Post by Shireen Parimoo

What's the science?

The hippocampus is a region of the brain that is central to processes like memory formation and imagination. New research shows that it may also play a role in transforming a continuous experience into separate memories. For example, increased hippocampal activity is observed during shifts from one context to another in experimental tasks like viewing picture sequences and film clips, and this activity is linked to subsequent memory performance. However, it is unclear whether the hippocampus is also involved in segmenting continuous events in more natural or realistic contexts. This week in the Journal of Neuroscience, Ben-Yakov & Henson used functional magnetic resonance imaging (fMRI) to examine the role of the hippocampus in representing event boundaries during continuous film-viewing.

How did they do it?

The authors used data from two datasets (Cam CAN and studyforrest) in which fMRI activity was recorded from 253 adults while they watched a short version of an Alfred Hitchcock film, and from 15 adults while they watched the movie Forrest Gump. Independent observers identified events in the film that they categorized as boundaries, resulting in 19 and 161 event boundaries in the Hitchcock and Forrest Gump films, respectively. These boundaries were categorized into low, medium, and high levels of salience (like importance) based on how many observers characterized the events as boundaries. To analyze the fMRI data, the authors first examined changes in hippocampal activity (i) in response to event boundaries, and (ii) in response to changes in angular gyrus activity at event boundaries and non-boundaries. Previous studies have shown that hippocampal activity is modulated by activity in the angular gyrus, so the second analysis (ii) was performed to ensure that hippocampal responses were driven by event boundaries rather than by angular gyrus activity. The authors also performed a different analysis (using a linear mixed effects model) in which hippocampal activity was assumed to vary across participants and event boundaries, instead of being fixed. Finally, a data-driven approach was used to locate events in the films that the hippocampus responded to most strongly and determine if these events were consistent with the event boundaries identified by the independent observers.

What did they find?

The authors found that the hippocampus responded strongly to event boundaries in both films, and this response was greater for high salience than medium and low salience boundaries. That is, when more observers rated an event as a boundary, the hippocampus showed stronger activation. This effect remained significant while participants viewed Forrest Gump even after perceptual factors such as visual features and changes in location across boundaries were added as predictors. Additionally, hippocampal activity was modulated by event boundaries independently of changes in angular gyrus activity. Finally, boundaries identified using hippocampal activity and those identified by independent observers revealed a 58% match for the Hitchcock film and a 38% match for Forrest Gump. These findings suggest that the hippocampus is sensitive to event boundaries in films and that this response scales with the salience of these boundaries. Further, these results show that greater hippocampal activity corresponds to boundaries occurring in a natural context (i.e. passively viewing a film).