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A Coordinated Temporal Interplay within the Language Network: Evidence from TMS-EEG during Sentence Processing

Poster A63 in Poster Session A, Thursday, October 6, 10:15 am - 12:00 pm EDT, Millennium Hall
Also presenting in Poster Slam A, Thursday, October 6, 10:00 - 10:15 am EDT, Regency Ballroom

Joëlle Schroën¹, Thomas Gunter¹, Leon Kroczek², Gesa Hartwigsen¹, Angela Friederici¹; ¹Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ²University of Regensburg, Regensburg, Germany

Introduction. Sentence comprehension is supported by the coordinated temporal interplay within a left-dominant brain network, including the posterior inferior frontal gyrus (pIFG), posterior superior temporal gyrus and sulcus (pSTG/STS), and angular gyrus (AG). In two transcranial magnetic stimulation (TMS) experiments, we used a condition-and-perturb approach to investigate how these distinct brain regions interact during auditory sentence processing. Combined with simultaneous electroencephalography (EEG) measurements, our experiments allowed for a time-sensitive investigation of how sentence-based semantic processing (i.e., as reflected by the N400 response) is affected by the combined perturbation of two language-related brain regions (i.e., AG-pIFG and AG-pSTG/STS). Methods. In each TMS-EEG experiment, the neural excitability of left AG was temporarily reduced (“conditioning”) using 40 seconds of offline continuous theta burst stimulation (cTBS). Next, native German speakers listened to short sentences (i.e., pronoun-verb-article-noun) in which the semantic expectancy for the sentence final noun was either high (e.g., He drinks the beer) or low (e.g., He sees the beer), depending on the prediction given by the verb. In Experiment 1, we applied three pulses (10 Hz) of online repetitive TMS (rTMS) over either left pIFG or left pSTG/STS at mid-sentence verb onset (“perturb”). In Experiment 2, online rTMS perturbation over left pIFG was shifted to a later point in time (i.e., 150 ms after verb onset). In both experiments, sham stimulation was included as an ineffective control condition. Using linear mixed models, we investigated how rTMS perturbation modulated the N400 response at the mid-sentence verb as well as at the sentence final noun. Results. Both experiments showed that cTBS over left AG (“unifocal perturbation”) was not sufficient to affect semantic-based processing, as reflected by a lack of modulation of the N400 response at both the verb and noun position. Nonetheless, cTBS over left AG did sensitize the language network to a disruptive effect of subsequent online rTMS over left pIFG and left pSTG/STS at the mid-sentence verb position. When applied at verb onset, online rTMS over the left pSTG/STS showed functional significance whereas stimulating the left pIFG did not. However, when rTMS was shifted 150 ms later in time, the left pIFG did show a clear functional effect, thereby giving insight into the temporal interplay of regions within the language network. Importantly, both online rTMS effects outlasted the stimulation duration (i.e., mid-sentence verb) and modulated the N400 effect at the sentence final noun, providing causal evidence for verb predictive processing (i.e. predictive coding) during auditory sentence processing. At the behavioral level, no significant modulation of rTMS was found. Conclusion. Together, these findings highlight the joint contribution of left pIFG, left pSTG/STS, and left AG to sentence comprehension, with a clear processing order of left pSTG/STS followed by left pIFG. Consequently, they provide interesting new insight into the temporal dynamics of single word processing in a sentence context. Moreover, our results show that neural markers are more sensitive to modulatory rTMS effects than behavioural measures.

Topic Areas: Speech Perception, Meaning: Combinatorial Semantics