Quanta spoke to Fedorenko about how the language network is like the digestive system, what she knows about how the language decoder works, and whether she really believes that people have LLMs inside their heads. The conversation has been condensed and edited for clarity.

What is the language network?

There’s a core set of areas in adult brains that acts as an interconnected system for computing linguistic structure. They store the mappings between words and meanings, and rules for how to put words together. When you learn a language, that’s what you learn: You learn these mappings and the rules. And that allows us to use this “code” in incredibly flexible ways. You can convert between a thought and a word sequence in any language that you know.

That sounds very abstract. But you call the language network a “natural kind” — does that mean it’s something physical you can point to, like the digestive system?

That’s exactly right. These systems that people have discovered [in the brain], including the language network and some parts of the visual system, are like organs. For example, the fusiform face area is a natural kind: It’s meaningfully definable as a unit. In the language network, there are basically three areas in the frontal cortex in most people. All three of them are on the side of the left frontal lobe. There’s also a couple of areas that fall along the side of the middle temporal gyrus, this big hunk of meat that goes along the whole temporal lobe. Those are the core areas.

You can see the unity in a few different ways. For example, if you put people in an [fMRI, or functional magnetic resonance imaging], scanner, you can look at responses to language versus some control condition. Those regions always go together. We’ve now scanned about 1,400 people, and we can build up a probabilistic map, which estimates where those regions will tend to be. The topography is a little bit variable across people, but the general patterns are very consistent. Somewhere within those broad frontal and temporal areas, everybody will have some tissue that is reliably doing linguistic computations.

How is this different from other parts of brain anatomy known to be associated with language, such as Broca’s area?

Broca’s area is actually incredibly controversial. I would not call it a language region; it’s an articulatory motor-planning region. Right now, it’s being engaged to plan the movements of my mouth muscles in a way that allows me to say what I’m saying. But I could say a bunch of nonsense words, and it would be just as engaged. So it’s an area that takes some sound-level representation of speech and figures out the set of motor movements you would need [to produce it]. It’s a downstream region that the language network sends information to.

You’ve also said that language isn’t the same as thought. So if the language network isn’t producing speech, and it’s also not involved in thinking, what is it doing?

The language network is basically an interface between lower-level perceptual and motor components and the higher-level, more abstract representations of meaning and reasoning.

There are two things we do with language. In language production, you have this fuzzy thought, and then you have a vocabulary — not just of words, but larger constructions, and rules for how to connect them. You search through it to find a way to express the meaning you’re trying to convey using a structured sequence of words. Once you have that utterance, then you go to the motor system to say it out loud, write it or sign it.