Time-course changes of psychological status and the masseteric reflex

Objectives: In symptom-free subjects, the present study examined time course changes of perceived pain intensity, and quantitative measures of the second exteroceptive suppression (ES2) period in the masseteric EMG after incremental electric stimulation in relation to a psychological evaluation.

Methods: Twelve males and 12 females (age range from 20 to 35 years) participated. The surface EMG was recorded from the left masseter muscle. Thirteen fixed stimulus intensities from 5 to 35 mA at 2.5 mA increments were applied to the skin above the left mental nerve. The stimulation intensity at which the late ES appeared first and the lowest intensity at which the subjects reported the stimulus intensity to be painful were defined as the reflex threshold (RT) and pain threshold (PT), respectively. Three experimental sessions were scheduled at one week intervals. The state and trait anxiety inventory was used to evaluate the psychological status at each experimental session. Repeated measured ANOVA was used to analyze the time-course effect.

Results: From the first to second and third session, the state anxiety inventory score significantly decreased (36.3 ± 4.2, 32.1 ± 3.6 and 31.3 ± 3.4, respectively, mean ± SD, P<0.01), and the RT also significantly decreased (16.87 ± 4.2, 13.3 ± 3.8 and 13.4 ± 3.2, respectively, P<0.01) . By contrast, the PT significantly increased (12.8 ± 5.9, 18.3 ± 7.6, 19.0 ± 6.7, respectively, P<0.01). No change was observed in the trait anxiety inventory score.

Conclusions: The present results illustrate that the reflex parameters and the pain threshold changed in line with the state anxiety. Possibly, habituation or decreased fear from the experimental set-up may play a role.

Acknowledgement: Supported by a 2006 collaborative study grant of the International Association for the Study of Pain (IASP) and a Nihon University Individual Research Grant for 2007 (07-101).