Evaluating Pupillometry as a Tool for Assessing Facial and Emotional Processing in Nonhuman Primates

Non-human primates (NHPs) are extensively utilized to investigate the neural mechanisms underlying face processing; however, measuring their brain activity necessitates a diverse array of technologies. Pupillometry emerges as a convenient, cost-effective, and non-invasive alternative for indirectly assessing brain activity. To evaluate the efficacy of pupillometry in assessing facial and emotional processing in NHPs, this study designed a face fixation task for experimental monkeys (Rhesus macaque) and recorded variations in their pupil size in response to face images with differing characteristics, such as species, emotional expression, viewing angles, and orientation (upright vs. inverted). All face images were balanced with luminance and spatial frequency. A sophisticated eye-tracking system (Eye-link 1000 plus) was employed to observe the pupils and track the viewing trajectories of monkeys as they examined images of faces. Our findings reveal that monkeys exhibited larger pupil sizes in response to carnivore faces (versus human faces, p = 0.035), negative conspecific faces (versus human faces, p = 0.018), and profile viewing angles (versus frontal view angles, p = 0.010). Notably, pupil size recorded during the 500–1000 ms post-stimulus interval was negatively correlated with their gaze durations directed at those images (r = −0.357, p = 0.016). Overall, this study demonstrates that pupillometry effectively captures subtle differences in facial and emotional processing, underscoring its potential as a valuable tool in future cognitive research and the diagnosis of disorders.