## 4 Discussion and Conclusions
The latest observation shows that it is the local temperature impact that leads to changes in BPA. These changes were not caused by additional light reflections from the pair of plastic-water interfaces introduced by the compress. Despite that the plastic bag with water affects the transmitted light, the AC/DC ratio used for evaluation of BPA in our algorithm (see Equation (1)) is almost independent from light intensity variations. Figure 3B shows that the features of the spatial distribution of BPA assessed through the plastic bag are similar (but with higher amplitude) as in Figure 3C after removing the compress. We suggest that the proposed heating test and its monitoring have important diagnostic value for local regulation of microcirculation in patients having different diseases of pathogenesis characterized by factors violating the neurogenic and endothelial regulation of microcirculation [20; 21]. It should be noted that our previous experiments, in which applicability of an IPPG system to assess the microvasculature response on the temperature changes [12] was demonstrated, were carried out in conditions under which the temperature of a preliminarily-cooled finger was increased due to internal heating. In contrast, here we assessed the microcirculation reaction to the external local thermal impact with the temperature exceeding the skin temperature. Our near-future plans include carrying out a study to monitor the microcirculation reaction to the application of a cold compress (below the forehead temperature). It is worth noting that the proposed technique allows measuring the vascular response to the local heat exposure in almost any area of human skin. This is also a valuable feature, since in many diseases the changes in microcirculation are local in nature, which makes it necessary to carry out diagnostic studies in specific regions. The observed correlation between the BPA increase and the intensity of the thermal impact could be explained by previous observations demonstrating that a larger number of different TRP channels are involved in the vascular reaction to the temperature increase [22]. The response of each type of channel is likely summarized with the increasing temperature of the impact. Therefore, the revealed differences in the hemodynamic response with the different intensity of the thermal exposure create the prerequisite for the differential assessment of the function of various types of TRP channels involved in thermoregulation. In conclusion, the method for assessing local microcirculation demonstrated in this work has the potential to be a valuable diagnostic tool that allows for the first time to reveal the direct response of microvessels to the temperature exposure at the place of its application. To realize this potential successfully, it is necessary to carry out further studies of the vasomotor response in various regions and under different temperature exposure in patients with diseases such as diabetes mellitus, systemic scleroderma, obliterating vascular diseases, as well as in patients with cardiovascular risk factors. Conceptualization, A.A.K., O.V.M. ; methodology, A.A.K., O.V.M. ; experimental setup design and manufacturing, A.A.K., M.A.V., N.B.M. ; software design, A.A.K. ; formal analysis, M.A.V., N.B.M. ; investigation, M.A.V., N.B.M., A.A.K. ; writing--original draft preparation, M.A.V., N.B.M., O.V.M. ; writing--review and editing, A.A.K. ; visualization, M.A.V. ; supervision, A.A.K. This research was funded by the Russian Science Foundation (grant number 15-15-20012) in terms of developing a conception and methodology of research, software development, experimental data collecting and processing. The Ministry of Science and Higher Education of the Russian Federation (project number 8.2501.2017/4.6) supported this research in terms of modifications of the experimental setup, including temperature control equipment and operational modes optimization for the optical-electronic part of the setup. The authors declare no conflict of interest.