Pupil dynamics in macaque recognition memory tasks: investigating physiological mechanisms

Cognitive deficits are common in primates, particularly in memory and emotional processes. Rhesus monkey (Macaca mulatta), widely used in cognitive and behavioral research, are central to memory studies. The relationship between recognition memory performance and pupillary dynamics in rhesus monkeys remains underexplored. This study investigated pupil dynamics during recognition memory tasks and their physiological correlates in five sexually mature male rhesus monkeys. We measured pupil diameter and oscillatory features during tasks and analyzed the relationship between behavioral performance and physiological indicators. We found that the average correct response rate exceeded the random success level, and reaction times were significantly shorter during successful recognition than failures, highlighting their cognitive efficiency. During recognition of familiar scenes, average pupil diameter increased, while maximum change in pupil size decreased, indicating reduced cognitive load. Both the frequency and amplitude of pupillary oscillation were lower during successful trials, reflecting decreased cognitive conflict and effective processing. This change reflects a decrease in cognitive conflict and suggests that information processing was more effective. The absolute value of the pupil peak slope decreased during successful recognition, indicating more stable cognitive state. These results support that pupillary dynamics can serve as physiological markers of cognitive effort in rhesus monkeys. Future studies should investigate how stimulus characteristics influence recognition and incorporate measures, such as intracranial EEG and fMRI, to enhance our understanding of their neural mechanisms. This research supports the rhesus monkey model in cognitive neuroscience and contributes to understanding primate cognition and its physiological foundations, with implications for clinical and translational research.