Abstracts Category : Other

Neural Mechanisms of Conceptual Relations

by Gwyneth A Lewis

An over-arching goal in neurolinguistic research is to characterize the neural bases of semantic representation. A particularly relevant goal concerns whether we represent features and events (a) together in a generalized semantic hub or (b) separately in distinct but complementary systems. While the left anterior temporal lobe (ATL) is strongly emphasized in representing both feature-based (taxonomic) knowledge and event-based (thematic) knowledge, recent evidence suggests that the temporal parietal junction (TPJ) plays a unique role in thematic semantics. The primary goal of this dissertation was to identify and characterize the neural mechanisms that support taxonomic and thematic semantics, and the general goal was to shed further light on neural stages of word comprehension. We conducted two magnetoencephalography (MEG) experiments to identify neural indices of visual representations (Chapter 1) and to examine ATL vs. TPJ involvement in taxonomic and thematic semantics (Chapter 2), respectively. We also conducted a functional magnetic resonance imaging (fMRI) experiment to characterize the role of the TPJ in thematic inhibition vs. thematic semantics (Chapter 3). The three experiments employed semantic judgment tasks, equated stimulus conditions on linguistic and psycholinguistic variables, and supplemented analyses with continuous variables as more sensitive hallmarks of lexical access. Chapter 1 demonstrated that initial stages of spoken word recognition involve contact with visual representations of features associated with the real-world referent. The early timing of the effect suggests that sensory aspects of meaning are not necessarily a product of lexical activation during speech recognition. Chapter 2 demonstrated ATL selectivity for taxonomic relations, and moderate TPJ selectivity for both taxonomic and thematic relations. Results for the TPJ could reflect either inhibition of irrelevant information or conceptual processing. Chapter 3 tested these possibilities by requiring inhibition of the opposite relation in two semantic judgment tasks. Results of this experiment indicate that the TPJ plays a role both in thematic semantics <i> and</i> in inhibitory processing when the conceptual computation requires it. In sum, this dissertation focuses on topics pertaining to neural encoding of words with respect to form and meaning. Across three neurolinguistic experiments, we addressed (1) contributions of visual representations during lexical access, (2) ATL and TPJ selectivity for thematic vs. taxonomic concepts, and (3) TPJ inhibition vs. specialization for thematic concepts, respectively.