The Role of Unimodal and Transmodal Cortex in Perceptually-Coupled and Decoupled Semantic Cognition: Evidence from fMRI

Semantic retrieval extends beyond the here-and-now, to draw on abstract knowledge that has been extracted across multiple experiences; for instance, we can easily bring to mind what a dog looks and sounds like, even when a dog is not present in our environment. However, a clear understanding of the neural substrates that support patterns of semantic retrieval that are not immediately driven by stimuli in the environment is lacking. This thesis sought to investigate the neural basis of semantic retrieval within unimodal and heteromodal networks, whilst manipulating the availability of information in the environment. Much of the empirical work takes inspiration from modern accounts of transmodal regions (Lambon Ralph et al. 2017; Margulies et al. 2016), which suggest the anterior temporal lobe (ATL) and default mode network (DMN) support both abstraction and perceptual decoupling. The first empirical chapter examines whether words and experiences activate common neural substrates in sensory regions and where, within the ATLs, representations are transmodal. The second empirical chapter investigates how perceptually-decoupled forms of semantic retrieval in imagination are represented across unimodal and transmodal regions. The third empirical chapter interrogates whether transmodal regions respond in a similar manner to conceptually-guided and perceptually-decoupled cognition, and whether these two factors interact. The data suggests ventrolateral ATL processes both abstract modality-invariant semantic representations (Chapter 3) and decoupled semantic processing during imagination (Chapter 4). In addition, this thesis found comparable networks recruited for both conceptual processing and perceptually-decoupled retrieval corresponding to the broader DMN (Chapter 5). Further interrogation of these sites confirmed lateral MTG and bilateral angular gyrus were pivotal in the combination of conceptual retrieval from memory. Collectively, this data suggests that brain regions situated farthest from sensory input systems in both functional and connectivity space are required for the most abstract forms of cognition.