Our memories are what makes each of us unique, but this unique set of experiences is precisely what makes scientific study of memory difficult. The cumbersome nature of neuroimaging tools doesn’t help. MRI (Magnetic Resonance Imaging) scanners are so large and heavy that they are often positioned in place and the housing is built around them. So how can the scientists study the brain during everyday realistic memory tasks? We have decided to use movies in our pursuit of brain mechanisms of memory for events, the so-called “episodic memory”.

We all love movies. We find them incredibly engaging. Movies simulate and condense real life experiences which makes them ideally suited for investigation of neural mechanisms underlying cognitive functions in more naturalistic settings than that of traditional experiments. Movies consist of a series of events, which in turn consist of individual elements such as people, locations, objects and activities. For this reason, they are excellent as a proxy for real life events and as a method for engaging neural mechanisms underlying episodic memory formation.

Our episodic memories are placed in spatial and temporal contexts (where did something happen and when did it happen). At the same time, we also use individual events to construct our own personal stories like “how we landed our dream job” or “how we met the love of our lives”. The events which form those personal stories can happen at a variety of locations and span long time periods, and can be interspersed by various other events which don’t feature in the story. So how does the brain organize our experiences so that we can access them at will?

Movies, and more generally stories in their various formats, provide a general context of sorts, unrestricted by space and time, that can be used to organise memories into networks of related events. By studying how events from a movie are organized in the brain, we can begin to understand how our brains sort and organize our real-life memories and how we create our own personal narratives out of them.

In this study, 24 participants watched a romantic comedy called Sliding Doors with two interleaved narratives, while their brain activity was monitored using fMRI. In this movie, the main storyline splits into two, when the main protagonist either catches a train in London or misses it. The movie then follows the ensuing events in these two parallel universes, switching back and forth between them. We used this movie feature to track how these storylines are represented by the brain. Dr Milivojevic said: “We show that the hippocampus, a part of the brain critically involved in representation of spatial and temporal contexts in episodic memory, also represents these storylines as separate narrative contexts.” Professor Doeller added: “These narrative contexts were not so different to begin with, but their representations gradually diverged, similarly how the representations of new spatial environments gradually diverge in the hippocampus of freely navigating rodents. Our findings suggest that the neural computations underlying episodic memory and spatial navigation share some striking similarities.”

More surprisingly though, we found that the hippocampus also represented particular elements from the movie such as people and locations. “Our results on people-specific representations are a bit surprising because studies often show that the hippocampus is not so sensitive to individual items” said Dr Milivojevic and then added “But there is a large difference between the dynamic movie we used, and the more traditional static images used in previous studies. In a movie, each “item” or character, was presented multiple times in different spatial locations at different times in the movie, and these multiple presentations may have given rise to abstraction of the items from the particular events in which they appeared in. This makes it more similar to how we actually encounter individual people in real life. We see them often in a variety of locations and at different times.” “That’s right!” Said Dr Doeller “Our brains, and more specifically the hippocampus, represents memories as networks of inter-related events with prominent spatial and non-spatial event elements, such as spatial locations and people, as nodes of these memory networks. We think these memory networks are the neural basis of navigation through space, but also through the landscape of our memories.”

Taken together, this study uses a novel approach to the study of episodic memory, by using movies to answer the question of how our brains process realistic events and integrate them into networks of events in memory. By doing this, the researchers bring a slice of reality to neuroimaging research of episodic memory.

Research published in Journal of Neuroscience

Research funded by ERC and NWO