Prosody meets syntax: the role of the corpus callosum

Contemporary neural models of auditory language comprehension proposed that the two hemispheres are differently specialized in the processing of segmental and suprasegmental features of language. While segmental processing of syntactic and lexical semantic information is predominantly assigned to the left hemisphere, the right hemisphere is thought to have a primacy for the processing of suprasegmental prosodic information such as accentuation and boundary marking. A dynamic interplay between the hemispheres is assumed to allow for the timely coordination of both information types. The present event-related potential study investigated whether the anterior and/or posterior portion of the corpus callosum provide the crucial brain basis for the online interaction of syntactic and prosodic information. Patients with lesions in the anterior two-thirds of the corpus callosum connecting orbital and frontal structures, or the posterior third of the corpus callosum connecting temporal, parietal and occipital areas, as well as matched healthy controls, were tested in a paradigm that crossed syntactic and prosodic manipulations. An anterior negativity elicited by a mismatch between syntactically predicted phrase structure and prosodic intonation was analysed as a marker for syntax-prosody interaction. Healthy controls and patients with lesions in the anterior corpus callosum showed this anterior negativity demonstrating an intact interplay between syntax and prosody. No such effect was found in patients with lesions in the posterior corpus callosum, although they exhibited intact, prosody-independent syntactic processing comparable with healthy controls and patients with lesions in the anterior corpus callosum. These data support the interplay between the speech processing streams in the left and right hemispheres via the posterior portion of the corpus callosum, building the brain basis for the coordination and integration of local syntactic and prosodic features during auditory speech comprehension.