Modulation of functional connectivity with the syntactic and semantic demands of a Noun Phrase Formation Task: A possible role for the Default Network

Neuroimaging studies of language processing have shown that different circuits within the language network are isolated by syntactic and semantic tasks. We examined differential activation in an fMRI study of language when a single task with two minimally distinct conditions is employed. 16 subjects participated in a Noun Phrase Formation Task that encompassed a minimal difference between syntactic and semantic conditions. In-magnet accuracy and response times were compared, imaging data were evaluated for effects of task conditions, and the functional connectivity between areas of differential response to conditions were evaluated using cross-subject, interregional correlations between measures of BOLD activity. Although no difference in performance was observed between conditions, random effects analysis pinpointed pivotal components of the syntactic and semantic circuits (p<0.01 corrected for multiple comparisons) usually isolated with more complex, sentence-level stimuli. These included bilateral Broca's area and left inferior parietal lobe in the syntactic condition and left BA47 and left middle temporal gyrus in the semantic. Correlations among these regions did not pattern straightforwardly with the two task conditions, contrary to our expectations. Two significantly correlated circuits were identified (p<0.01): the well-documented connection between the left inferior parietal lobe and Broca's area and a connection between the medial frontal lobe and BA 47. An intriguing pattern was revealed in how the two sets of regions correlated with three areas of significantly differential deactivation to task, areas that are co-extensive with the Default Network. The deactivating regions, plus those areas that inversely correlated with them, formed one coordinated sub-circuit, encompassing the regions identified with syntax. Areas that positively correlated with the deactivating regions encompassed the semantic circuit. We suggest that by leveraging the Default Network, a partial bifurcation in connectivity was obtained that appeared to be conditioned externally. These regions could be viewed as an interface, balancing activation between the two overlapping, condition-dependent sub-circuits to enhance differential brain response within a complex cognitive network.

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