Disentangling hierarchical and sequential computations during sentence processing

Sentences in natural language have a hierarchical structure that can be described in terms of nested trees. To compose sentence meaning, the human brain needs to link successive words into complex syntactic structures. However, such hierarchical-structure processing could co-exist with a simpler, shallower, and perhaps evolutionarily older mechanism for local, word-by-word sequential processing. Indeed, classic work from psycholinguistics suggests the existence of such non-hierarchical processing, which can interfere with hierarchical processing and lead to sentence-processing errors in humans. Here, we conducted a combined magnetoencephalography and electroencephalography experiment to study hierarchical versus sequential computations during sentence processing in the human brain. We studied whether the two processes have distinct neural signatures. Our results show a large dominance of hierarchical processing in the human brain compared to sequential processing, pointing to a major difference between how humans process sentences and sequences of non-linguistic items. This shows that once words enter the language system, computations are dominated by structure-based processing and largely robust to sequential effects.