Correlation and cluster analysis of sensory, pain, and reflex thresholds to various stimulus modalities in symptom-free subjects

Objective In order to evaluate the possible relation between the psychophysical response and a motor reflex, sensory and pain thresholds to various stimuli were analyzed in combination with the occurrence threshold of the late masseteric exteroceptive suppression (ES2) period. Methods Twenty men and 20 women participated. The tactile detection threshold and the filament-prick pain detection threshold were measured on the cheek skin overlying the left masseter muscles. The pressure pain threshold and pressure pain tolerance threshold were measured at the left masseter muscle. The surface EMG was recorded from the left masseter muscle, while electrical stimuli with 13 fixed intensities were applied to the skin above the left mental nerve. The stimulation intensity at which the ES2 appeared for the first time and the lowest stimulus intensity at which the subjects reported to be painful were defined as the ES2 and pain threshold, respectively. Results There were significant positive correlations between the tactile detection threshold and the pain thresholds determined using the different stimulus modalities, and the ES2 threshold was also significantly correlated with the pain thresholds (P < 0.05). Cluster analysis could significantly discriminate two distinct groups with high versus low tactile, pain and ES2 thresholds (P < 0.05). Conclusions The present findings suggested that the ES2 reflex response has a relation with the individual sensory and pain sensitivity in symptom-free subjects. Significance Combined examination of sensory, pain, and ES2 thresholds might provide complementary information on the pathophysiology underlying orofacial pain.