Abstract
Humans spend hours each day spontaneously engaging with visual content, free from specific tasks and at their own pace, such as when scrolling through social media. In line with the increasing interest in naturalistic and ecologically valid research, here we aimed to understand how the brain supports such spontaneous perceptual experiences. To that aim, participants freely viewed naturalistic images and self-paced each image’s viewing duration while undergoing encephalography (EEG) monitoring and eye-tracking. Across two independent datasets we found that while viewing duration was influenced by the specific image content in an idiosyncratic manner, the images’ presentation order strongly influenced viewing duration similarly across most participants: Images presented earlier in the experiment were viewed for longer compared with images presented later in the experiment. We next sought for the neural mechanisms controlling self-paced viewing duration. To this end, we ran a regression analysis on viewing duration that controlled for presentation order, and used the residuals of this regression as a proxy for spontaneous viewing duration. The images’ presentation order, and separately, the regression’s residuals, were then correlated with EEG and eye-tracking metrics. The results revealed that the neurophysiological underpinnings of spontaneous viewing duration are doubly dissociable from those of presentation order: While presentation order was selectively correlated with baseline alpha power and with baseline pupil size, spontaneous viewing durations (i.e., regression residuals) were specifically associated with the magnitude of evoked EEG activity and evoked pupil-size changes. Furthermore, spontaneous viewing durations, estimated by the residuals, were also associated with neural stability, measured by similarity in spatial activity patterns across time points. As early as 500ms after image onset, higher neural stability predicted longer viewing durations. Together, these findings broaden our understanding of the neurophysiological dynamics contributing to the ebb and flow of spontaneous human perceptual behavior in naturalistic settings.