Evaluating Pupillometry as a Tool for Assessing Facial and Emotional Processing in Nonhuman Primates  
[PERSON]  
1 Shenzhen Technological Research Center for Primate Translational Medicine, Shenzhen-Hong Kong Institute of Brain Science, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China; [EMAIL_ADDRESS] (X.L. ); [EMAIL_ADDRESS] (Z.Z. )1 Shenzhen Technological Research Center for Primate Translational Medicine, Shenzhen-Hong Kong Institute of Brain Science, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China; sh. the temporal cortex contains neurons responsive to both face orientation [6] and viewing angles [7; 8; 9; 10]. Moreover, pupil size in primates is influenced by neural activity, offering a unique \"window\" into brain function [11; 12; 13]. Key neural substrates that regulate pupil size, primarily located in the brain stem, include the locus coeeruleus (LC) and pretectal olivary nucleus (PON), which receive input from retinal ganglion cells [11; 14]. In NHPs, the LC receives projections from both the prefrontal cortex (PFC) and anterior cingulate cortex (ACC) [15; 16], influencing PFC activity through noradrenergic pathways [17]. The PON connects to the inferior temporal and visual cortices [18; 19]. Electrophysiological studies have revealed that variations in pupil size correspond to neural activity in the PFC and ACC during passive fixation and unexpected events in NHPs [20]. Additionally, microstimulation of the PFC has been shown to affect the pupil light reflex in these animals [21; 22]. In humans, similar MRI studies have identified correlations between blood oxygenation level-dependent (BOLD) activity in the ACC and orbitofrontal cortex and changes in pupil size [23]. Collectively, these findings suggest that changes in pupil size can reflect underlying brain activity in primates. Given this context, pupillometry has emerged as a valuable tool for assessing emotional and cognitive processing in NHPs [11; 24]. This study aims to evaluate whether pupillometry can be applied to investigate facial processing in NHPs. Compared to traditional methods for studying primate brain activity--such as multi-channel electrophysiological recording and fMRI--pupillometry offers a cost-effective, convenient, and non-invasive alternative [20]. As NHPs lack the ability to communicate verbally with humans, pupillometry presents an ideal approach for uncovering their internal states [24]. The study developed a face fixation task (FFT) featuring various face images, differing in species, expressions, orientation, and viewpoints--all known to affect neural activity in NHPs [4; 5; 6; 25; 26; 27; 28; 29; 30]. By employing an eye-tracking system, it will monitor the pupil area and gaze location of NHPs while they observe these facial stimuli. Through this research, it aims to determine whether pupillometry effectively reveals psychological differences in facial processing among NHPs. The key contributions of this research can be outlined as follows:  
1. This study investigates the potential of pupil size variations as indicators of emotional responses in NHPs. Furthermore, it aims to evaluate the effectiveness of changes in pupil size as a method for interpreting the emotional states of animals. 2. By analyzing the pupillary reactions of NHPs to facial imagery, it can differentiate their emotional responses to faces from those of humans. 3. Ultimately, this research seeks to explore the viability of pupil responses as a biomarker for deficits in face processing, as such impairments are a significant hallmark of several neurological disorders. The structure of this paper is organized as follows: Section 1 (Introduction) offers a review of recent related literature in this field. Section 2 (Related Works) reviews recent related works. Section 3 (Materials and Methods) outlines the animal subjects, experimental procedures, and materials utilized throughout the experiments, while also covering data and statistical analysis methods.