Spoken language comprehension relies on context-dependent neural weighting of syntax and prosody

Comprehending spoken language requires extracting syntactic structure and meaning from a continuous acoustic signal. Psycholinguistic evidence points to a key role for prosody in this process. However, because interactions between prosody and syntax are typically demonstrated using carefully controlled sentence constructions, it remains unclear whether such interactions constitute a general, default property of spoken language processing. Addressing this question requires naturalistic, broad-coverage data. Here, we combined experimental psycholinguistics with naturalistic neuroscience to investigate the joint neural encoding of syntax and prosody in MEG recordings of 26 participants (19 female) listening to narrative speech. Linguistic annotations revealed that strong prosodic boundaries tend to coincide with syntactic phrase closures, and that the strength of prosodic boundaries scales with the number of phrases closed. Using time-resolved regression with temporal response functions, we modeled how these features relate to delta-band neural activity during continuous speech processing. Our results reveal a bidirectional interaction between syntax and prosody. Prosodic marking modulates the neural processing of syntax, with phrase closures being encoded more strongly when accompanied by strong prosodic boundaries. Conversely, syntactic structure shapes prosodic encoding, such that boundary strength is neurally encoded primarily at points of syntactic phrase closure. These findings demonstrate that syntax and prosody are encoded not independently, but in a context-dependent and mutually constraining manner, highlighting their joint role in guiding structure building during naturalistic spoken language comprehension.