Facial temperature and pupil size as indicators of internal state in primates

•Facial temperature and pupil size changes are sympathetic responses to emotions.•Facial temperature decreases in response to emotionally salient stimuli.•Non-luminance-mediated changes in pupil size track cognitive processes.•Facial temperature and pupil size may aid in understanding mental disorders. Studies in human subjects have revealed that autonomic responses provide objective and biologically relevant information about cognitive and affective states. Measures of autonomic responses can also be applied to studies of non-human primates, which are neuro-anatomically and physically similar to humans. Facial temperature and pupil size are measured remotely and can be applied to physiological experiments in primates, preferably in a head-fixed condition. However, detailed guidelines for the use of these measures in non-human primates are lacking. Here, we review the neuronal circuits and methodological considerations necessary for measuring and analyzing facial temperature and pupil size in non-human primates. Previous studies have shown that the modulation of these measures primarily reflects sympathetic reactions to cognitive and emotional processes, including alertness, attention, and mental effort, over different time scales. Integrated analyses of autonomic, behavioral, and neurophysiological data in primates are promising methods that reflect multiple dimensions of emotion and could provide tools for understanding the mechanisms underlying neuropsychiatric disorders and vulnerabilities characterized by cognitive and affective disturbances.