Memory and Hippocampal Responses to Event Boundaries are Modulated by Global Brain States

Abstract

Our daily experiences unfold as a continuous stream, yet we perceive and remember them as discrete events. Event boundaries, the moments of transition between these events, are known to elicit increases in hippocampal activity believed to reflect memory encoding. However, it remains unknown how this hippocampal response relates to large-scale brain dynamics. Here, using fMRI data from two independent datasets (Sherlock and StudyFor-rest), we applied the Greedy State Boundary Search (GSBS) algorithm to whole-brain activity patterns and identified two recurring global brain states corresponding to the Default Mode Network (DMN) and Task-Positive Network (TPN). We found that event boundaries were associated with an increased probability of being in the TPN state, and that hippocampal activity was generally higher during TPN states. The hippocampal response to event boundaries appeared predominantly during TPN states. When overall state-related differences in baseline hippocampal activity were controlled for, event boundaries elicited a hippocampal response regardless of the concurrent global state. Critically, individual differences in the tendency to shift toward the TPN state at event boundaries; as well as overall time spent at the DMN state predicted subsequent memory for narrative content, whereas univariate hippocampal activity at boundaries did not. These findings demonstrate that hippocampal event boundary responses are modulated by global brain state dynamics, and suggest that the interplay between large-scale network configurations and event segmentation plays a key role in how continuous experience is encoded into memory.