Left-hemisphere parieto-frontal interactions during phonologic and semantic word decisions. A conditioning-and-perturb TMS study

Introduction: Left supramarginal gyrus (SMG) and posterior inferior frontal gyrus (pIFG) have been identified as key areas for phonologic decisions while angular gyrus (ANG) and anterior IFG (aIFG) have been associated with semantic processing (1,2). We investigated the relevance of intact interactions between these areas by means of a conditioning-and-perturb repetitive transcranial magnetic stimulation (rTMS) approach. Methods: 17 healthy right-handers underwent 4 sessions of conditioning and perturbing rTMS, combining offline rTMS before a task and online rTMS during a task over different areas. In one session, offline rTMS of ANG preceded online rTMS of aIFG. Another session combined offline rTMS of ANG with online rTMS of pIFG. Thirdly, offline rTMS over SMG preceded online rTMS of aIFG. Finally, offline rTMS of SMG was combined with online rTMS of pIFG. During online rTMS, subjects performed phonologic and semantic decisions. Results: A significant 3-way interaction for reaction times (RTs) indicated that combined offline and online rTMS differentially affected both tasks: Relative to semantic RTs, phonologic RTs were equally delayed after combined rTMS of SMG and pIFG and rTMS over either area alone but there was no effect of rTMS over ANG or aIFG. In contrast, semantic compared with phonologic RTs were increased after combined rTMS over both ANG and aIFG, without any lesion effect after rTMS over either area alone. rTMS over neither SMG nor pIFG affected semantic RTs. Conclusions: Our results extend previous studies (1,2) in showing that phonologic and semantic decisions are differentially integrated in parieto-frontal networks. We found that both SMG and pIFG contribute to efficient phonologic decisions, without any evidence that both areas can compensate a virtual lesion of the respective other area. In contrast, our finding that semantic decisions were only delayed after combined rTMS of ANG and aIFG suggests a greater redundancy within the semantic network. Literatur: (1) Gough, P.M., Nobre, A.C., Devlin, J.T. (2005) Dissociating linguistic processes in the left inferior frontal cortex with transcranial magnetic stimulation. J Neurosci 25:8010-8016 (2) Hartwigsen, G., Baumgaertner, A., Price, C.J., Koehnke, M., Ulmer, S., Siebner, H.R. (2010a) Phonological decisions require both the left and right supramarginal gyri. Proc Nat Sci USA 107:16494-16499