Slide Slam G11
Transcranial magnetic stimulation over the IFG facilitates action naming, but is modulated by language lateralization and handedness
Tatiana Bolgina1, Vidya Somashekarappa1, Zoya Cherkasova1, Anna Sapuntsova2, Matteo Feurra1, Stefano F. Cappa3, Yuri Shtyrov4, Olga Dragoy1; 1Higher School of Economics, Moscow, Russia, 2Lomonosov Moscow State University, Moscow, Russia, 3Institute of Advanced Study, Pavia, Italy, 4Aarhus University, Denmark
Neuropsychological studies suggest functional and neuroanatomical differences between noun and verb processing. Empirical data show that the inferior frontal gyrus (IFG) is specifically involved in action-related word processing. One vital question is whether the difference between verbs and nouns is conceptual or grammatical in nature. In the current study, we examined whether repetitive transcranial magnetic stimulation (rTMS) over the IFG would only influence verb production if a grammatical class is at stake, or would influence both noun and verb production, if it is involved in processing of action-related words. Healthy Russian adults (n = 31) with a different direction and degree of handedness, as measured with the Edinburgh Handedness Inventory, participated in the study. Handedness quotient (HQ) for each participant ranged from -100 to +100. Individual language-related functional activation maps and language lateralization were previously assessed in them in a functional magnetic resonance imaging (fMRI) study with a sentence completion task. Laterality indices (LIs) for functional maps were calculated with LI-Toolbox in SPM and ranged from -1(right hemisphere) to 1(left hemisphere). We applied rTMS over participants’ left and right IFG, as well as sham vertex stimulation (control condition), in an object and action naming task. rTMS bursts were delivered for 1000 ms and 300 ms after an onset of the picture presentation. The task was to name an object or to say what a hero is doing in a picture, with one word. We fitted generalized (for accuracy) and linear mixed effect models (for log-transformed reaction times (RTs)) separately for nouns and verbs with stimulated site, HQ and LI as predictors. During stimulation over the left IFG, compared to the control sham condition, verbs were produced significantly more accurately (p = 0.03) and there were significant interactions between RTs, LI and HQ: increased LIs predicted decreased RTs (p=0.03) and increased HQ predicted increased RTs (p < 0.01) in action naming, while increased LI and increased HQ predicted increased RTs (p = 0.03) in object naming. After stimulation over the right IFG compared to the control condition we did not find the main effect of stimulated site, but several significant interactions were found: in object naming accuracy was significantly higher with increased LIs (p < 0.05), increased LIs predicted increased RTs (p = 0.001) and increased HQ predicted decreased RTs (p < 0.001); in action naming increased LIs and increased HQ predicted increased RTs (p < 0.001). The results demonstrate TMS-induced facilitation effect for action naming in the left IFG stimulation condition. The accuracy scores for object naming, as well as RTs for both action and object naming were affected by stimulation depending on handedness and language lateralization of our participants. Individuals with more left lateralized language representation were faster in verb production when the left IFG was stimulated. This result confirmed the specificity of the left IFG for processing verbs as a grammatical class. The hypothesis about involvement of these regions in processing of action-related words irrespective of their grammatical class was not confirmed.