The neural architecture of semantic retrieval with and without cues: evidence from neuropsychology and neuroimaging

Everyday situations are conceptually rich, but not all of this knowledge is relevant at a given time. At the heart of adaptive cognition is flexibility, which allows us to focus on particular mental representations in a way that suits the changing context and goals. Previous work has highlighted the importance of semantic control mechanisms in retrieval, which allow cognition to diverge from dominant associations (Lambon Ralph et al., 2016). However, a clear understanding of the cognitive and neural substrates of semantic flexibility is currently lacking. This work collects evidence from different methods and experimental populations to tackle this broad question. We use novel multimodal semantic cues (i.e. affect and spatial locations) to examine the mechanisms that support flexible patterns of retrieval when the context is helpful or unhelpful. The first two empirical chapters examine behavioural effects of cues and miscues in patients with semantic aphasia (Chapter 2) and investigate whether patients with SA show greater benefits of coherent cue combinations compared to minimal levels of cueing (Chapter 3). The third chapter explores the neural bases of cued semantic retrieval, and tests the predictions of another recent framework which situates the default mode network at the top of a cortical hierarchy of abstraction (Margulies et al., 2016). The final chapter investigates whether the intrinsic connectivity of the brain at rest is predictive of the behavioural efficiency in cued semantic retrieval. Our findings provide evidence for the existence of two qualitatively distinct mechanisms for semantic flexibility, one driven by control processes (impaired in SA) and one driven by the integration of contextual information with long-term semantic knowledge (relatively intact in SA). In line with a growing body of work suggesting a role of default mode network in information integration, we show that more coherent patterns of retrieval which are driven by the context recruit this network. In contrast, multiple-demand regions appear to support more executive aspects of cued retrieval required for the maintenance of cue information. Finally, this thesis provide evidence that affect and location cues are both effective at shaping the activation of semantic knowledge. In summary, this thesis suggests that semantic flexibility is a complex and multi-faceted process which requires an interplay of different cognitive and neural components.