| We propose a conceptually bipartite model of sentence comprehension that includes grammatical processes and cognitive resources. This is tested in a functional neuroimaging study with young adults, revealing a pattern of recruitment including left inferior frontal and bilateral posterolateral temporal cortices, depending on the grammatical or working memory (WM) demands of the sentence type. The role of WM in sentence comprehension is further assessed as we administer this experiment in the context of a limited WM “preparation,” namely, to a group of performance-matched healthy seniors. Both young and senior adults reliably recruited posterolateral temporal and ventral inferior frontal regions, suggesting a “core” sentence-processing network. Differences in the pattern of activation in seniors are attributed to age-related WM limitations. While we initially adopt a linear approach to WM by manipulating the distance between antecedent noun phrase and gap in center-embedded relative clauses to increase WM load, further study of young adults reveals the additional role of information processing speed and it's interaction with a load-based WM, via frontal-striatal-thalamic connections. With a provisional neural model of sentence comprehension in hand, we assess its interaction with various syntactic agreements hypothesized to require varying levels of automatic and effort-related processing. An on-line word monitoring paradigm assesses the automaticity of grammatical processes and a dual task paradigm extracts cognitive resource aspects of supposedly automatic grammatical processes. We use an event-related functional neuroimaging protocol to assess the commitment by inferior frontal and posterolateral temporal brain regions. We show the spatial-temporal flexibility with which these regions contribute to various grammatical processes. To assess the role of left hemisphere regions in sentence comprehension from another perspective, we test patients with frontotemporal dementia (FTD), in whom neural substrates for grammatical and WM processes appear to be selectively compromised. FTD patient subgroups with distinct behavioral deficits show selective difficulty recruiting brain regions for grammatical and WM functions. In sum, grammatical and cognitive resource aspects of sentence comprehension are highly interactive and multidimensional, adding layers of complexity to the brain's obligation to meet gradations of linguistic demands. |
