Changes in prefrontal cerebral hemodynamics during intermittent pain stimulation to gingiva: Preliminary study using functional near infrared spectroscopy

Background/purpose: Elucidating the transmission mechanism of pain signals from the orofacial area and the corresponding modification mechanism will not only aid in the understanding of pain mechanisms but also provide useful information regarding the development of pain mitigation methods. In this study, the involvement of the pain suppression system in the trigeminal area was investigated through an analysis of the activation status over time in the prefrontal cortex using functional near-infrared spectroscopy (fNIRS). Materials and methods: In 28 healthy, right-handed male volunteers (average age, 30.1 +/- 4.2 years) as subjects, a mild, intermittent, acute pain stimulus was administered through the implementation of pocket probing of the gingiva surrounding the right maxillary central incisor. In the prefrontal cortex, the levels of hemoglobin (Hb) were measured using the fNIRS measurement system. Average values of both oxy-Hb and deoxy-Hb were calculated at four stages: rest stage, 20 s prior to the pain stimulus application, and three stages at 20-s intervals within 1 min of stimulation. One-way analysis of variance and multiple comparisons were used to compare representative values to investigate the changes due to pain. Results: Oxy-Hb levels decreased the most during the 20 s stage directly after stimulus application. This change was seen mainly on the contralateral side, after which it returned to the resting baseline level before the stimulus application. Conclusion: Our data demonstrate that in healthy males, a mechanism exists to mitigate pain involving the pain suppression system in the 20 s after feeling mild pain to the gingiva.