Spatiotemporal dynamics of lexical and morphosyntactic planning during language production revealed by magnetoencephalography

Fluent sentence production relies on the precise and timely coordination of lexical retrieval, morphosyntactic planning, and motor execution. While a left-dominant fronto-parieto-temporal network has been identified for these subcomponents of sentence production, less is known about the time course of engagement of these regions. One key issue is the extent to which the engagement of neural regions is sequential. The aim of the present study was to delineate spatiotemporal patterns of subcomponents of sentence production, namely lexical retrieval, inflection, and constituent assembly, and identify any lexical category differences (nouns versus verbs) in these processes. Magnetoencephalographic (MEG) responses were measured using a novel picture-icon paradigm in which verbal responses were elicited using object or action pictures followed by an icon that indicated the subcomponent sentence processes: lexical access, inflection (plurals for nouns, tense for verbs), and constituent assembly (adjective + object or pronoun + action). MEG data from twenty native English-speaking right-handed neurotypical adults was analysed for both stimulus-locked and response-locked timelines. We focused on responses in left hemisphere speech-language regions of interest (ROI): precentral gyrus (M1), inferior frontal gyrus (IFG), anterior temporal lobe (ATL), posterior temporal lobe (PTL), and inferior parietal lobe (IPL). The results revealed the following: 1) early and largely simultaneous engagement of all ROIs for lexical access; 2) compared to noun lexical access, verb lexical access is associated with a larger neural response across multiple language ROIs; 3) morphosyntactic planning engaged all ROIs, showing commonalities as well as unique patterns for each morphosyntactic condition; 4) noun morphosyntax vs noun lexical access showed a larger response across multiple ROIs compared to verb morphosyntax vs verb lexical access; 5) M1 was coactivated with other ROIs early in the planning stage, suggesting simultaneity of language and motor planning. The findings support a network model of language in which the syntactic, semantic, and phonological properties of a word are bound to a single anatomically distributed representation.