Using physiological markers to measure sympathetic activation can be used to infer pain and stress in humans. To date, the only methods in humans that reproducibly do this are invasive and poses an undesired risk to participants. Previous work on cattle (Bos taurus) has used infrared thermography to measure the temperature of the lacrimal caruncle region and provides a promise of a novel method for measuring stress and pain non-invasively. The current study attempted to determine if this method could be transferred for use in humans. Sixteen participants were recruited for the study and underwent temporary painful stimuli using validated methods (that have previously been shown to induce sympathetic activity known as the cold pressor test and the muscle chemoreflex). During the trials, measurements included temperature of the lacrimal caruncle, heart rate, mean arterial blood pressure and pulse transit time. Following each trial, blood was drawn to measure concentrations of norepinephrine and epinephrine in the plasma. An enzyme-linked immunosorbent assay was then performed to attempt to quantify the catecholamine concentrations, however, the standards of the concentrations were not reliably determined so these results were excluded from the study. A two-way repeated measures analysis of variance was performed with the factors of condition and time. A condition x time interaction was observed in heart rate (df= 8, F= 8.020, p<0.01), mean arterial pressure (df= 6 F= 12.6, p<0.01), and pulse transit time (df= 8, F= 2.269, p= 0.03), but not temperature of the lacrimal caruncle. The results of this study suggest that infrared thermography is not a reliable tool to measure sympathetic activation in humans. This study also suggests that changes in blood flow at the lacrimal caruncle region in response to sympathetic activation may be more complicated than previously proposed.
