What constitutes a language has been of interest to diverse disciplines — from philosophy and linguistics to psychology, anthropology, and sociology. An empirical approach is to test whether the system in question recruits the brain system that processes natural languages. Despite their similarity to natural languages, math and programming languages recruit a distinct brain system. Using functional magnetic resonance imaging (fMRI), MIT neuroscientists tested brain responses to constructed languages (conlangs) — which share features with both natural languages and programming languages — and found that they are processed by the same brain network as natural languages. Thus, an ability for a symbolic system to express diverse meanings about the world — but not the recency, manner, and purpose of its creation, or a large user base — is a defining characteristic of a language.

“We find that constructed languages very much recruit the same system as natural languages, which suggests that the key feature that is necessary to engage the system may have to do with the kinds of meanings that both kinds of languages can express,” said Dr. Evelina Fedorenko, a neuroscientist at MIT.

“The findings help to define some of the key properties of language and suggest that it’s not necessary for languages to have naturally evolved over a long period of time or to have a large number of speakers.”

“It helps us narrow down this question of what a language is, and do it empirically, by testing how our brain responds to stimuli that might or might not be language-like,” added Saima Malik-Moraleda, a postdoctoral researcher at MIT.

Unlike natural languages, which evolve within communities and are shaped over time, conlangs are typically created by one person who decides what sounds will be used, how to label different concepts, and what the grammatical rules are.

To explore how the brain processes conlangs, the researchers invited speakers of Esperanto, Klingon, Na’vi (from Avatar), and High Valyrian and Dothraki (two languages from Game of Thrones) MIT for a weekend conference in November 2022.

For all of these languages, there are texts available for people who want to learn the language, and for Esperanto, Klingon, and High Valyrian, there is even a Duolingo app available.

“It was a really fun event where all the communities came to participate, and over a weekend, we collected all the data,” Malik-Moraleda said.

During that event, which also featured talks from several of the conlang creators, the scientists used fMRI to scan 44 conlang speakers as they listened to sentences from the constructed language in which they were proficient.

The creators of these languages helped construct the sentences that were presented to the participants.

While in the scanner, the participants also either listened to or read sentences in their native language, and performed some nonlinguistic tasks for comparison.

The authors found that when people listened to a conlang, the same language regions in the brain were activated as when they listened to their native language.

The findings help to identify some of the key features that are necessary to recruit the brain’s language processing areas.

One of the main characteristics driving language responses seems to be the ability to convey meanings about the interior and exterior world — a trait that is shared by natural and constructed languages, but not programming languages.

“All of the languages, both natural and constructed, express meanings related to inner and outer worlds. They refer to objects in the world, to properties of objects, to events,” Dr. Fedorenko said.

“Whereas programming languages are much more similar to math. A programming language is a symbolic generative system that allows you to express complex meanings, but it’s a self-contained system: the meanings are highly abstract and mostly relational, and not connected to the real world that we experience.”

Some other characteristics of natural languages, which are not shared by constructed languages, don’t seem to be necessary to generate a response in the language network.

“It doesn’t matter whether the language is created and shaped over time by a community of speakers, because these constructed languages are not,” Malik-Moraleda said.

“It doesn’t matter how old they are, because conlangs that are just a decade old engage the same brain regions as natural languages that have been around for many hundreds of years.”

The findings appear in the Proceedings of the National Academy of Sciences.

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Saima Malik-Moraleda et al. 2025. Constructed languages are processed by the same brain mechanisms as natural languages. PNAS 122 (12): e2313473122; doi: 10.1073/pnas.2313473122