Thermography reveals emotional arousal in rats encountering highly preferred food

Thermography is increasingly used in studies on animal emotions. To gain information on physiological processes linked to emotions, an infrared camera can be used to measure subtle changes in skin and eye temperatures that correlate with changes in blood flow and muscle activity. Most thermographic studies on animal emotions have focused on detecting a temperature decrease in the nose and eyes, which indicates vasoconstriction caused by sympathetic activity during emotional arousal. Development of further thermographic methods to measure animal emotions will require identifying indicators for other physiological processes during emotional arousal as well, as they may be linked to partly different emotional experiences. The aim of this study was to test a potential indicator for which there is evidence from human studies: temperature increase on the forehead. It is thought to be caused by a rise in core body temperature due to increased thermogenesis (“emotional fever”) and by increased blood flow to periorbital muscles. Another aim was to contribute to practical design of thermographic experiments for freely moving animals. We tested whether exposing rats to highly preferred food would elicit a measurable rise in their forehead temperature. The animals were six outbred female rats, living in a large enriched communal cage, fed ad libitum. Each rat was tested separately, moving freely while it was presented alternatingly with ordinary food (whole grains) or highly preferred food (salmon paté). Three repetitions per rat were carried out for each type of food, counterbalanced for the order of presentation. During the first 60 seconds of eating, the facial temperature was recorded with a FLIR T620 thermal video camera. Data were collected from the thermal videos at 5-second intervals (+- 2.5 s) from the forehead and from a standard reference point on the nose bridge. During the first 30 seconds, exposure to preferred food elicited a significantly higher forehead temperature than exposure to ordinary food: the mean differences from the reference point were 3.0°C (SEM 0.2) vs. 2.6°C (SEM 0.2), respectively (F=8.4, p<0.01, linear mixed-effects model). From 35 seconds onwards the difference was no longer significant. This change may reflect a transition from an appetitive to a consummative phase of pleasure; the former is more plausible to elicit arousal. In conclusion, our results suggest forehead temperature is a promising indicator for emotional arousal in rats. The experimental setup proved to be successful and contributes to the practical design of thermographic experiments for fast-moving unrestrained animals.