## 5 Discussion
### Pupil Size Changes in Face Processing  
Firstly, it was found that NHPs exhibited larger pupil sizes in the late phase when viewing images of carnivores (tigers, leopards, and lions) compared to human faces. The subject monkeys had been trained to interact with humans, receiving food, water, and fruits from them, and had visual exposure to both monkeys and humans within their environments. As a result, these monkeys can be considered familiar with human faces. In contrast, they had no prior exposure to carnivores or snakes, which are recognized as primary predators of primates [53; 54]. [PERSON] et al. [4] reported that neurons in the prefrontal cortex show stronger responses to images of snakes and carnivores than to humans in NHPs. Significant pupil size changes were not observed in response to snake images; this may be attributed to our control of low-level image properties, such as color, spatial frequency, and luminance. Snakeskin is characterized by a distinctive color and texture (smoothy and hairless) that differs from that of common mammals. By manipulating these low-level attributes of the images, it may have complicated the monkeys' recognition of snakes, ultimately reducing their salience.  
Secondly, it identified that NHPs displayed larger pupil sizes in response to negative facial expressions of monkeys, but not humans. The ability to recognize negative emotions from faces is crucial for survival [1], and imaging studies have shown that multiple brain regions in NHPs exhibit increased BOLD signals when viewing conspecific negative expressions. Electrophysiological research suggests that the orbitofrontal cortex and medial prefrontal cortex contain neurons responsive to these expressions [4; 5; 28]. Our findings align with this biological evidence regarding pupil size modulation by conspecific negative expressions. However, a similar pupil response to negative human expressions was not observed, potentially because NHPs and humans communicate emotional facial expressions differently [41; 49; 55], making it challenging for NHPs to infer negative emotions from human faces.  
Thirdly, viewing angle influenced pupil size, with NHPs responding with larger pupil sizes to profile views compared to frontal views. Research has shown that neurons in the temporal cortex of NHPs are sensitive to viewing angles [8; 10]. The recognition of emotions and identities from frontal views occurs more quickly and accurately than from profile views [56; 57; 58; 59]. Therefore, we propose that viewing faces from a frontal perspective allows NHPs to more efficiently gather social information, such as identity and emotions, thereby reducing uncertainty in facial recognition [56; 57; 58; 59].  
These findings indicate that changes in pupil size during the middle and late phases (200-1000 ms post-stimulus presentation) reflect the processing of facial information. Human studies have shown that pupil sizes increase under high working memory load and attentional control [38; 39]. It is plausible that NHPs also require increased cognitive effort to recognize threatening or uncertain face information quickly. Moreover, our discovery of a negative correlation between gaze duration and pupil size in the late phase (500-1000 ms post-stimulus presentation) suggests that smaller pupils correspond to longer gaze durations, indicating that NHPs might allocate less attentional control and engage in prolonged viewing to comprehend facial information.