Cingulate control of fronto-temporal integration reflects linguistic demands: A three-way interaction in functional connectivity

In a recent fMRI language comprehension study, we asked participants to listen to word-pairs and to make same/different judgments for regularly and irregularly inflected word forms [Tyler, L.K., Stamatakis, E.A., Post, B., Randall, B., Marslen-Wilson, W.D., in press. Temporal and frontal systems in speech comprehension: an fMRI study of past tense processing. Neuropsychologia, available online.]. We found that a fronto-temporal network, including the anterior cingulate cortex (ACC), left inferior frontal gyrus (LIFG), bilateral superior temporal gyrus (STG) and middle temporal gyrus (MTG), is preferentially activated for regularly inflected words. We report a complementary re-analysis of the data seeking to understand the behavior of this network in terms of inter-regional covariances, which are taken as an index of functional connectivity. We identified regions in which activity was predicted by ACC and LIFG activity, and critically, by the interaction between these two regions. Furthermore, we determined the extent to which these inter-regional correlations were influenced differentially by the experimental context (i.e. regularly or irregularly inflected words). We found that functional connectivity between LIFG and left MTG is positively modulated by activity in the ACC and that this effect is significantly greater for regulars than irregulars. These findings suggest a monitoring role for the ACC which, in the context of processing regular inflected words, is associated with greater engagement of an integrated fronto-temporal language system.