A new study is the first to show that two of our most sophisticated cognitive functions, using and understanding language and being able to sense how other people feel, have distinct origins in the brain in young children - matching what we know about the adult brain.
The findings suggest that these separate but related ways of processing complex concepts, both uniquely human skills, do not originate from overlapping brain areas and grow more distinct as the mind matures, which challenges prior theories. Instead, our brains appear to have evolved with discrete architecture and wiring enabling these different kinds of thinking.
Using fMRI to scan the brains of children while they listened to spoken language and watched a short movie, the researchers found that parts of the brain responsible for language and mentalizing, known as theory of mind, are separate and do not overlap. Additional analysis of how these regions communicate with other brain areas at rest reinforced the imaging data.
"It seems that these processors that help us mentalize and that help us speak and understand were dissociated very, very early in the evolutionary process, such that we can't even see traces of overlap right now in human development," said Zeynep Saygin, senior author of the study and an associate professor of psychology at The Ohio State University.
"It's a fundamental question humans ask themselves: 'What is it that makes us human? How does human cognition emerge?' I think this sheds some light on that."
Kelly Hiersche, a doctoral student in Saygin's lab, led the study, published April 23 in Communications Biology. David Osher, assistant professor of psychology, was also a co-author and collaborator.
The two communication skills of focus originate from a region of the brain called the superior temporal lobe, located near each temple - with language based in the left hemisphere and theory of mind based in the right.
The researchers first confirmed with fMRI scans of the brain in 28 adults what has been found before - that separate and distinct regions associated with language and theory of mind did, indeed, respond strongly to stimuli intended to activate those areas.
The team then worked with 42 children between ages 3 and 9, scanning their brains with 2 fMRI scans, one while they listened to sentences and another while they watched a silent cartoon, observing which brain regions were activated for each task. Control conditions included nonsense words for the language assessment and, for the mentalization evaluation, signs of pain in cartoon characters - which elicits a pain response rather than theory of mind.
Results of the scans and additional analysis - imaging at the 2-3 millimeter, or 3D voxel, level of the brain across both hemispheres - showed that the regions responding to language stimuli and theory of mind stimuli were separate, with no overlap.
"That was our first question: Are these skills distinct in both their function and location? And we see really broadly, yes," Hiersche said. "We demonstrate this for the first time in kids, extending an adult finding to development. They're really distinct there, which is pretty cool."
To tap further into the evolutionary question, the researchers took fMRI scans of the adults' and children's brains at rest - when the brain is still busy, but not being asked to respond to specific stimuli - to observe what other brain regions these separate language and mentalization regions were connecting with.
"If you observe a voxel's connectivity, or how it talks to the rest of the brain, that's going to give you an idea about how that voxel is going to function," Hiersche said. This is the idea of a connectivity fingerprint: a unique connectivity pattern that determines the unique function of a brain region.
Using predictive modeling to characterize these connectivity fingerprints, the researchers found that there was more to the language and theory of mind distinctions than their locations on separate sides of the brain.
"Regions of the brain that are functionally specific should be communicating in a unique way," Saygin said. "We knew these regions were localized in different parts of the brain, but also showed that there's nothing in how they communicate with the rest of the brain that indicates that they were at any point overlapping."
Looking for changes in the kids' connectivity fingerprints over time further drove home the point that the regional and functional distinctions don't change during childhood brain development.
"We were able to not just look across different kids, but look within the same child to see what happened over time," Hiersche said. "And we showed that it's not the case that when you're 3 years old, you see a lot of overlap in these functions, but then when you get to 5 years old, they pull apart and become more separate.
"The connections we're seeing that support these tasks - and that also separate them - are stable within the same person over time."
In fact, comparing the differences in connectivity fingerprints between children and adults showed that while these functions and connectivity patterns are quite clearly separate and distinct in kids, there is some overlap across brain networks in adults - a sign of change in how we make use of the complementary skills.
"In adults, the mentalizing theory of mind network starts to talk to slightly similar regions as the language areas. In children, as those skills keep developing, maybe those networks are talking to each other more," Saygin said.
These results challenge the idea that language and mentalizing have similar origins and instead support distinct mechanisms for these communicative skills, she said.
This work was supported by the U.S. National Science Foundation, the Alfred P. Sloan Foundation, the National Institutes of Health, and Ohio State's College of Arts and Sciences, Center for Brain Injury Recovery and Discovery, and Women in Philanthropy award.
