Verifying the physiological significance of the preference for low facial temperature in humans

Regional differences in thermal pleasantness have been reported; local cooling of the head induces pleasantness during mild heat exposure. However, the physiological significance of these effects remains unclear. This study tested the hypothesis that facial or neck cooling could selectively decrease brain temperature (selective brain cooling; SBC).

Eight male volunteers participated in normothermic and hyperthermic protocols, comprising a 10-min baseline, 15-min cooling, and 15-min post-cooling period. During cooling, facial fanning (FAN_face), forehead conductive cooling (COND_head), neck conductive cooling (COND_neck), or control without cooling (NO-C) was performed. In the hyperthermic protocol, the participants performed cycling exercises at 60 % heart rate reserve until oesophageal temperature (T_es) rose by 1 °C. The difference between tympanic temperature (T_ty) and T_es was used as an index of brain cooling.

Under normothermic conditions, no significant difference was observed between T_ty and T_es in any trial. Under hyperthermic conditions, T_ty was significantly lower than T_es after facial fanning, with the largest difference 10 min after the end of facial fanning (T_ty = 37.6 ± 0.3 °C; T_es = 37.7 ± 0.3 °C; P = 0.001, Cohen's d = 2.021); the brain-cooling index (T_es-T_ty) was 0.08 ± 0.04 °C. No significant differences occurred in the NO-C, COND_head, or COND_neck trials. These results suggest that facial fanning should contribute slightly to brain cooling in hyperthermic individuals.

A preference for low facial temperature should help cool the brain. However, this brain cooling effect should be minimal compared with SBC in animals possessing carotid rete. SBC in humans could not be validated by the results in this study.