Signals of memory building-blocks: Pupil-linked brain arousal predicts event segmentation and episodic memory

We automatically segment the continuous stream of our experiences into events with discrete boundaries, and these events serve then as building blocks for later episodic memories. How our brain creates these discrete event representations remains elusive. Here we investigated the role of noradrenergic (NA) transmission, heavily linked to change-detection, in identifying event boundaries during listening to auditorily presented narratives. Specifically, we investigated how pupil dilation, an indirect physiological proxy of NA activity, tracks event boundaries while listening to short stories of everyday events. In a series of three studies, we could demonstrate that event boundaries trigger significant pupil dilation, and the magnitude of these dilations predicts later memory of narratives. Furthermore, we also showed that pupil dilation magnitude at each time point during the narratives scaled as a function of how frequently a set of independent raters indicated the presence of an event boundary preceding that specific story location. This set of results suggests that NA transmission influences episodic memory encoding by signaling unexpected changes in the flow of continuous experience, which then lead to the identification of event boundaries structuring long-term memory representations.