How do we store and retrieve knowledge
Over the last few years, neuropsychological, functional neuroimaging and transcranial magnetic stimulation (TMS) studies have supported the view that a complex, distributed neural network underpins semantic cognition. This talk traces the putative roles of each region within this network. Comparisons of patients who have semantic dementia (SD) and multimodal semantic impairment following stroke aphasia (SA) indicate that semantic cognition draws on at least two interacting components – semantic representations (degraded in SD) and semantic control processes (deficient in SA). To explore the first of these components, we have employed distortion-corrected fMRI and TMS in healthy volunteers: these studies convergently indicate that the anterior temporal lobes (ATL; atrophied in SD) combine information from different modalities within an amodal semantic “hub”. This brain region contributes to the default mode network, which may support information integration. Nevertheless, this capacity to combine different types of information to form conceptual representations may not be sufficient for successful semantic cognition, as we have many features and associations for any given concept, and we need a mechanism to promote the retrieval of relevant aspects of knowledge. To achieve flexible retrieval, semantic representations interact with semantic control processes reliant on left inferior frontal cortex (LIFC) and posterior middle temporal gyrus (pMTG). SA patients with damage to these regions have difficulty focusing on aspects of knowledge that are relevant to the current goal or context, in both verbal and nonverbal semantic tasks (such as object use). Convergent evidence is again provided by fMRI and TMS: both these methods show that LIFC and pMTG act together as a distributed network that lies between domain-general executive regions and the default mode network. In addition, when semantic control demands increase, connectivity between executive and default mode areas also increases.