Asymmetric binarity as a cognitive universal: The rhythm of syntactic structures

In syntactic and rhythmic structure, universal rules group elements hierarchically and asymmetrically. In syntax, the operation ‘merge’ is theorized to combine elements in phrasal structures, with one element governing the other, recursively, to form sentences. In rhythm, a similar asymmetric hierarchy of beats is proposed in the Generative Theory of Tonal Music. Just like syntactic processing, assigning beats into rhythmic strings, ‘beat induction,’ is automatic and subconscious. Merge and beat induction assign structure not only to grammatical phrases, but also to inherently unstructured sequences. Prompted by overlapping neural support for the processing of both language and music, this study tested whether the process of assigning asymmetric binarity has a domain-general neural substrate. Structurally comparable syntactic and rhythmic stimuli were presented in a functional MRI experiment, with region-of-interest analyses focusing on core regions of the language network, inferior frontal and posterior superior temporal cortex. Results support prior research suggesting overlap in brain regions responsible for both rhythmic and syntactic processing, specifically in the inferior frontal operculum. However, the function of these regions for syntactic and rhythmic input is likely different, as syntactic and rhythmic grammaticality were not processed symmetrically within each region. These results suggest that the left inferior frontal operculum and the posterior superior temporal gyrus are domain general in their overall cortical roles, yet domain-specific in the type of processes they support within the domains of syntactic and rhythmic processing.