## 4 Discussion  
Referred pain is explained by the converging projection between peripheral nociceptive and sensitive fibres of the reference pain areas on the same interneuron in the spinal marrow. Pain is referred to be felt in a different place from its actual origin site because  
Figure 3: Example of the IR imaging and the corresponding VAS template. nociceptive stimuli, both those originating at the trigger points and those of the target area, reach upper nervous centres through just one neuron of second-order [8]. For this reason, the stimulation of the target area does not increase the symptomatology, while the stimulation of the trigger points increases both local and referred pain. It is also possible that the patient feels the referred pain without perceiving pain at the trigger point location. Given the central role of the sternocleidomastoid for mastication, swallowing, and control of head movements and the fact that trigger points for referred pain are more commonly found in the sternocleidomastoid, we decided to focus our research only on its trigger points and referred pain areas. According to the maps of referred pain, the stimulation of the sternocleidomastoid breastbone's head induces pain in the occup; then, it radiates in the eyebrow, cheek, and throat, whereas the stimulation of the collarbone's head is linked to a frontal headache, to a mastoid and auricular pain. Our investigation wanted to examine thermal reactions in facial areas of referred pain after the stimulation of sternocleidomastoid trigger points. Previous studies focused on the thermographic assessment of facial temperature distribution in patients suffering from myofasical pain [18]. However, to the best of our knowledge, no one examined whether the temperature changes eventually found in those regions correlates with the intensity of the perceived pain concomitantly with the stimulation of sternocleidomastoid. No significant local temperature changes were found to be associated with a 0 or 1 VAS score. This result agrees with the literature which reports hyperthermal distribution in the presence of referred pain [25]. The use of thermographic examination in the diagnosis of TMDs have been largely investigated in the literature [26; 27; 28; 29], and several studies focused on myogenous pain [20; 25] but only a few studies applied superimposition of ROI images [18; 30]. A recent survey also assessed that facial nerve function is one of the possible biomedical applications of infrared thermography [31]. Starting from these evidences, the present study seems to state the efficiency of the IR thermography in investigating facial nerve response related to TMDs. Our study refers to a small sample of subjects. There is no clear evidence supported by strong statistical analysis to support our findings, and therefore it might be considered as a pilot study. However, our results seem to prove that the amplitude of the temperature variation is related to the self-reported perceived pain and introduce several elements of novelty such as the stimulation scheme, the focus on the sternocleidomastoid muscle, and the image processing of thermal data. We found that the temperature variation 2 s after the stimulation (t2) was not significantly different from the pre-stimulation temperature, while the largest temperature differences occurred after several seconds (t3). This result suggests that the thermal processes associated with the pain perception in the referred pain areas are slow and delayed compared to the stimulus. The absence of previous studies investigating IR imaging the referred pain evocated by stimulation of TrP locations along the sternocleidomastoid belly makes it difficult to compare our results with the existing literature. [PERSON] et al. [18] correlated the referred and local pain evocated by TrP situated on masseter and anterior temporalis muscles using thermography and algometry by ROI images, concluding that IR imaging measurement could be an aid in the diagnosis of myofasical trigger points in masticatory muscles.