasks, subjects were instructed to use their analog force signal (lip or finger) to continuously perform a visually guided precision force task by tracking a 2 Hz sinusoidally moving target calibrated to a peak-to-peak force load of 0.2 N. Both the analog force signal and the target-tracking signal were displayed in real-time on an oscilloscope. Results showed a statistically significant elevation of LL vibrotactile detection thresholds for test frequencies below 50 Hz during the simultaneous performance of the lip force control task. Disassociating the site of motor control from the location of sensory stimulation (active control task) was effective in normalizing the elevations in LL vibrotactile thresholds, thus demonstrating that the threshold elevation during the lip force task was not solely an artifact of the added attentional load resulting from combining a perceptual task with a motor control requirement. These findings are discussed in relation to published reports of movement-related sensory gating in limb systems and the possible significance that this phenomenon may have for perception and proprioception in the orofacial system.