Watching the 1995 film Forrest Gump can elicit sincere emotion and pleasure or more negative responses in viewers, depending on one's subjective cinematic tastes. It can also teach neuroscientists something about how the brain encodes everyday events into long-term memory, according to a recent study published in the Journal of Neuroscience. The brain seems most interested in tracking transitions between distinct events, the better to segment and store them.

The hippocampus is the brain region most closely associated with forming new memories. Most experiments focusing on memory use the most minimal, simplified stimuli possible to better control for variables, according to co-author Aya Ben-Yakov of the University of Cambridge. But in reality, the brain actually processes a huge amount of continually incoming stimuli. This is one of the first studies to specifically investigate how the hippocampus operates during so-called "natural experiences."

Films turn out to be ideal for simulating that kind of natural continuous input, mimicking our daily lived experience. And Forrest Gump is one of the most popular with neuroscientists, thanks in large part to an open source dataset called studyforrest. Founded in 2013, the project is a repository for experiments that study the brain's natural behavior in response to watching the film, using fMRI, eye tracking, structural brain scans, and more.

"The most important thing that a film captures is that the events unfold over time."

According to the studyforrest website, the film is ideal for such purposes given the many different kinds of complex sensory stimuli it offers. Its timeline spans several decades with references to historical events. There is a great variety of landscapes and plenty of popular songs spanning decades and genres. There's also tremendous emotional range as the story unfolds. Plus, there are audio descriptions for the blind available in several languages, making it possible to "present the same story using different sensory inputs."

"The most important thing about using films is that the events unfold over time," says Ben-Yakov. "Each event is an entire sequence, not just a moment in time." And frankly, it's also much more fun for study subjects to watch a movie while confined in an MRI machine.

Keeping it short

Ben-Yakov started using short film clips back in 2011. Previous memory research had focused on how brain activity during events was linked to memory, but she had a hunch that something else was going on at the end of such events. The clips she used in this initial experiment were just 8 seconds each, carefully edited so that each represented a single event or experience, separated by a gray screen. The hippocampus only responded when a clip had ended. And the response was more pronounced for clips that participants subsequently remembered when they were tested an hour after they initially viewed the clips.

In 2013, a second study presented subjects with pairs of short clips, one immediately after the other. Once again, the hippocampus responded when the clips ended, but there was a weaker response at the end of the first clip. Participants remembered the first clip less often than the second, suggesting that showing two clips back to back interfered somehow with hippocampal response. According to Ben-Yakov, this could be why we need short breaks to boost memory.

The following year, Ben-Yakov explored how the hippocampus responded as the clips became more familiar after repeated viewings. So they showed subjects the same clips six times in a row. The result: there was a lower hippocampal response at the end of the clip, "likely because there's less need to encode, as the clip is already familiar," says Ben-Yakov. However, there was also a strong response at the start of previously viewed clips, possibly constituting what she calls the brain's "Hey, I've seen that before" response.

Based on those findings, Ben-Yakov concluded that we don't really grasp an event until it has ended, and how strongly the hippocampus responds to a given event is linked to how well we remember it. To take a closer look, she and her University of Cambridge colleague Richard Henson did a meta-analysis of sorts on existing datasets: specifically, the studyforrest project and a second open source dataset called Cam-CAN (part of the research program at the Cambridge Center for Aging and Neuroscience), which monitors brain activity in subjects while they watch the 1961 Alfred Hitchcock Presents episode "Bang You're Dead."

It's about the transitions

As expected, they found that the strongest hippocampal responses to the films occurred at points where participants indicated they perceived the ending of one event and the beginning of a new one. Ben-Yakov and Henson showed a separate study group the film and asked the subjects to indicate whenever they perceived a transition between one event and the next. Then, they compared those responses to the existing studyforrest and CamCAN datasets, finding the two groups largely identified the same transition points and segmented the films the same way.

To confirm that the hippocampus was responding specifically to the transitions and not some other kind of sensory input, the researchers conducted a blind analysis of the two datasets. They just looked for places where there was a strong response from the hippocampus, and they compared that to where subjects had perceived transitions between events. Once again, there was a very strong correspondence.

"I think film editors know a lot about the hippocampus without knowing what they know."

Clearly, this emphasis on transitions has something to do with how we shape our memories. "Whenever we experience a transition, that's a cue for the hippocampus to say 'Wait. Pause. Let me wrap up what just happened and start encoding the next part as a different event,'" says Ben-Yakov. For instance, if you chat with someone on the phone and then hang up and have breakfast, the brain will naturally segment this and store them as separate events, even though they happened in succession.

Obviously, for a film like Forrest Gump or the Hitchcock TV episode, there is a film editor employing various perceptual tricks to give the illusion of transitioning between scenes. In some sense, they are intuitive neuroscientists. "I think film editors know a lot about the hippocampus without knowing what they know," says Ben-Yakov. "They use film editing tricks to determine how they want the audience to respond."

But this did not actually influence how the hippocampus responded, the researchers found. The strongest activity correlated to where each participant subjectively perceived the event boundaries to be, irrespective of the film editor's intent. The brain is not responding to filmic perceptual cues, but to "meaningful units of experience," Ben-Yakov says.

DOI: JNeurosci, 2018. 10.1523/JNEUROSCI.0524-18.2018 (About DOIs).