The neural correlates of verbs and nouns: a MEG study on Italian homophones

The neural correlates of nouns and verbs are the central issue of a long-standing debate in neuropsychology. The double dissociability of the two grammatical categories and of the respective morphosyntactic processes has been firmly established in neuropsychological studies (eg, Shapiro & Caramazza, 2003a; Drucks, 2002) leading to the hypothesis that verbs and nouns, as well as the involved morphosyntactic operations, have distinct neural representations. However, neuroimaging findings provide only inconsistent support to these hypotheses, which could be due in part to the limitations intrinsic to the various neuroimaging tools. In the present study, the neural representation of Noun/Verb distinctions is studied by means of a novel technique - magnetoencephalography (MEG), which is characterized by excellent temporal resolution and good spatial resolution. The goal of the study is to establish whether distinct neural substrates are involved in processing nouns and verbs in the context of phrases. In an adaptation paradigm we presented Italian homophonous noun and verbs. Homophones were selected because they share the same word form but belong to different grammatical categories. The homophones nouns and verbs were presented in minimal syntactical context: article and noun, pronoun and verb (eg, il ballo/i balli, the dance/the dances; io ballo/tu ballii, I dance/you dance). Twelve healthy participants performed a silent reading task, and their spatiotemporal information was measured during the processing of homophones. The analysis conducted on the evoked responses of noun and verb phrases identified the response components of lexical processing and the differences between the two grammatical categories. A localization technique was employed to isolate the corresponding differences in the cerebral topographies. The resultant differences revealed partially matching neural substrates during the early processing of both grammatical categories that diverged in the later stages of processing. Both nouns and verbs activated the occipital, left temporal and parietal regions but only verbs engaged the left inferior frontal gyrus during the late time window. Our results are comparable to findings of the clinical reports: verbs were mainly processed at the left frontal cortex while nouns activated the left temporal lobe. These findings support the hypothesis that at least partially distinct neural structures are involved in the processing/representation of verbs and nouns and of their respective morphosyntactic operations. MEG appears to be a promising tool for the analysis of the spatiotemporal dynamics of the networks implicated in language processing.