Using TMS to evaluate a causal role for right posterior temporal cortex in talker-specific phonetic processing

Available online 21 April 2023Theories suggest that speech perception is informed by listeners’ beliefs of what phonetic variation is typical of a talker. A previous fMRI study found right middle temporal gyrus (RMTG) sensitivity to whether a phonetic variant was typical of a talker, consistent with literature suggesting that the right hemisphere may play a key role in conditioning phonetic identity on talker information. The current work used transcranial magnetic stimulation (TMS) to test whether the RMTG plays a causal role in processing talker-specific phonetic variation. Listeners were exposed to talkers who differed in how they produced voiceless stop consonants while TMS was applied to RMTG, left MTG, or scalp vertex. Listeners subsequently showed near-ceiling performance in indicating which of two variants was typical of a trained talker, regardless of previous stimulation site. Thus, even though the RMTG is recruited for talker-specific phonetic processing, modulation of its function may have only modest consequences.This research was supported by NSF 1554810 (PI: EBM), NIH NIDCD 2R01 DC013064 (PI: EBM) and NSF NRT 1747486 (PI: JSM). This research was supported in part by the Basque Government through the BERC 2022–2025 program, by the Spanish State Research Agency through BCBL Severo Ochoa excellence accreditation CEX2020-001010- S and award PID2020-119131 GB-I000

Effects of linguistic properties in preoperative language mapping: Insights from VLSM and navigated TMS

Tumors in the left hemisphere of the brain commonly cause language disturbances, prompting the recommendation of surgery for the removal of the tumor. Pre- and intraoperative language mapping methods are typically used to identify and preserve functional tissue. Previous work demonstrated that different semantic categories (e.g., actions vs objects) affect the outcomes of language mapping methods, yet it remains unclear whether words within the same semantic category but with different features also influence mapping outcomes. To this end, the present thesis explores the impact of words within the same semantic category but with different linguistic characteristics on the outcomes of language mapping. In Chapter 1, I reviewed studies that employed magnetic stimulation of the human cortex (Transcranial magnetic Stimulation) and verb or sentences, to highlight that the same brain regions are related to multiple linguistic processes without clear-cut differences between linguistic processing levels. In chapter 2, Lesion Symptom Mapping was used to demonstrate that words with various linguistic characteristics are differentially affected by gliomas in different regions. In chapters 3 and 4, navigated Transcranial Magnetic Stimulation was employed to investigate whether actions with varying numbers of arguments (eat vs hike) are differentially affected by the stimulation. Findings in neurotypical individuals and people with gliomas revealed that the production of transitive verbs (a verb that takes an object, e.g., eat) is more affected when posterior temporal regions of the left hemisphere are stimulated. The key point of this thesis is that items with small linguistic variations are differentially affected by tumor locations, and they can also influence the results of language mapping methods

Perturbation of left posterior prefrontal cortex modulates top-down processing in sentence comprehension

Communication is an inferential process. In particular, language comprehension constantly requires top-down efforts, as often multiple interpretations are compatible with a given sentence. To assess top-down processing in the language domain, our experiment employed ambiguous sentences that allow for multiple interpretations (e.g., The client sued the murderer with the corrupt lawyer., where the corrupt lawyer could either belong to The client or the murderer). Interpretation thus depended on whether participants chunk the words of the sentence into short or long syntactic phrases. In principle, bottom-up acoustic information (i.e., the presence or absence of an intonational phrase boundary at the offset of the murderer) indicates one of the two possible interpretations. Yet, acoustic information often indicates interpretations that require words to be chunked into overly long phrases that would overburden working memory. Processing is biased against these demands, reflected in a top-down preference to chunk words into short rather than long phrases. It is often proposed, but also hotly debated, that the ability to chunk words into short phrases is subserved by the left inferior frontal gyrus (IFG). Here, we employed focal repetitive transcranial magnetic stimulation to perturb the left IFG, which resulted in a further decrease of the aptitude to tolerate long phrases, indicating the inability of the left IFG to assist the chunking of words into phrases. In contrast, the processing of auditory information was not affected. Our findings support a causal top-down role of the left inferior frontal gyrus in the chunking of words into phrases

Online neurostimulation of Broca's area does not interfere with syntactic predictions: A combined TMS-EEG approach to basic linguistic combination

Categorical predictions have been proposed as the key mechanism supporting the fast pace of syntactic composition in human language. Accordingly, grammar-based expectations facilitate the analysis of incoming syntactic information—e.g., hearing the determiner “the” enhances the prediction of a noun—which is then checked against a single or few other word categories. Previous functional neuroimaging studies point towards Broca’s area in the left inferior frontal gyrus (IFG) as one fundamental cortical region involved in categorical prediction during on-line language processing. Causal evidence for this hypothesis is however still missing. In this study, we combined Electroencephalography (EEG) and Transcranial Magnetic Stimulation (TMS) to test whether Broca’s area is functionally relevant in predictive mechanisms for language. Specifically, we transiently perturbed Broca’s area during the categorical prediction phase in two-word constructions, while simultaneously measuring the Event-Related Potential (ERP) correlates of syntactic composition. We reasoned that if Broca’s area is involved in predictive mechanisms for syntax, disruptive TMS during the processing of the first word (determiner/pronoun) would mitigate the difference in ERP responses for predicted and unpredicted categories when composing basic phrases and sentences. Contrary to our hypothesis, perturbation of Broca’s area at the predictive stage did not affect the ERP correlates of basic composition. The correlation strength between the electrical field induced by TMS and the magnitude of the EEG response on the scalp further confirmed this pattern. We discuss the present results in light of an alternative account of the role of Broca’s area in syntactic composition, namely the bottom-up integration of words into constituents