Bridging the Causality Gap: Insights into the Spatiotemporal Dynamics of Language Comprehension

Abstract

Language comprehension is a fascinating and complex ability unique to humans. Its neurobiology has yielded profound insights through decades of electrophysiological and neuroimaging studies (Chapter 1). Despite these significant advances on the neurobiology of speech comprehension, understanding the precise timing and causal relevance of brain regions within the human language network remains one of the key challenges. This dissertation addresses this challenge by combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) measurements, providing a means to identify the causal relevance of particular brain regions with high temporal precision. By focusing on the causality of both the time and place of comprehension, this innovative approach offers new insights for developing adequate functional-anatomical models of the neurobiology of language. This dissertation reports data from three distinct studies. Going beyond correlative ev idence, Study 1 (Chapter 2) describes a set of three concurrent TMS-EEG experiments investigating the bottom-up and top-down information flow within the core language network while German native speakers listen to single sentences. Together, the findings of these three experiments indicated that speech comprehension is subserved by the temporally-coordinated interplay between the left inferior frontal and posterior temporal cortex, which may be supported by the left inferior parietal cortex. Going beyond the core system, Study 2 (Chapter 3) describes two EEG experiments that investigated the neurocognitive mechanisms involved in multi-sentence discourse comprehension. Their findings showed that a distinction between a set of positive-going event-related potentials (ERP) reflect general processing principles relevant for interpreting speech in everyday communitive settings. Going beyond this ERP-based timing information, Study 3 (Chapter 4) describes a sequential TMS-EEG probing the causal relevance of brain regions of the semantic (meaning-related) network in the processing of discourse contexts. The findings of this study highlights that the critical contribution of the left inferior parietal cortex in achieving a higher-level interpretation of speech input. In the General Discussion (Chapter 5), the findings of these studies are placed in a wider perspective, addressing their limitations and providing a number of future directions for an even finer-grained understanding of the neurobiology of speech comprehension