“Our work is the first attempt to get a layout of different areas and networks of the brain during naturalistic conditions,” says first author and neuroscientist Reza Rajimehr of Massachusetts Institute of Technology (MIT).

Different areas of the brain are highly interconnected, and these connections form functional networks that relate to how we perceive stimuli and behave. Most studies of brain functional networks have been based on fMRI scans of people at rest, but many parts of the brain or cortex are not fully active in the absence of external stimulation.

In this study, the researchers wanted to investigate whether screening movies during fMRI scanning could provide insight into how the brain’s functional networks respond to complex audio and visual stimuli.

“With resting-state fMRI, there is no stimulus — people are just thinking internally, so you don’t know what has activated these networks,” says Rajimehr. “But with our movie stimulus, we can go back and figure out how different brain networks are responding to different aspects of the movie.”

To map the brain during movie watching, the researchers leveraged a previously collected fMRI dataset from the Human Connectome Project, consisting of whole brain scans from 176 young adults that were obtained while the participants watched 60 minutes’ worth of short clips from a range of independent and Hollywood films.

The researchers averaged the brain activity across all participants and used machine learning techniques to identify brain networks, specifically within the cerebral cortex. Then, they examined how activity within these different networks related to the movie’s scene-by-scene content — which included people, animals, objects, music, speech, and narrative.

Their analysis revealed 24 different brain networks that were associated with specific aspects of sensory or cognitive processing, for example recognizing human faces or bodies, movement, places and landmarks, interactions between humans and inanimate objects, speech, and social interactions.

They also showed an inverse relationship between “executive control domains” — brain regions that enable people to plan, solve problems, and prioritize information — and brain regions with more specific functions. When the movie’s content was difficult to follow or ambiguous, there was heightened activity in executive control brain regions, but during more easily comprehendible scenes, brain regions with specific functions, like language processing, predominated.

“Executive control domains are usually active in difficult tasks when the cognitive load is high,” says Rajimehr. “It looks like when the movie scenes are quite easily comprehendible, for example if there’s a clear conversation going on, the language areas are active, but in situations where there is a complex scene involving context, semantics, and ambiguity in the meaning of the scene, more cognitive effort is required, and so the brain switches over to using general executive control domains.”

Since the analyses in this paper were based on average brain activities, the researchers say that future research could investigate how brain network function differs between individuals, between individuals of different ages, or between individuals with developmental or psychiatric disorders.

“In future studies, we can look at the maps of individual subjects, which would allow us to relate the individualized map of each subject to the behavioral profile of that subject,” says Rajimehr. “Now, we’re studying in more depth how specific content in each movie frame drives these networks — for example, the semantic and social context, or the relationship between people and the background scene.”