TSST-home - A pilot study investigating a home version of the 'Trier Social Stress Test'.

Disgust and fear are two emotions that evolved to serve different adaptive functions. While disgust primarily protects from disease by motivating the avoidance of pathogens, parasites, and toxins, fear facilitates escape from immediate threats, such as predators, via fight-or-flight responses. The differing consequences of these threats—from acute or chronic disease and reduced fitness in the case of infection to immediate death in the case of predation—may have led to the evolution of distinct responses. In humans, disgust is expressed by facial expressions that block the entry of potential toxins or pathogens (narrowed eyes, nose wrinkling, and raised upper lip), oculomotor avoidance after initial processing, and physiological reactions such as pupil constriction—or dilation—an inconsistency, which could be due to differences in stimulus type. Disgust also disrupts gastric rhythm, reduces heart rate, and activates the anterior insula. By contrast, fear manifests through expressions that enhance sensory perception (raised eyebrows, nose elongation, and stretched lips), early and sustained attention to the threat, along with pupil dilation, elevated heart rate, adrenaline release, and amygdala activation. Disparities in perception and outcomes may have contributed to the research gap between these two emotions, with fear historically receiving more attention, though interest in disgust has markedly increased since the 2000s. Disgust has been understudied not only in humans but across species. Moreover, these two emotions are frequently examined in isolation, even though threats from pathogens and predators often co-occur within the same environments. This siloed approach limits the ecological validity of current findings. Understanding how disgust and fear interact—or diverge—is crucial for uncovering the evolutionary mechanisms underlying threat detection and behavioral responses, particularly in our closest phylogenetic relatives, where emotional functioning remains largely unexplored. Recent research suggests that non-human primates display core disgust responses similar to those observed in humans, including characteristic facial expressions, physiological reactions like nausea, anterior insula activation, and avoidance behaviors. Despite increasing evidence of emotional expressions and physiological responses associated with disgust in non-human primates, its cognitive dimension—how it modulates attention and decision-making—has received little empirical investigation. In contrast, fear has been extensively studied behaviorally, cognitively, and physiologically. Notably, both emotions have been primarily examined through visual stimuli in primates, while threat perception may involve sensory diversity. In this study, we explored the effect of pathogen-versus predator-related stimuli on cognitive processes in captive trichromatic chimpanzees (; ), building on previous findings of chimpanzees’ avoidance behaviors toward pathogen-associated cues. We conducted four experiments to address the following questions: (1) Do pathogen-related images influence cognitive performance, here defined as the accuracy and latency with which individuals process and respond to task stimuli? (2) Do pathogen-associated odors influence cognitive performance? (3) Does cognitive performance differ under pathogen-versus predator-related stimuli? (4) Do attentional patterns differ between pathogen- and predator-related stimuli? To test these, chimpanzees completed a number ordering task, a standard touchscreen test of working memory and attention, while being presented with visual or olfactory stimuli. Attentional responses were measured using eye-tracking. In line with the hypothesis that disgust promotes the avoidance of physical and perceptual contact with pathogen sources, we predicted that pathogen-related stimuli would impair cognitive performance (e.g., reduce accuracy, reactivity and/or efficiency), while predator-related stimuli would enhance it due to heightened vigilance (). According to the eye-mind hypothesis—that gaze reflects attention and thought—we also expected that pathogen-related images would divert gaze and reduce attentional focus, while predator images would sustain visual attention. Understanding how these emotions modulate cognition offers insight into their adaptive roles in guiding behavior under threat.