[alew1]

The article presents the fact that we appear to treat non-constituents (eg “in the middle of the”) as “units” to mean that language is more like “snapping legos together” than “building trees.”

But linguists have proposed the possibility that we store “fragments” to facilitate reuse—essentially trees with holes, or equivalently, functions that take in tree arguments and produce tree results. “In the middle of the” could take in a noun-shaped tree as an argument and produce a prepositional phrase-shaped tree as a result, for instance. Furthermore, this accounts for the way we store idioms that are not just contiguous “Lego block” sequences of words (like “a ____ and a half” or “the more ___, the more ____”). See e.g. work on “fragment grammars.”

Can’t access the actual Nature Human Behavior article so perhaps it discusses the connections.

[lupire]

There's no reason to assume that an human word begins and ends with a space. Compound words exist. The existence of both "Aforementioned" and "previously spoken of" isn't based on a deep neurological construct of compound words.

[mcswell]

Sorry, I'm not following. What do spaces have to do with this? Grammar is dependent on concepts like lexemes (sort of like words), but there aren't any spaces between lexemes in spoken language.