Neural dynamics underlying object and action naming across languages: Insights from health and disease

Abstract

Words representing objects (nouns) and words representing actions (verbs) are essential components of speech across languages. Previous studies have shown that the lexico-semantic processing of object and action knowledge depends on segregated ventral and dorsal pathways, respectively involving inferotemporal and frontoparietal nodes. Despite extensive research, a comprehensive understanding of the neural dynamics (including spatial, temporal, and spectral patterns) underlying this process and its potential variation across languages is currently missing. Furthermore, evidence supporting its functional reorganization in the presence of brain damage is scarse. This doctoral thesis addresses two critical questions: (1) What are the neural signatures of object and action naming across different languages? (2) How does the presence of brain tumors affecting ventral and dorsal pathways impact on the lexico-semantic processing of object and action knowledge? To answer these questions, we collected behavioral and MEG data while participants performed a picture-naming task, the gold standard for investigating speech production. We evaluated healthy monolingual and bilingual speakers, and brain tumor patients with low-grade gliomas in ventral and dorsal regions, both before and after surgery for tumor resection. Results from healthy participants showed segregated beta (13¿28 Hz) power decreases in left ventral and dorsal pathways for object and action naming. These modulations occurred during a time-window associated with lexico-semantic processing (~250-500ms), supporting the notion that speech production involves partially different networks depending on the semantic category being processed. Interestingly, we found that these neural patterns overlapped across different languages (Spanish and Basque) in highly proficient bilingual speakers, suggesting that when individuals master both languages to a similar extent, category-based representations overlap at the semantic level. Finally, when longitudinally comparing patients' responses (post- vs. pre-surgery) we found beta modulations mimicking the category¿based segregation showed by healthy individuals, with ventral and dorsal damage leading to selective compensation for object and action naming, respectively. A similar pattern was also observed in bilingual patients. Overall, these findings have important theoretical and clinical implications for language (re)organization in health and disease.