A Pupil-Dilation Method to Test Developmental Differences in Visual Synchrony During Free Viewing

Visual synchrony, a form of coordinated behavior wherein observers look at displays in a similar manner, is important for understanding how coordinated visual attention influences cognitive, emotional, and social development. Traditional developmental research tested visual synchrony using gaze location metrics—assessing the convergence of children's visual focus at any given moment. However, gaze location is not the only looking measure linked to attention and cognitive states. Pupil dilation—the process of the pupils increasing in size as a physiological response to stimuli—offers a window into the autonomic nervous system, providing insights into cognitive load, emotional arousal, and attention shifts. Our aim was to develop and validate a new technique—time-resolved pupillometry pipeline—that tests developmental differences in visual synchrony by quantifying within-group physiological coupling during free viewing. Rather than comparing raw pupil size between children and adults, we identify when each group exhibits coordinated pupillary responses and whether the temporal profile of synchrony differs across development. This approach complements gaze-based measures by capturing autonomic co-fluctuations linked to attention and arousal, yielding a scalable tool for naturalistic developmental research. We demonstrate our approach in previously collected data from preschoolers and adults during free viewing of cartoon videos. We found stable and significant time windows where the two age groups differed in synchrony, suggesting different physiological responses to the videos within each age group. We shared all our analyses and tutorials for other researchers to use. Findings highlight the potential for using pupil dilation to explore how individuals, from children to adults, synchronize their attention and emotions. Such a technique offers a richer picture of what and how children share visual information during observation.