Functional near infrared spectroscopy (fNIRS) was used to explore hemodynamic responses in the human frontal cortex to noxious thermal stimulation over the right temporomandibular joint (TMJ). fNIRS experiments were performed on nine healthy volunteers under both low-pain stimulation (LPS) and high-pain stimulation (HPS), using a temperature-controlled thermal stimulator. By analyzing the temporal profiles of changes in oxy-hemoglobin concentration (HbO) using cluster-based statistical tests, several regions of interest in the prefrontal cortex, such as the dorsolateral prefrontal cortex and the anterior prefrontal cortex, were identified, where significant differences (p < .05) between HbO responses to LPS and HPS were shown. In order to classify these two levels of pain, a neural network-based classification algorithm was utilized. With leave-one-out cross-validation, the two levels of pain were identified with 99% mean accuracy to high pain. Furthermore, the “internal mentation hypothesis” and the default-mode network were introduced to explain our observations of the contrasting trend, as well as the rise and fall of HbO responses to HPS and LPS.